Past Missions
Luna 2
impact on the surface of the Moon 1959 (USSR)
Luna 3
first photos of the far side of the Moon 1959 (USSR)
Mariner 2
the first successful probe to flyby Venus in December of 1962, and it returned information which confirmed that Venus is a very hot (800 degrees Fahrenheit, now revised to 900 degrees F.) world with a cloud-covered atmosphere composed primarily of carbon dioxide.
(more info from NASA Spacelink)
Mariner 3
launched on November 5, 1964, was lost when its protective shroud failed to eject as the craft was placed into interplanetary space. Unable to collect the Sun's energy for power from its solar panels, the probe soon died when its batteries ran out and is now in solar orbit. It was intended for a Mars flyby with Mariner 4.
Mariner 4
the sister probe to Mariner 3, did reach Mars in 1965 and took the first close-up images of the Martian surface (22 in all) as it flew by the planet. The probe found a cratered world with an atmosphere much thinner than previously thought. Many scientists concluded from this preliminary scan that Mars was a "dead" world in both the geological and biological sense.
Mariner 9
Mariner 9, the sister probe to Mariner 8 which failed on launch, became the first craft to orbit Mars in 1971. It returned information on the Red Planet that no other probe had done before, revealing huge volcanoes on the Martian surface, as well as giant canyon systems, and evidence that water once flowed across the planet. The probe also took the first detailed closeup images of Mars' two small moons, Phobos and Deimos.
Apollo
6 manned landings on the Moon and sample returns 1969-72. (The seventh landing, Apollo 18, was canceled for political reasons)
(Apollo "home page"; Apollo Missions)
Luna 16
automated sample return from the Moon 1970 (USSR)
Pioneer 10 and Pioneer 11
Pioneer 10 was the first spacecraft to flyby Jupiter in 1973. Pioneer 11 followed it in 1974, and then went on to become the first probe to study Saturn in 1979. The Pioneers were designed to test the ability of spacecraft to survive passage thru the asteroid belt and Jupiter's magnetosphere. The asteroid belt was easy, but they were nearly fried by ions trapped in Jupiter's magnetic field. This information was crucial to the success of the Voyager missions.
Pioneer 11's RTG power supply is dead. Its last communication with Earth was in November 1995. Pioneer 10 is still functioning (barely) but is no longer being tracked regularly due to budget cutbacks. The last data was received from it on 1997 March 31. They are heading off into interstellar space, the first craft ever to do so.
As the first two spacecraft to leave our solar system, Pioneer 10 & 11 carry a graphic message in the form of a 6- by 9-inch gold anodized plaque bolted to the spacecraft's main frame.
(Pioneer Project Home Page and more about Pioneer 10 and Pioneer 11 from NASA Spacelink )
Mariner 10
used Venus as a gravity assist to Mercury in 1974. The probe did return the first close-up images of the Venusian atmosphere in ultraviolet, revealing previously unseen details in the cloud cover, plus the fact that the entire cloud system circles the planet in four Earth days. Mariner 10 eventually made three flybys of Mercury from 1974 to 1975 before running out of attitude control gas. The probe revealed Mercury as a heavily cratered world with a mass much greater than thought. This would seem to indicate that Mercury has an iron core which makes up 75 percent of the entire planet.
(more from JPL and JPL)
Venera 7
First probe to return data from the surface of another planet (Venus) in 1970.
Venera 9
soft landing on Venus, pictures of the surface 1975. (USSR) This was the first spacecraft to land on the surface of another planet.
Pioneer Venus
1978; orbiter and four atmospheric probes; made the first high-quality map of the surface of Venus.
(more info from NASA Spacelink; and NSSDC a tutorial from UCLA)
Viking 1
Viking 1 was launched from Cape Canaveral, Florida on August 20, 1975 on a TITAN 3E-CENTAUR D1 rocket. The probe went into Martian orbit on June 19, 1976, and the lander set down on the western slopes of Chryse Planitia on July 20, 1976. It soon began its programmed search for Martian micro-organisms (there is still debate as to whether the probes found life there or not), and sent back incredible color panoramas of its surroundings. One thing scientists learned was that Mars' sky was pinkish in color, not dark blue as they originally thought (the sky is pink due to sunlight reflecting off the reddish dust particles in the thin atmosphere). The lander set down among a field of red sand and boulders stretching out as far as its cameras could image.
Viking 2
Viking 2 was launched on September 9, 1975, and arrived in Martian orbit on August 7, 1976. The lander touched down on September 3, 1976 in Utopia Planitia. It accomplished essentially the same tasks as its sister lander, with the exception that its seismometer worked, recording one marsquake.
The last data from Viking (Lander 1) made its final transmission to Earth Nov. 11, 1982. Controllers at JPL tried unsuccessfully for another six and one-half months to regain contact with Viking Lander 1. The overall mission came to an end May 21, 1983.
An interesting side note: Viking 1's lander has been designated the Thomas A. Mutch Memorial Station in honor of the late leader of the lander imaging team. The National Air and Space Museum in Washington, DC is entrusted with the safekeeping of the Mutch Station Plaque until it can be attached to the lander by a manned expedition.
(more info (pdf) and an web page from JPL)
Voyager 1
Voyager 1 (image at top) was launched September 5, 1977, and flew past Jupiter on March 5, 1979 and by Saturn on November 13, 1980. Voyager 2 was launched August 20, 1977 (before Voyager 1), and flew by Jupiter on August 7, 1979, by Saturn on August 26, 1981, by Uranus on January 24, 1986, and by Neptune on August 8, 1989. Voyager 2 took advantage of a rare once-every-189-years alignment to slingshot its way from outer planet to outer planet. Voyager 1 could, in principle, have headed towards Pluto, but JPL opted for the sure thing of a Titan close up.
Between the two probes, our knowledge of the 4 giant planets, their satellites, and their rings has become immense. Voyager 1 & 2 discovered that Jupiter has complicated atmospheric dynamics, lightning and aurorae. Three new satellites were discovered. Two of the major surprises were that Jupiter has rings and that Io has active sulfurous volcanoes, with major effects on the Jovian magnetosphere.
When the two probes reached Saturn, they discovered over 1000 ringlets and 7 satellites, including the predicted shepherd satellites that keep the rings stable. The weather was tame compared with Jupiter: massive jet streams with minimal variance (a 33-year great white spot/band cycle is known). Titan's atmosphere was smoggy. Mimas's appearance was startling: one massive impact crater gave it the Death Star appearance. The big surprise here was the stranger aspects of the rings. Braids, kinks, and spokes were both unexpected and difficult to explain.
Voyager 2
Voyager 2, thanks to heroic engineering and programming efforts, continued the mission to Uranus and Neptune. Uranus itself was highly monochromatic in appearance. One oddity was that its magnetic axis was found to be highly skewed from the already completely skewed rotational axis, giving Uranus a peculiar magnetosphere. Icy channels were found on Ariel, and Miranda was a bizarre patchwork of different terrains. 10 satellites and one more ring were discovered.
In contrast to Uranus, Neptune was found to have rather active weather, including numerous cloud features. The ring arcs turned out to be bright patches on one ring. Two other rings, and 6 other satellites, were discovered. Neptune's magnetic axis was also skewed. Triton had a canteloupe appearance and geysers. (What's liquid at 38K?)
If no unforeseen failures occur, we will be able to maintain communications with both spacecraft until at least the year 2030. Both Voyagers have plenty of hydrazine fuel -- Voyager 1 is expected to have enough propellant until 2040 and Voyager 2 until 2034. The limiting factor is the RTGs (radio-isotope thermal generators). The power output from the RTGs is slowly dropping each year. By 2000, there won't be enough power for the UVS (ultraviolet spectrometer) instrument. By 2010, the power will have dropped low enough such that not all of the fields and particles instruments can be powered on at the same time. A power sharing plan will go into effect then, where some of the F & P instruments are powered on, and others off. The spacecraft can last in this mode for about another 10 years, and after that the power will probably be too low to maintain the spacecraft.
(the Voyager Project Home Page from JPL; another nice "home page" at NSSDC; fact sheets and a web page from JPL )
Vega
International project VENUS-HALLEY, launched in 1984, carried a Venus orbiter and lander and did a fly-by of Comet Halley.
(Vega Mission Home page
Phobos
Two spacecraft were launched by the USSR in 1988. One failed without a trace. A few images were returned before the second one failed, too.
(Phobos Mission Home page
Giotto
Giotto was launched by an Ariane-1 by ESA on July 2 1985, and approached within 540 km +/- 40 km of the nucleus of Comet Halley on March 13, 1986. The spacecraft carried 10 instruments including a multicolor camera, and returned data until shortly before closest approach, when the downlink was temporarily lost. Giotto was severely damaged by high-speed dust encounters during the flyby and was placed into hibernation shortly afterwards.
