Robotic Exploration of Space

One of the most impressive results of the Space Age has been the robotic exploration of the Solar System. Tiny points of light have been transformed into worlds by detailed close range observation. Astronomers use space telescopes to explore the whole universe more thoroughly than ever before, and earth scientists have become planetary scientists, able to compare and contrast a broader range of atmospheric and geological processes than ever before. A brief summary is given here.

What can we see from Earth?

The unaided eye can see five planets move across the sky, as well as spots on the Moon. The telescope, invented in 1609, revealed these objects to be worlds. But distance places limitations on telescopic observation. Even the best telescopes today can only see fuzzy markings on Mercury, Pluto and smaller worlds like the moons of the outer planets. Radar reflecting off other worlds can reveal more than light in some cases. Even Mars, close enough to see clearly from Earth, was not at all understood before spacecraft visited it. Close-up observations revolutionized our understanding of other worlds, and continue to do so.
Hubble Space Telescope images of solar system targets
Planetary radar.

Flyby, orbit, landing

This is the typical sequence of spacecraft exploration of another world. The easiest mission - still very difficult - is to fly past a planet, observing it during the flyby. Mariner 4 (1965) was the first successful Mars flyby, returning 22 small images. The Voyager missions (1979-1989) involved two spacecraft and flights past all four of the giant outer planets. The first Pluto flyby (New Horizons) is on its way. Next, for detailed observations over a long period, an orbiting mission may be flown. Mariner 9 was the first Mars orbiter (1971-1972). Right now the first Saturn orbiter, Cassini, is sending back data. The first Mercury orbiter, Messenger, is on its way. Finally, a landing is the most difficult type of mission to fly. Luna 9 (January 1966) was the first spacecraft to land on the