In April, 1990, Giotto was reactivated. 3 of the instruments proved fully operational, 4 partially damaged but usable, and the remainder, including the camera, were unusable. On July 2, 1990, Giotto made a close encounter with Earth and was retargeted to a successful flyby of comet Grigg-Skjellerup on July 10, 1992.
(more info from NSSDC)
Clementine
a joint mission of the Ballistic Missile Defense Organization (formerly SDIO) and NASA to flight test sensors developed by Lawrence Livermore for BMDO. The spacecraft, built by the Naval Research Lab, was launched on January 25 1994 to a 425 km by 2950 km orbit of the Moon for a 2 month mapping mission. Instruments onboard include UV to mid-IR imagers, including an imaging lidar that may be able to also obtain altimetric data for the middle latitudes of the Moon. In early May the spacecraft was to have been sent out of lunar orbit toward a flyby of the asteroid 1620 Geographos but a failure prevented the attempt.
Ground controllers have regained control of the spacecraft, however. Its potential future mission is being considered.
(for more information see the Clementine Mission Home page from USGS and the Clementine page from NASA PDS or The Clementine Mission from LPI.)
Mars Observer
Mars orbiter including 1.5 m/pixel resolution camera. Launched 9/25/92 on a Titan III/TOS booster. Contact was lost with MO on 8/21/93 while it was preparing for entry into Mars orbit. The spacecraft has been written off (postmortem analysis). Mars Global Surveyor, a replacement mission to achieve most of MO's science goals, has been very successful.
Magellan
Launched in May 1989, Magellan has mapped 98% of the surface of Venus at better than 300 meter resolution and obtained a comprehensive gravity field map for 95 percent of the planet. Magellan recently executed an 80-day aerobraking program to lower and circularize its orbit. Magellan has completed its radar mapping and gravity data collection. In the fall of 1994, just before it would have failed due to deterioration in its solar panels, Magellan was deliberately sent into Venus' atmosphere to further study aerobraking techniques which can make major savings in fuel for future missions.
(more info (pdf), a web page and another web page from JPL; fact sheet from NSSDC)
Galileo
Jupiter orbiter and atmosphere probe. It made extensive surveys of the Jovian moons and the probe descended into Jupiter's atmosphere to provide our first direct evidence of the interior of a gas giant.
In addition, Galileo has returned the first resolved images of two asteroids, 951 Gaspra and 243 Ida, while in transit to Jupiter. It also returned pictures of the impact of Comet SL9 onto Jupiter from its unique vantage point.
Galileo was deliberately crashed in to Jupiter in 2003 to prevent any possibility that it might crash into Europa and contaminate any life that might be there.
(Education and Public Outreach (images!); Galileo Home Page; Galileo Probe Home Page from ARC; web page; NSSDC page; preliminary Galileo Probe Results from JPL and LANL)
Mars 96
a large orbiter with several landers originally known as Mars 94. Launch failed 1996 November 17. (The original Mars 96 was known for a while as Mars 98 and then cancelled.) (more info from MSSS and from IKI (Russia))
Pathfinder
The Mars Pathfinder (formerly known as the Mars Environmental Survey, or MESUR, Pathfinder) is the second of NASA's low-cost planetary Discovery missions. The mission consists of a stationary lander and a surface rover known as Sojourner. The mission has the primary objective of demonstrating the feasibility of low-cost landings on and exploration of the Martian surface. This objective will be met by tests of communications between the rover and lander, and the lander and Earth, and tests of the imaging devices and sensors.
The scientific objectives include atmospheric entry science, long-range and close-up surface imaging, with the general objective being to characterize the Martian environment for further exploration. The spacecraft will enter the Martian atmosphere without going into orbit around the planet and land on Mars with the aid of parachutes, rockets and airbags, taking atmospheric measurements on the way down. Prior to landing, the spacecraft will be enclosed by three triangular solar panels (petals), which will unfold onto the ground after touchdown.
Mars Pathfinder was launched 1996 December 4 and landed successfully on Mars on 1997 July 4.
( MPF Home Page from JPL; more info from NSSDC; images and press releases from MSFC; Mars Watch, Linking Amateur and Professional Mars Observing Communities for Observational Support of the Mars Pathfinder Mission)
NEAR
The Near Earth Asteroid Rendezvous (NEAR) mission promises to answer fundamental questions about the nature of near-Earth objects such as asteroids and comets.
Launched on 1996 February 17 aboard a Delta 2 rocket, the NEAR spacecraft should arrive in orbit around asteroid 433 Eros in early January 1999. It will then survey the rocky body for a minimum of one year, at altitudes as close as 15 miles (24 kilometers). Eros is one of the largest and best-observed asteroids whose orbits cross Earth's path. These asteroids are closely related to the more numerous "Main Belt" asteroids that orbit the Sun in a vast doughnut-shaped ring between Mars and Jupiter.
(NEAR Home Page; more info from NSSDC; more from John Hopkins Univ.; Curriculum materials)
Lunar Prospector
Lunar Prospector, the first NASA mission to the Moon in almost 30 years, was launched Jan 6th, 1998. Within a month it will begin returning answers to long-standing questions about the Moon, its resources, its structure and its origins. (Welcome to the Moon, Lunar Prospector home page); more from NSSDC
Ongoing Missions
Voyager 1 and 2
still operational after more than 15 years in space and are traveling out of the Solar System. The two Voyagers are expected to last until at least the year 2015 when their radioisotope thermoelectric generators (RTG) power supplies are expected for fail. Their trajectories give negative evidence about possible planets beyond Pluto. Their next major scientific discovery should be the location of the heliopause. Low-frequency radio emissions believed to originate at the heliopause have been detected by both Voyagers.
Both Voyagers are using their ultraviolet spectrometers to map the heliosphere and study the incoming interstellar wind. The cosmic ray detectors are seeing the energy spectra of interstellar cosmic rays in the outer heliosphere
Voyager 1 has passed the Pioneer 10 spacecraft and is now the most distant human-made object in space.
(more info from JPL)
Hubble Space Telescope
launched April 1990; fixed December 1993. HST can provide pictures and spectra over a long period of time. This provides an important extra dimension to the higher resolution data from the planetary probes. For example, recent HST data show that Mars is colder and drier than during the Viking missions; and HST images of Neptune indicate that its atmospheric features change rapidly.
Named for the American astronomer Edwin Hubble.
Much, much more information about HST and HST pictures are available at the Space Telescope Science Institute. HST's latest images are posted regularly. (Here is a brief history of the HST project. There's also some more HST info at JPL.)
Ulysses
now investigating the Sun's polar regions (European Space Agency/NASA). Ulysses was launched by the Space Shuttle Discovery in October 1990. In February 1992, it got a gravity boost from Jupiter to take it out of the plane of the ecliptic. It has now completed its main mission of surveying both of the Sun's poles. Its mission has been extended for another orbit so that it can survey the Sun's poles near the maximum of the sunspot cycle, too. Its aphelion is 5.2 AU, and, surprisingly, its perihelion is about 1.5 AU-- that's right, a solar-studies spacecraft that's always further from the Sun than the Earth is! It is expected to provide a much better understanding of the Sun's magnetic field and the solar wind.
(Ulysses Home Pages from JPL and ESA)
Wind
After its November 1, 1994, launch, NASA's Wind satellite will take up a vantage point between the Sun and the Earth, giving scientists a unique opportunity to study the enormous flow of energy and momentum known as the solar wind.
The main scientific goal of the mission is to measure the mass, momentum and energy of the solar wind that somehow is transferred into the space environment around the Earth. Although much has been learned from previous space missions about the general nature of this huge transfer, it is necessary to gather a great deal of detailed information from several strategic regions of space around the Earth before scientists understand the ways in which the planet's atmosphere responds to changes in the solar wind.
The launch also marks the first time a Russian instrument will fly on an American spacecraft. The Konus Gamma-Ray Spectrometer instrument, provided by the Ioffe Institute, Russia, is one of two instruments on Wind which will study cosmic gamma-ray bursts, rather than the solar wind. A French instruments is also aboard.
At first, the satellite will have a figure-eight orbit around the Earth with the assistance of the Moon's gravitational field. Its furthest point from the Earth will be up to 990,000 miles (1,600,000 kilometers), and its closest point will be at least 18,000 miles (29,000 kilometers).
Later in the mission, the Wind spacecraft will be inserted into a special halo orbit in the solar wind upstream from the Earth, at the unique distance which allows Wind to always remain between the Earth and the Sun (about 930,000 to 1,050,000 miles, or 1,500,000 to 1,690,000 kilometers, from the Earth).
Mars Surveyor Program
Launched with a Delta II expendable vehicle from Cape Canaveral, Fla., on November 7 1996, the spacecraft is now in orbit around Mars. The spacecraft circles Mars once every two hours, maintaining a "sun synchronous" orbit that will put the sun at a standard angle above the horizon in each image and allow the mid-afternoon lighting to cast shadows in such a way that surface features will stand out. The spacecraft will carry a portion of the Mars Observer instrument payload and will use these instruments to acquire data of Mars for a full Martian year, the equivalent of about two Earth years. The spacecraft will then be used as a data relay station for signals from U.S. and international landers and low-altitude probes for an additional three years.
(MGS Home Page from JPL; Planned Missions from 1996 to 2003)
Cassini
Saturn orbiter and Titan atmosphere probe. Cassini is a joint NASA/ESA project designed to accomplish an exploration of the Saturnian system with its Cassini Saturn Orbiter and Huygens Titan Probe. Cassini was launched aboard a Titan IV/Centaur 1997 Oct 15.
(Cassini Home Page from JPL; Huygens Home Page; another Cassini page from JPL)
Stardust
Scheduled for launch in February 1999, Stardust will fly close to a comet and, for the first time ever, bring material from the comets coma back to Earth for analysis by scientists worldwide. Scheduled to fly-by Comet Wild-2 in 2004, return to Earth in 2006.
(home page) (All missions not otherwise labeled are NASA)
วันอังคารที่ 18 กันยายน พ.ศ. 2550
Seeing the Solar System
You don't need your own Voyager to see the solar system. You can see much of it from your own back yard. Of course, you don't see the fantastic closeup views that NASA gets, but you can see it first-hand with your own eyes. If you enjoyed The Nine Planets, go outside and take a look at what you just read about. You'll be amazed how rewarding such a simple thing it can be.
Hardcopy
Touring the Universe through Binoculars A personal tour of the universe using nothing more than a pair of binoculars.
Turn Left at Orion A guide to the night sky perfect for those with no previous knowledge of astronomy and in any age group. Shows how to explore the sky with a small telescope.
Nightwatch: A Practical Guide to Viewing the Universe A classic handbook combines a text both meaty and hard to put down with graphics and dazzling full-color photos.
To find the planets, you'll need to know where to look. Refer to Sky & Telescope or a similar magazine for up to date positions or check one of the several Web sites that show planetary positions. A planetarium program can also be useful, especially for fast moving objects like moons and comets. A simple chart or planisphere is a nice way to find the bright stars and constellations but isn't much help for planets.
The tables below are ordered by visual magnitude ("Vo"; bigger numbers are dimmer); this is the maximum brightness that the object attains (approximately when it is closest to Earth). "Date" is the date of discovery.
Unaided Eye
You can see 99.99% of the mass of the solar system with no instruments whatsoever:
Name
Vo
Sun
-27
Earth
Moon
-13
Venus
-4.4
Jupiter
-2.7
Mars
-2.0
Mercury
-1.9
Saturn
+0.7
Never look directly at the Sun! Always use a special solar filter designed specifically for solar observing.
Solar Observing FAQ by Jeff Medkeff
Does the Earth really count? Only the Apollo astronauts have ever seen the Earth from far enough away to perceive it as a globe.
Those with good eyes (especially children) and dark skies may be able to see a few of the binocular objects below, too.
Binoculars
A simple pair of binoculars is by far the most cost-effective optical aid available. For $200 you can get a far better optical instrument than Galileo or Newton had. You will find it much easier if you arrange a stable support for your binoculars (such as a tripod):
Name
Date
Vo
Discoverer
Ganymede
1610
4.6
Galileo Galilei
Io
1610
5.0
Galileo Galilei
Europa
1610
5.3
Galileo Galilei
Uranus
1781
5.5
William Herschel
Callisto
1610
5.6
Galileo Galilei
Neptune
1846
7.8
Johann Gottfried Galle
Titan
1655
8.3
Christiaan Huygens
Looking at the Sun with binoculars even for a fraction of a second can burn a hole in your retina. Be very careful, especially when looking for Mercury.
Amateur Telescopes
If you're more serious a modest telescope will reveal many more moons. The first few below are pretty easy, the last few are considerably more difficult. Good dark skies are essential:
Name
Date
Vo
Discoverer
Rhea
1672
9.7
Giovanni Domenico Cassini
Tethys
1684
10.2
Giovanni Domenico Cassini
Iapetus
1671
10.2
Giovanni Domenico Cassini
Dione
1684
10.4
Giovanni Domenico Cassini
Phobos
1877
11.3
Asaph Hall
Enceladus
1789
11.7
William Herschel
Deimos
1877
12.4
Asaph Hall
Mimas
1789
12.9
William Herschel
Triton
1846
13.5
William Lassell
Pluto
1930
13.6
Clyde W. Tombaugh
Titania
1787
13.7
William Herschel
Oberon
1787
13.9
William Herschel
Amalthea
1892
14.1
Edward Emerson Barnard
Ariel
1851
14.2
William Lassell
Hyperion
1848
14.2
William Cranch Bond
Janus
1966
14.5
Audouin Dollfus
Umbriel
1851
14.8
William Lassell
Himalia
1904
14.8
C. Perrine
Phobos and Deimos are harder to see than it might appear since they are so close to Mars (and the above magnitudes are for a favorable opposition)
The same holds for Amalthea and Janus.
Iapetus' brightness varies greatly as it rotates, from 10.2 to 11.9 or less.
The order of discovery may be a better guide to what is easy to see than magnitude.
Mars FAQ for amateur astronomers
With a small telescope you can easily see the phases of Venus and even the phases of Mercury when conditions are right.
Don't buy your first telescope without first reading Information for Beginning Astronomers
Other objects
Of course, the solar system has more than just planets and moons. Every year there are comets that can be seen with small telescopes and usually one or two that can be seen with binoculars. Occasionally there are comets visible to the unaided eye such as Hale-Bopp which was so spectacular in 1997.
It's easy to see a few of the brighter asteroids with binoculars. Several hundred can be seen with small telescopes. And even today, many asteroids and comets are still discovered by amateur astronomers.
If you're out at night under a clear sky, you are likely to see a meteor. You may see dozens of meteors if you catch one of the regular meteor showers.
You can even see the interplanetary medium if you're close enough to the poles to see an aurora or if you see the zodiacal light or the gegenschein.
You can also see the stars 51 Pegasi, 70 Virginis and 47 Ursae Majoris which probably have their own planets, though of course, you can't see the planets themselves.
Photography
Hardcopy
Touring the Universe through Binoculars A personal tour of the universe using nothing more than a pair of binoculars.
Turn Left at Orion A guide to the night sky perfect for those with no previous knowledge of astronomy and in any age group. Shows how to explore the sky with a small telescope.
Nightwatch: A Practical Guide to Viewing the Universe A classic handbook combines a text both meaty and hard to put down with graphics and dazzling full-color photos.
To find the planets, you'll need to know where to look. Refer to Sky & Telescope or a similar magazine for up to date positions or check one of the several Web sites that show planetary positions. A planetarium program can also be useful, especially for fast moving objects like moons and comets. A simple chart or planisphere is a nice way to find the bright stars and constellations but isn't much help for planets.
The tables below are ordered by visual magnitude ("Vo"; bigger numbers are dimmer); this is the maximum brightness that the object attains (approximately when it is closest to Earth). "Date" is the date of discovery.
Unaided Eye
You can see 99.99% of the mass of the solar system with no instruments whatsoever:
Name
Vo
Sun
-27
Earth
Moon
-13
Venus
-4.4
Jupiter
-2.7
Mars
-2.0
Mercury
-1.9
Saturn
+0.7
Never look directly at the Sun! Always use a special solar filter designed specifically for solar observing.
Solar Observing FAQ by Jeff Medkeff
Does the Earth really count? Only the Apollo astronauts have ever seen the Earth from far enough away to perceive it as a globe.
Those with good eyes (especially children) and dark skies may be able to see a few of the binocular objects below, too.
Binoculars
A simple pair of binoculars is by far the most cost-effective optical aid available. For $200 you can get a far better optical instrument than Galileo or Newton had. You will find it much easier if you arrange a stable support for your binoculars (such as a tripod):
Name
Date
Vo
Discoverer
Ganymede
1610
4.6
Galileo Galilei
Io
1610
5.0
Galileo Galilei
Europa
1610
5.3
Galileo Galilei
Uranus
1781
5.5
William Herschel
Callisto
1610
5.6
Galileo Galilei
Neptune
1846
7.8
Johann Gottfried Galle
Titan
1655
8.3
Christiaan Huygens
Looking at the Sun with binoculars even for a fraction of a second can burn a hole in your retina. Be very careful, especially when looking for Mercury.
Amateur Telescopes
If you're more serious a modest telescope will reveal many more moons. The first few below are pretty easy, the last few are considerably more difficult. Good dark skies are essential:
Name
Date
Vo
Discoverer
Rhea
1672
9.7
Giovanni Domenico Cassini
Tethys
1684
10.2
Giovanni Domenico Cassini
Iapetus
1671
10.2
Giovanni Domenico Cassini
Dione
1684
10.4
Giovanni Domenico Cassini
Phobos
1877
11.3
Asaph Hall
Enceladus
1789
11.7
William Herschel
Deimos
1877
12.4
Asaph Hall
Mimas
1789
12.9
William Herschel
Triton
1846
13.5
William Lassell
Pluto
1930
13.6
Clyde W. Tombaugh
Titania
1787
13.7
William Herschel
Oberon
1787
13.9
William Herschel
Amalthea
1892
14.1
Edward Emerson Barnard
Ariel
1851
14.2
William Lassell
Hyperion
1848
14.2
William Cranch Bond
Janus
1966
14.5
Audouin Dollfus
Umbriel
1851
14.8
William Lassell
Himalia
1904
14.8
C. Perrine
Phobos and Deimos are harder to see than it might appear since they are so close to Mars (and the above magnitudes are for a favorable opposition)
The same holds for Amalthea and Janus.
Iapetus' brightness varies greatly as it rotates, from 10.2 to 11.9 or less.
The order of discovery may be a better guide to what is easy to see than magnitude.
Mars FAQ for amateur astronomers
With a small telescope you can easily see the phases of Venus and even the phases of Mercury when conditions are right.
Don't buy your first telescope without first reading Information for Beginning Astronomers
Other objects
Of course, the solar system has more than just planets and moons. Every year there are comets that can be seen with small telescopes and usually one or two that can be seen with binoculars. Occasionally there are comets visible to the unaided eye such as Hale-Bopp which was so spectacular in 1997.
It's easy to see a few of the brighter asteroids with binoculars. Several hundred can be seen with small telescopes. And even today, many asteroids and comets are still discovered by amateur astronomers.
If you're out at night under a clear sky, you are likely to see a meteor. You may see dozens of meteors if you catch one of the regular meteor showers.
You can even see the interplanetary medium if you're close enough to the poles to see an aurora or if you see the zodiacal light or the gegenschein.
You can also see the stars 51 Pegasi, 70 Virginis and 47 Ursae Majoris which probably have their own planets, though of course, you can't see the planets themselves.
Photography
Other Planetary Systems?
Are there planets orbiting other stars beyond our solar system? We do not know for sure, but with the recent discoveries about 51 Pegasi, 70 Virginis and 47 Ursae Majoris the weight of evidence is now so strong that only a "devil's advocate" denies the conclusions. Here is some of what we do know (this is somewhat incomplete; please see the references below for more info):
Facts
Three small bodies have been found in orbit around the pulsar PSR 1257+12. They have been designated "PSR1257+12 A, ..B, and ..C". One is about the size of the Moon, the other two are about 2 to 3 times the mass of Earth.
They were discovered by measuring variations in the pulsation speed of the pulsar which can be interpreted as gravitational effects of three small planets. The original observation has been confirmed but, of course, no direct images have been made -- that is way beyond the capabilities of our best telescopes.
These planets are believed to have formed after the supernova that produced the pulsar. The present planets would have originally been within the envelope of the progenitor star and therefore wouldn't have stood much chance of surviving the supernova explosion, and wouldn't have remained in circular orbits after the explosion.
Several decades of timing data on the pulsar PSR 0329+54 (PKS B0329+54) by Tatiana Shabanova (Lebedev Physics Institute) shows evidence of a planet with a 16.9 year period and mass greater than 2 Earth masses.
But, while the evidence for these is pretty good, they aren't really what we're looking for when we talk about 'solar systems'.
It has been known since 1983 that the star Beta Pictoris is surrounded by a disk of gas and dust. Spectra of Beta Pictoris show absorption features which are currently believed to be due to cometary like clouds of gas occultating the star from the debris left over from planetary formation. Though it's far from certain it is believed by some that planets may already have formed around Beta Pictoris.
HST has observed Beta Pictoris (right) and found the disk to be significantly thinner than previously thought. Estimates based on the Hubble image place the disk's thickness as no more than one billion miles (1600 million kilometers), or about 1/4 previous estimates from ground-based observations. The disk is tilted nearly edge-on to Earth. Because the dust has had enough time to settle into a flat plane, the disk may be older than some previous estimates. A thin disk also increases the probability that comet-sized or larger bodies have formed through accretion in the disk. Both conditions are believed to be characteristic of a hypothesized circumstellar disk around our own Sun, which was a necessary precursor to the planet-building phase of our Solar Systems, according to current theory.
More recent HST observations have shown the disk to be slightly warped as might be expected from the gravitational influence of a planet. This has been confirmed by observations at ESO.
Recent observations at radio wavelengths of a gas cloud known as Bok Globule B335 have produced images of material collapsing onto a newly born star (only about 150,000 years old). These observations are helping to understand how stars and planets form. The phenomena observed matches the theory of the formation of the solar system -- that is, a large gas cloud collapsed to form a star with an attendant circumstellar disk in which, over time, planets accreted from the matter in the disk and orbited the Sun.
The IRAS satellite found that Vega had too much infrared emission, and that has been attributed to a dust shell (with a mass of maybe Earth's moon).
Observations of the very nearby Barnard's Star were once thought to be evidence of gravitational effects of planets but they now seem to have been in error.
The star Gl229 seems to contain a 20 Jupiter mass object orbiting at a distance of 44 AU. An object this large is probably a brown-dwarf rather than an ordinary planet.
What may be the first discovery of a planet orbiting a normal, Sun-like star other than our own has been announced by astronomers studying 51 Pegasi, a spectral type G2-3 V main-sequence star 42 light-years from Earth. At a recent conference in Florence, Italy, Michel Mayor and Didier Queloz of Geneva Observatory explained that they observed 51 Pegasi with a high-resolution spectrograph and found that the star's line-of-sight velocity changes by some 70 meters per second every 4.2 days. If this is due to orbital motion, these numbers suggest that a planet lies only 7 million kilometers from 51 Pegasi -- much closer than Mercury is to the Sun -- and that the planet has a mass at least half that of Jupiter. These physical characteristics hinge on the assumption that our line of sight is near the planet's orbital plane. However, other evidence suggests that this is a good bet. A world merely 7 million km from a star like 51 Pegasi should have a temperature of about 1,000 degrees Celsius, just short of red hot. It was initially thought that it might be a solid body like a very big Mercury but the concensus now seems to be that it is a "hot Jupiter", a gas planet formed much farther from its star that migrated inward.
These observations have now been confirmed by several independent observers. And there is some evidence for a second planet much farther out that is not yet confirmed.
[ The 5.5-magnitude 51 Pegasi is easily visible in binoculars between Alpha and Beta Pegasi, the western pair of stars in the Great Square of Pegasus. The star's equinox-2000 coordinates are R.A. 22 hours 57 minutes, Dec. +20 degrees 46 minutes. ]
On 1/17/96 Geoffrey Marcy andPaul Butler announced the discovery of planets orbiting the stars 70 Virginis and 47 Ursae Majoris. 70 Vir is a G5V (main sequence) star about 78 light-years from Earth; 47 UMa is a G0V star about 44 light-years away. These were discovered using the same doppler shift technique that found the planet orbiting 51 Pegasi.
The planet around 70 Vir orbits the star in an eccentric, elongated orbit every 116 days and has a mass about nine times that of Jupiter. Using standard formulas that balance the sunlight absorbed and the heat radiated, Marcy and Butler calculated the temperature of the planet at about 85 degrees Celsius (185 degrees Fahrenheit), cool enough to permit water and complex organic molecules to exist. The star 70 Vir is nearly identical to the Sun, though several hundred degrees cooler and perhaps three billion years older.
The planet around 47 UMa was discovered after analysis of eight years of observations at Lick Observatory. Its period is a little over three years (1100 days), its mass about three times that of Jupiter, and its orbital radius about twice the Earth's distance from the Sun. This planet too probably has a region in its atmosphere where the temperature would allow liquid water.
As of April 1996, Drs. Marcy and Butler have discovered yet another planet this time around the star HR3522 (aka Rho 1 Cancri, 55 Cancri) about 45 light years from the Earth. The planet is estimated to be about 0.8 Jupiter masses. It is likely that several more planets will show up in the initial set of 120 stars that they have monitored.
Several more extra-solar planets have now been discovered by the Butler/Marcy method. It seems likely that there are a very large number of such planets out there.
Another extra-solar planet has been discovered orbiting 16 Cygni B. But unlike all other previously known planets this one has a very large orbital eccentricity (0.6); its orbit carries it from a closest distance of 0.6 AU from its star to 2.7 AU. This calls into question many theories of planetary formation.
Detecting extra-solar planets directly is very difficult. Even the Hubble Space Telescope wouldn't be able to image planets at the expected sizes and distances from their suns.
What HST did find were disks of matter around stars seen in silhouette against the Orion Nebula (called 'proplyds', for 'proto-planetary disks' (right). This is great evidence for how common these objects are, but the scale is way too small to say anything directly about planets there. More detailed HST images are now available, too.
Nevertheless, it might be possible to detect the infra-red radiation of very large planets (Jupiter-sized or more) in some circumstances.
By a stroke of good luck, HST has taken an image of what appears to be a planet escaping from a double star system. See the 1998 May 28 announcement. If this is confirmed, the existence of extrasolar planets will be undeniable.
Facts
Three small bodies have been found in orbit around the pulsar PSR 1257+12. They have been designated "PSR1257+12 A, ..B, and ..C". One is about the size of the Moon, the other two are about 2 to 3 times the mass of Earth.
They were discovered by measuring variations in the pulsation speed of the pulsar which can be interpreted as gravitational effects of three small planets. The original observation has been confirmed but, of course, no direct images have been made -- that is way beyond the capabilities of our best telescopes.
These planets are believed to have formed after the supernova that produced the pulsar. The present planets would have originally been within the envelope of the progenitor star and therefore wouldn't have stood much chance of surviving the supernova explosion, and wouldn't have remained in circular orbits after the explosion.
Several decades of timing data on the pulsar PSR 0329+54 (PKS B0329+54) by Tatiana Shabanova (Lebedev Physics Institute) shows evidence of a planet with a 16.9 year period and mass greater than 2 Earth masses.
But, while the evidence for these is pretty good, they aren't really what we're looking for when we talk about 'solar systems'.
It has been known since 1983 that the star Beta Pictoris is surrounded by a disk of gas and dust. Spectra of Beta Pictoris show absorption features which are currently believed to be due to cometary like clouds of gas occultating the star from the debris left over from planetary formation. Though it's far from certain it is believed by some that planets may already have formed around Beta Pictoris.
HST has observed Beta Pictoris (right) and found the disk to be significantly thinner than previously thought. Estimates based on the Hubble image place the disk's thickness as no more than one billion miles (1600 million kilometers), or about 1/4 previous estimates from ground-based observations. The disk is tilted nearly edge-on to Earth. Because the dust has had enough time to settle into a flat plane, the disk may be older than some previous estimates. A thin disk also increases the probability that comet-sized or larger bodies have formed through accretion in the disk. Both conditions are believed to be characteristic of a hypothesized circumstellar disk around our own Sun, which was a necessary precursor to the planet-building phase of our Solar Systems, according to current theory.
More recent HST observations have shown the disk to be slightly warped as might be expected from the gravitational influence of a planet. This has been confirmed by observations at ESO.
Recent observations at radio wavelengths of a gas cloud known as Bok Globule B335 have produced images of material collapsing onto a newly born star (only about 150,000 years old). These observations are helping to understand how stars and planets form. The phenomena observed matches the theory of the formation of the solar system -- that is, a large gas cloud collapsed to form a star with an attendant circumstellar disk in which, over time, planets accreted from the matter in the disk and orbited the Sun.
The IRAS satellite found that Vega had too much infrared emission, and that has been attributed to a dust shell (with a mass of maybe Earth's moon).
Observations of the very nearby Barnard's Star were once thought to be evidence of gravitational effects of planets but they now seem to have been in error.
The star Gl229 seems to contain a 20 Jupiter mass object orbiting at a distance of 44 AU. An object this large is probably a brown-dwarf rather than an ordinary planet.
What may be the first discovery of a planet orbiting a normal, Sun-like star other than our own has been announced by astronomers studying 51 Pegasi, a spectral type G2-3 V main-sequence star 42 light-years from Earth. At a recent conference in Florence, Italy, Michel Mayor and Didier Queloz of Geneva Observatory explained that they observed 51 Pegasi with a high-resolution spectrograph and found that the star's line-of-sight velocity changes by some 70 meters per second every 4.2 days. If this is due to orbital motion, these numbers suggest that a planet lies only 7 million kilometers from 51 Pegasi -- much closer than Mercury is to the Sun -- and that the planet has a mass at least half that of Jupiter. These physical characteristics hinge on the assumption that our line of sight is near the planet's orbital plane. However, other evidence suggests that this is a good bet. A world merely 7 million km from a star like 51 Pegasi should have a temperature of about 1,000 degrees Celsius, just short of red hot. It was initially thought that it might be a solid body like a very big Mercury but the concensus now seems to be that it is a "hot Jupiter", a gas planet formed much farther from its star that migrated inward.
These observations have now been confirmed by several independent observers. And there is some evidence for a second planet much farther out that is not yet confirmed.
[ The 5.5-magnitude 51 Pegasi is easily visible in binoculars between Alpha and Beta Pegasi, the western pair of stars in the Great Square of Pegasus. The star's equinox-2000 coordinates are R.A. 22 hours 57 minutes, Dec. +20 degrees 46 minutes. ]
On 1/17/96 Geoffrey Marcy andPaul Butler announced the discovery of planets orbiting the stars 70 Virginis and 47 Ursae Majoris. 70 Vir is a G5V (main sequence) star about 78 light-years from Earth; 47 UMa is a G0V star about 44 light-years away. These were discovered using the same doppler shift technique that found the planet orbiting 51 Pegasi.
The planet around 70 Vir orbits the star in an eccentric, elongated orbit every 116 days and has a mass about nine times that of Jupiter. Using standard formulas that balance the sunlight absorbed and the heat radiated, Marcy and Butler calculated the temperature of the planet at about 85 degrees Celsius (185 degrees Fahrenheit), cool enough to permit water and complex organic molecules to exist. The star 70 Vir is nearly identical to the Sun, though several hundred degrees cooler and perhaps three billion years older.
The planet around 47 UMa was discovered after analysis of eight years of observations at Lick Observatory. Its period is a little over three years (1100 days), its mass about three times that of Jupiter, and its orbital radius about twice the Earth's distance from the Sun. This planet too probably has a region in its atmosphere where the temperature would allow liquid water.
As of April 1996, Drs. Marcy and Butler have discovered yet another planet this time around the star HR3522 (aka Rho 1 Cancri, 55 Cancri) about 45 light years from the Earth. The planet is estimated to be about 0.8 Jupiter masses. It is likely that several more planets will show up in the initial set of 120 stars that they have monitored.
Several more extra-solar planets have now been discovered by the Butler/Marcy method. It seems likely that there are a very large number of such planets out there.
Another extra-solar planet has been discovered orbiting 16 Cygni B. But unlike all other previously known planets this one has a very large orbital eccentricity (0.6); its orbit carries it from a closest distance of 0.6 AU from its star to 2.7 AU. This calls into question many theories of planetary formation.
Detecting extra-solar planets directly is very difficult. Even the Hubble Space Telescope wouldn't be able to image planets at the expected sizes and distances from their suns.
What HST did find were disks of matter around stars seen in silhouette against the Orion Nebula (called 'proplyds', for 'proto-planetary disks' (right). This is great evidence for how common these objects are, but the scale is way too small to say anything directly about planets there. More detailed HST images are now available, too.
Nevertheless, it might be possible to detect the infra-red radiation of very large planets (Jupiter-sized or more) in some circumstances.
By a stroke of good luck, HST has taken an image of what appears to be a planet escaping from a double star system. See the 1998 May 28 announcement. If this is confirmed, the existence of extrasolar planets will be undeniable.
Small Solar-System Bodies
The title The Nine Planets is somewhat misleading. In addition to the (eight) planets and their satellites the solar system contains a large number of smaller but interesting objects.
There are thousands of known asteroids and comets and undoubtedly many more unknown ones. Most asteroids orbit between Mars and Jupiter. A few (e.g. 2060 Chiron) are farther out. There are also some asteroids whose orbits carry them closer to the Sun than the Earth (Aten, Icarus, Hephaistos). Most comets have highly elliptical orbits which spend most of their time in the outer reaches of the solar system with only brief passages close to the Sun. And there is a large and important class of Trans-Neptunian Objects or Kuiper Belt Objects (including Pluto) that orbit (mostly) beyond Neptune.
The distinction between comets and asteroids is somewhat controversial. The main distinction seems to be that comets have more volatiles and more elliptical orbits. But there are interesting ambiguous cases such as 2060 Chiron (aka 95 P/Chiron) and 3200 Phaethon which seem to share some aspects of both categories.
Asteroids are sometimes also referred to as minor planets or planetoids (not to be confused with "lesser planets" which refers to Mercury and Pluto). Some of the largest asteroids and Kuiper Belt objects may be classified as dwarf planets. Very small rocks orbiting the Sun are sometimes called meteoroids to distinguish them from the larger asteroids. When such a body enters the Earth's atmosphere it is heated to incandescence and the visible streak in the sky is known as a meteor. If a piece of it survives to reach the Earth's surface it is known as a meteorite.
Millions of meteors bright enough to see strike the Earth every day (amounting to hundreds of tons of material). All but a tiny fraction burn up in the atmosphere before reaching the ground. The few that don't are our major source of physical information about the rest of the solar system.
Finally, the space between the planets is not empty at all. It contains a great deal of microscopic dust and gas as well as radiation and magnetic fields.
There are thousands of known asteroids and comets and undoubtedly many more unknown ones. Most asteroids orbit between Mars and Jupiter. A few (e.g. 2060 Chiron) are farther out. There are also some asteroids whose orbits carry them closer to the Sun than the Earth (Aten, Icarus, Hephaistos). Most comets have highly elliptical orbits which spend most of their time in the outer reaches of the solar system with only brief passages close to the Sun. And there is a large and important class of Trans-Neptunian Objects or Kuiper Belt Objects (including Pluto) that orbit (mostly) beyond Neptune.
The distinction between comets and asteroids is somewhat controversial. The main distinction seems to be that comets have more volatiles and more elliptical orbits. But there are interesting ambiguous cases such as 2060 Chiron (aka 95 P/Chiron) and 3200 Phaethon which seem to share some aspects of both categories.
Asteroids are sometimes also referred to as minor planets or planetoids (not to be confused with "lesser planets" which refers to Mercury and Pluto). Some of the largest asteroids and Kuiper Belt objects may be classified as dwarf planets. Very small rocks orbiting the Sun are sometimes called meteoroids to distinguish them from the larger asteroids. When such a body enters the Earth's atmosphere it is heated to incandescence and the visible streak in the sky is known as a meteor. If a piece of it survives to reach the Earth's surface it is known as a meteorite.
Millions of meteors bright enough to see strike the Earth every day (amounting to hundreds of tons of material). All but a tiny fraction burn up in the atmosphere before reaching the ground. The few that don't are our major source of physical information about the rest of the solar system.
Finally, the space between the planets is not empty at all. It contains a great deal of microscopic dust and gas as well as radiation and magnetic fields.
Charon
Charon ( "KAIR en" ) is Pluto's largest satellite: orbit: 19,640 km from Pluto
diameter: 1212 km
mass: 1.90e21 kg
Charon is named for the mythological figure who ferried the dead across the River Acheron into Hades (the underworld).
(Though officially named for the mythological figure, Charon's discoverer was also naming it in honor of his wife, Charlene. Thus, those in the know pronounce it with the first syllable sounding like 'shard' ("SHAHR en").
Charon was discovered in 1978 by Jim Christy. Prior to that it was thought that Pluto was much larger since the images of Charon and Pluto were blurred together.
Charon is unusual in that it is the largest moon with respect to its primary planet in the Solar System (a distinction once held by Earth's Moon). Some prefer to think of Pluto/Charon as a double planet rather than a planet and a moon.
Charon's radius is not well known. JPL's value of 586 has an error margin of +/-13, more than two percent. Its mass and density are also poorly known.
Pluto and Charon are also unique in that not only does Charon rotate synchronously but Pluto does, too: they both keep the same face toward one another. (This makes the phases of Charon as seen from Pluto very interesting.)
Charon's composition is unknown, but its low density (about 2 gm/cm3) indicates that it may be similar to Saturn's icy moons (i.e. Rhea). Its surface seems to be covered with water ice. Interestingly, this is quite different from Pluto.
Unlike Pluto, Charon does not have large albedo features, though it may have smaller ones that have not been resolved.
It has been proposed that Charon was formed by a giant impact similar to the one that formed Earth's Moon.
It is doubtful that Charon has a significant atmosphere.
diameter: 1212 km
mass: 1.90e21 kg
Charon is named for the mythological figure who ferried the dead across the River Acheron into Hades (the underworld).
(Though officially named for the mythological figure, Charon's discoverer was also naming it in honor of his wife, Charlene. Thus, those in the know pronounce it with the first syllable sounding like 'shard' ("SHAHR en").
Charon was discovered in 1978 by Jim Christy. Prior to that it was thought that Pluto was much larger since the images of Charon and Pluto were blurred together.
Charon is unusual in that it is the largest moon with respect to its primary planet in the Solar System (a distinction once held by Earth's Moon). Some prefer to think of Pluto/Charon as a double planet rather than a planet and a moon.
Charon's radius is not well known. JPL's value of 586 has an error margin of +/-13, more than two percent. Its mass and density are also poorly known.
Pluto and Charon are also unique in that not only does Charon rotate synchronously but Pluto does, too: they both keep the same face toward one another. (This makes the phases of Charon as seen from Pluto very interesting.)
Charon's composition is unknown, but its low density (about 2 gm/cm3) indicates that it may be similar to Saturn's icy moons (i.e. Rhea). Its surface seems to be covered with water ice. Interestingly, this is quite different from Pluto.
Unlike Pluto, Charon does not have large albedo features, though it may have smaller ones that have not been resolved.
It has been proposed that Charon was formed by a giant impact similar to the one that formed Earth's Moon.
It is doubtful that Charon has a significant atmosphere.
Pluto
Pluto orbits beyond the orbit of Neptune (usually). It is much smaller than any of the official planets and now classified as a "dwarf planet". Pluto is smaller than seven of the solar system's moons (the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton). orbit: 5,913,520,000 km (39.5 AU) from the Sun (average)
diameter: 2274 km
mass: 1.27e22 kg
In Roman mythology, Pluto (Greek: Hades) is the god of the underworld. The planet received this name (after many other suggestions) perhaps because it's so far from the Sun that it is in perpetual darkness and perhaps because "PL" are the initials of Percival Lowell.
Pluto was discovered in 1930 by a fortunate accident. Calculations which later turned out to be in error had predicted a planet beyond Neptune, based on the motions of Uranus and Neptune. Not knowing of the error, Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto anyway.
After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. The search for Planet X continued but nothing was found. Nor is it likely that it ever will be: the discrepancies vanish if the mass of Neptune determined from the Voyager 2 encounter with Neptune is used. There is no Planet X. But that doesn't mean there aren't other objects out there, only that there isn't a relatively large and close one like Planet X was assumed to be. In fact, we now know that there are a very large number of small objects in the Kuiper Belt beyond the orbit of Neptune, some roughly the same size as Pluto.
Pluto has not yet been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface (left and above). A spacecraft called New Horizons was launched in January 2006. If all goes well it should reach Pluto in 2015.
Fortunately, Pluto has a satellite, Charon. By good fortune, Charon was discovered (in 1978) just before its orbital plane moved edge-on toward the inner solar system. It was therefore possible to observe many transits of Pluto over Charon and vice versa. By carefully calculating which portions of which body would be covered at what times, and watching brightness curves, astronomers were able to construct a rough map of light and dark areas on both bodies.
In late 2005, a team using the Hubble Space Telescope discovered two additional tiny moons orbiting Pluto. Provisionally designated S/2005 P1 and S/2005 P2, they are now known as Nix and Hydra. They are estimated to be between 60 and 200 kilometers in diameter.
Pluto's radius is not well known. JPL's value of 1137 is given with an error of +/-8, almost one percent.
Though the sum of the masses of Pluto and Charon is known pretty well (it can be determined from careful measurements of the period and radius of Charon's orbit and basic physics) the individual masses of Pluto and Charon are difficult to determine because that requires determining their mutual motions around the center of mass of the system which requires much finer measurements -- they're so small and far away that even HST has difficulty. The ratio of their masses is probably somewhere between 0.084 and 0.157; more observations are underway but we won't get really accurate data until a spacecraft is sent.
Pluto is the second most contrasty body in the Solar System (after Iapetus).
There has recently been considerable controversy about the classification of Pluto. It was classified as the ninth planet shortly after its discovery and remained so for 75 years. But on 2006 Aug 24 the IAU decided on a new definition of "planet" which does not include Pluto. Pluto is now classified as a "dwarf planet", a class distict from "planet". While this may be controversial at first (and certainly causes confusion for the name of this website) it is my hope that this ends the essentially empty debate about Pluto's status so that we can get on with the real science of figuring out its physical nature and history.
Pluto has been assigned number 134340 in the minor planet catalog.
Pluto's orbit is highly eccentric. At times it is closer to the Sun than Neptune (as it was from January 1979 thru February 11 1999). Pluto rotates in the opposite direction from most of the other planets.
Pluto is locked in a 3:2 resonance with Neptune; i.e. Pluto's orbital period is exactly 1.5 times longer than Neptune's. Its orbital inclination is also much higher than the other planets'. Thus though it appears that Pluto's orbit crosses Neptune's, it really doesn't and they will never collide. (Here is a more detailed explanation.)
Like Uranus, the plane of Pluto's equator is at almost right angles to the plane of its orbit.
The surface temperature on Pluto varies between about -235 and -210 C (38 to 63 K). The "warmer" regions roughly correspond to the regions that appear darker in optical wavelengths.
Pluto's composition is unknown, but its density (about 2 gm/cm3) indicates that it is probably a mixture of 70% rock and 30% water ice much like Triton. The bright areas of the surface seem to be covered with ices of nitrogen with smaller amounts of (solid) methane, ethane and carbon monoxide. The composition of the darker areas of Pluto's surface is unknown but may be due to primordial organic material or photochemical reactions driven by cosmic rays.
Little is known about Pluto's atmosphere, but it probably consists primarily of nitrogen with some carbon monoxide and methane. It is extremely tenuous, the surface pressure being only a few microbars. Pluto's atmosphere may exist as a gas only when Pluto is near its perihelion; for the majority of Pluto's long year, the atmospheric gases are frozen into ice. Near perihelion, it is likely that some of the atmosphere escapes to space perhaps even interacting with Charon. NASA mission planners want to arrive at Pluto while the atmosphere is still unfrozen.
The unusual nature of the orbits of Pluto and of Triton and the similarity of bulk properties between Pluto and Triton suggest some historical connection between them. It was once thought that Pluto may have once been a satellite of Neptune's, but this now seems unlikely. A more popular idea is that Triton, like Pluto, once moved in an independent orbit around the Sun and was later captured by Neptune. Perhaps Triton, Pluto and Charon are the only remaining members of a large class of similar objects the rest of which were ejected into the Oort cloud. Like the Earth's Moon, Charon may be the result of a collision between Pluto and another body.
Pluto can be seen with an amateur telescope but it is not easy. There are several Web sites that show the current position of Pluto (and the other planets) in the sky, but much more detailed charts and careful observations over several days will be required to reliably find it. Suitable charts can be created with many planetarium programs.
diameter: 2274 km
mass: 1.27e22 kg
In Roman mythology, Pluto (Greek: Hades) is the god of the underworld. The planet received this name (after many other suggestions) perhaps because it's so far from the Sun that it is in perpetual darkness and perhaps because "PL" are the initials of Percival Lowell.
Pluto was discovered in 1930 by a fortunate accident. Calculations which later turned out to be in error had predicted a planet beyond Neptune, based on the motions of Uranus and Neptune. Not knowing of the error, Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto anyway.
After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. The search for Planet X continued but nothing was found. Nor is it likely that it ever will be: the discrepancies vanish if the mass of Neptune determined from the Voyager 2 encounter with Neptune is used. There is no Planet X. But that doesn't mean there aren't other objects out there, only that there isn't a relatively large and close one like Planet X was assumed to be. In fact, we now know that there are a very large number of small objects in the Kuiper Belt beyond the orbit of Neptune, some roughly the same size as Pluto.
Pluto has not yet been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface (left and above). A spacecraft called New Horizons was launched in January 2006. If all goes well it should reach Pluto in 2015.
Fortunately, Pluto has a satellite, Charon. By good fortune, Charon was discovered (in 1978) just before its orbital plane moved edge-on toward the inner solar system. It was therefore possible to observe many transits of Pluto over Charon and vice versa. By carefully calculating which portions of which body would be covered at what times, and watching brightness curves, astronomers were able to construct a rough map of light and dark areas on both bodies.
In late 2005, a team using the Hubble Space Telescope discovered two additional tiny moons orbiting Pluto. Provisionally designated S/2005 P1 and S/2005 P2, they are now known as Nix and Hydra. They are estimated to be between 60 and 200 kilometers in diameter.
Pluto's radius is not well known. JPL's value of 1137 is given with an error of +/-8, almost one percent.
Though the sum of the masses of Pluto and Charon is known pretty well (it can be determined from careful measurements of the period and radius of Charon's orbit and basic physics) the individual masses of Pluto and Charon are difficult to determine because that requires determining their mutual motions around the center of mass of the system which requires much finer measurements -- they're so small and far away that even HST has difficulty. The ratio of their masses is probably somewhere between 0.084 and 0.157; more observations are underway but we won't get really accurate data until a spacecraft is sent.
Pluto is the second most contrasty body in the Solar System (after Iapetus).
There has recently been considerable controversy about the classification of Pluto. It was classified as the ninth planet shortly after its discovery and remained so for 75 years. But on 2006 Aug 24 the IAU decided on a new definition of "planet" which does not include Pluto. Pluto is now classified as a "dwarf planet", a class distict from "planet". While this may be controversial at first (and certainly causes confusion for the name of this website) it is my hope that this ends the essentially empty debate about Pluto's status so that we can get on with the real science of figuring out its physical nature and history.
Pluto has been assigned number 134340 in the minor planet catalog.
Pluto's orbit is highly eccentric. At times it is closer to the Sun than Neptune (as it was from January 1979 thru February 11 1999). Pluto rotates in the opposite direction from most of the other planets.
Pluto is locked in a 3:2 resonance with Neptune; i.e. Pluto's orbital period is exactly 1.5 times longer than Neptune's. Its orbital inclination is also much higher than the other planets'. Thus though it appears that Pluto's orbit crosses Neptune's, it really doesn't and they will never collide. (Here is a more detailed explanation.)
Like Uranus, the plane of Pluto's equator is at almost right angles to the plane of its orbit.
The surface temperature on Pluto varies between about -235 and -210 C (38 to 63 K). The "warmer" regions roughly correspond to the regions that appear darker in optical wavelengths.
Pluto's composition is unknown, but its density (about 2 gm/cm3) indicates that it is probably a mixture of 70% rock and 30% water ice much like Triton. The bright areas of the surface seem to be covered with ices of nitrogen with smaller amounts of (solid) methane, ethane and carbon monoxide. The composition of the darker areas of Pluto's surface is unknown but may be due to primordial organic material or photochemical reactions driven by cosmic rays.
Little is known about Pluto's atmosphere, but it probably consists primarily of nitrogen with some carbon monoxide and methane. It is extremely tenuous, the surface pressure being only a few microbars. Pluto's atmosphere may exist as a gas only when Pluto is near its perihelion; for the majority of Pluto's long year, the atmospheric gases are frozen into ice. Near perihelion, it is likely that some of the atmosphere escapes to space perhaps even interacting with Charon. NASA mission planners want to arrive at Pluto while the atmosphere is still unfrozen.
The unusual nature of the orbits of Pluto and of Triton and the similarity of bulk properties between Pluto and Triton suggest some historical connection between them. It was once thought that Pluto may have once been a satellite of Neptune's, but this now seems unlikely. A more popular idea is that Triton, like Pluto, once moved in an independent orbit around the Sun and was later captured by Neptune. Perhaps Triton, Pluto and Charon are the only remaining members of a large class of similar objects the rest of which were ejected into the Oort cloud. Like the Earth's Moon, Charon may be the result of a collision between Pluto and another body.
Pluto can be seen with an amateur telescope but it is not easy. There are several Web sites that show the current position of Pluto (and the other planets) in the sky, but much more detailed charts and careful observations over several days will be required to reliably find it. Suitable charts can be created with many planetarium programs.
Neptune
Neptune is the eighth planet from the Sun and the fourth largest (by diameter). Neptune is smaller in diameter but larger in mass than Uranus. orbit: 4,504,000,000 km (30.06 AU) from Sun
diameter: 49,532 km (equatorial)
mass: 1.0247e26 kg
Hardcopy
The New Solar SystemSummarizes what we have learned from interplanetary explorations in the last 25 years. My primary reference for The Nine Planets.
Encyclopedia of the Solar SystemA more scholarly introduction the planetary science for those who want to dig a little deeper.
The Compact NASA Atlas of the Solar SystemThis road map of the solar system contains lots of maps and data as well as photos.
In Roman mythology Neptune (Greek: Poseidon) was the god of the Sea.
After the discovery of Uranus, it was noticed that its orbit was not as it should be in accordance with Newton's laws. It was therefore predicted that another more distant planet must be perturbing Uranus' orbit. Neptune was first observed by Galle and d'Arrest on 1846 Sept 23 very near to the locations independently predicted by Adams and Le Verrier from calculations based on the observed positions of Jupiter, Saturn and Uranus. An international dispute arose between the English and French (though not, apparently between Adams and Le Verrier personally) over priority and the right to name the new planet; they are now jointly credited with Neptune's discovery. Subsequent observations have shown that the orbits calculated by Adams and Le Verrier diverge from Neptune's actual orbit fairly quickly. Had the search for the planet taken place a few years earlier or later it would not have been found anywhere near the predicted location.
More than two centuries earlier, in 1613, Galileo observed Neptune when it happened to be very near Jupiter, but he thought it was just a star. On two successive nights he actually noticed that it moved slightly with respect to another nearby star. But on the subsequent nights it was out of his field of view. Had he seen it on the previous few nights Neptune's motion would have been obvious to him. But, alas, cloudy skies prevented obsevations on those few critical days.
Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. Much of we know about Neptune comes from this single encounter. But fortunately, recent ground-based and HST observations have added a great deal, too.
Because Pluto's orbit is so eccentric, it sometimes crosses the orbit of Neptune making Neptune the most distant planet from the Sun for a few years.
Neptune's composition is probably similar to Uranus': various "ices" and rock with about 15% hydrogen and a little helium. Like Uranus, but unlike Jupiter and Saturn, it may not have a distinct internal layering but rather to be more or less uniform in composition. But there is most likely a small core (about the mass of the Earth) of rocky material. Its atmosphere is mostly hydrogen and helium with a small amount of methane.
Neptune's blue color is largely the result of absorption of red light by methane in the atmosphere but there is some additional as-yet-unidentified chromophore which gives the clouds their rich blue tint.
Like a typical gas planet, Neptune has rapid winds confined to bands of latitude and large storms or vortices. Neptune's winds are the fastest in the solar system, reaching 2000 km/hour.
Like Jupiter and Saturn, Neptune has an internal heat source -- it radiates more than twice as much energy as it receives from the Sun.
At the time of the Voyager encounter, Neptune's most prominent feature was the Great Dark Spot (left) in the southern hemisphere. It was about half the size as Jupiter's Great Red Spot (about the same diameter as Earth). Neptune's winds blew the Great Dark Spot westward at 300 meters/second (700 mph). Voyager 2 also saw a smaller dark spot in the southern hemisphere and a small irregular white cloud that zips around Neptune every 16 hours or so now known as "The Scooter" (right). It may be a plume rising from lower in the atmosphere but its true nature remains a mystery.
However, HST observations of Neptune (left) in 1994 show that the Great Dark Spot has disappeared! It has either simply dissipated or is currently being masked by other aspects of the atmosphere. A few months later HST discovered a new dark spot in Neptune's northern hemisphere. This indicates that Neptune's atmosphere changes rapidly, perhaps due to slight changes in the temperature differences between the tops and bottoms of the clouds.
Neptune also has rings. Earth-based observations showed only faint arcs instead of complete rings, but Voyager 2's images showed them to be complete rings with bright clumps. One of the rings appears to have a curious twisted structure (right).
Like Uranus and Jupiter, Neptune's rings are very dark but their composition is unknown.
Neptune's rings have been given names: the outermost is Adams (which contains three prominent arcs now named Liberty, Equality and Fraternity), next is an unnamed ring co-orbital with Galatea, then Leverrier (whose outer extensions are called Lassell and Arago), and finally the faint but broad Galle.
Neptune's magnetic field is, like Uranus', oddly oriented and probably generated by motions of conductive material (probably water) in its middle layers.
Neptune can be seen with binoculars (if you know exactly where to look) but a large telescope is needed to see anything other than a tiny disk. There are several Web sites that show the current position of Neptune (and the other planets) in the sky, but much more detailed charts will be required to actually find it. Such charts can be created with a planetarium program.
Neptune's Satellites Neptune has 13 known moons; 7 small named ones and Triton plus four discovered in 2002 and one discovered in 2003 which have yet to be named. Distance Radius Mass
Satellite (000 km) (km) (kg) Discoverer Date
--------- -------- ------ ------- ---------- -----
Naiad 48 29 ? Voyager 2 1989
Thalassa 50 40 ? Voyager 2 1989
Despina 53 74 ? Voyager 2 1989
Galatea 62 79 ? Voyager 2 1989
Larissa 74 96 ? Voyager 2 1989
Proteus 118 209 ? Voyager 2 1989
Triton 355 1350 2.14e22 Lassell 1846
Nereid 5509 170 ? Kuiper 1949
Neptune's Rings Distance Width
Ring (km) (km) aka
------- -------- ----- -------
Diffuse 41900 15 1989N3R, Galle
Inner 53200 15 1989N2R, LeVerrier
Plateau 53200 5800 1989N4R, Lassell, Arago
Main 62930 < 50 1989N1R, Adams
(distance is from Neptune's center to the ring's inner edge)
diameter: 49,532 km (equatorial)
mass: 1.0247e26 kg
Hardcopy
The New Solar SystemSummarizes what we have learned from interplanetary explorations in the last 25 years. My primary reference for The Nine Planets.
Encyclopedia of the Solar SystemA more scholarly introduction the planetary science for those who want to dig a little deeper.
The Compact NASA Atlas of the Solar SystemThis road map of the solar system contains lots of maps and data as well as photos.
In Roman mythology Neptune (Greek: Poseidon) was the god of the Sea.
After the discovery of Uranus, it was noticed that its orbit was not as it should be in accordance with Newton's laws. It was therefore predicted that another more distant planet must be perturbing Uranus' orbit. Neptune was first observed by Galle and d'Arrest on 1846 Sept 23 very near to the locations independently predicted by Adams and Le Verrier from calculations based on the observed positions of Jupiter, Saturn and Uranus. An international dispute arose between the English and French (though not, apparently between Adams and Le Verrier personally) over priority and the right to name the new planet; they are now jointly credited with Neptune's discovery. Subsequent observations have shown that the orbits calculated by Adams and Le Verrier diverge from Neptune's actual orbit fairly quickly. Had the search for the planet taken place a few years earlier or later it would not have been found anywhere near the predicted location.
More than two centuries earlier, in 1613, Galileo observed Neptune when it happened to be very near Jupiter, but he thought it was just a star. On two successive nights he actually noticed that it moved slightly with respect to another nearby star. But on the subsequent nights it was out of his field of view. Had he seen it on the previous few nights Neptune's motion would have been obvious to him. But, alas, cloudy skies prevented obsevations on those few critical days.
Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. Much of we know about Neptune comes from this single encounter. But fortunately, recent ground-based and HST observations have added a great deal, too.
Because Pluto's orbit is so eccentric, it sometimes crosses the orbit of Neptune making Neptune the most distant planet from the Sun for a few years.
Neptune's composition is probably similar to Uranus': various "ices" and rock with about 15% hydrogen and a little helium. Like Uranus, but unlike Jupiter and Saturn, it may not have a distinct internal layering but rather to be more or less uniform in composition. But there is most likely a small core (about the mass of the Earth) of rocky material. Its atmosphere is mostly hydrogen and helium with a small amount of methane.
Neptune's blue color is largely the result of absorption of red light by methane in the atmosphere but there is some additional as-yet-unidentified chromophore which gives the clouds their rich blue tint.
Like a typical gas planet, Neptune has rapid winds confined to bands of latitude and large storms or vortices. Neptune's winds are the fastest in the solar system, reaching 2000 km/hour.
Like Jupiter and Saturn, Neptune has an internal heat source -- it radiates more than twice as much energy as it receives from the Sun.
At the time of the Voyager encounter, Neptune's most prominent feature was the Great Dark Spot (left) in the southern hemisphere. It was about half the size as Jupiter's Great Red Spot (about the same diameter as Earth). Neptune's winds blew the Great Dark Spot westward at 300 meters/second (700 mph). Voyager 2 also saw a smaller dark spot in the southern hemisphere and a small irregular white cloud that zips around Neptune every 16 hours or so now known as "The Scooter" (right). It may be a plume rising from lower in the atmosphere but its true nature remains a mystery.
However, HST observations of Neptune (left) in 1994 show that the Great Dark Spot has disappeared! It has either simply dissipated or is currently being masked by other aspects of the atmosphere. A few months later HST discovered a new dark spot in Neptune's northern hemisphere. This indicates that Neptune's atmosphere changes rapidly, perhaps due to slight changes in the temperature differences between the tops and bottoms of the clouds.
Neptune also has rings. Earth-based observations showed only faint arcs instead of complete rings, but Voyager 2's images showed them to be complete rings with bright clumps. One of the rings appears to have a curious twisted structure (right).
Like Uranus and Jupiter, Neptune's rings are very dark but their composition is unknown.
Neptune's rings have been given names: the outermost is Adams (which contains three prominent arcs now named Liberty, Equality and Fraternity), next is an unnamed ring co-orbital with Galatea, then Leverrier (whose outer extensions are called Lassell and Arago), and finally the faint but broad Galle.
Neptune's magnetic field is, like Uranus', oddly oriented and probably generated by motions of conductive material (probably water) in its middle layers.
Neptune can be seen with binoculars (if you know exactly where to look) but a large telescope is needed to see anything other than a tiny disk. There are several Web sites that show the current position of Neptune (and the other planets) in the sky, but much more detailed charts will be required to actually find it. Such charts can be created with a planetarium program.
Neptune's Satellites Neptune has 13 known moons; 7 small named ones and Triton plus four discovered in 2002 and one discovered in 2003 which have yet to be named. Distance Radius Mass
Satellite (000 km) (km) (kg) Discoverer Date
--------- -------- ------ ------- ---------- -----
Naiad 48 29 ? Voyager 2 1989
Thalassa 50 40 ? Voyager 2 1989
Despina 53 74 ? Voyager 2 1989
Galatea 62 79 ? Voyager 2 1989
Larissa 74 96 ? Voyager 2 1989
Proteus 118 209 ? Voyager 2 1989
Triton 355 1350 2.14e22 Lassell 1846
Nereid 5509 170 ? Kuiper 1949
Neptune's Rings Distance Width
Ring (km) (km) aka
------- -------- ----- -------
Diffuse 41900 15 1989N3R, Galle
Inner 53200 15 1989N2R, LeVerrier
Plateau 53200 5800 1989N4R, Lassell, Arago
Main 62930 < 50 1989N1R, Adams
(distance is from Neptune's center to the ring's inner edge)
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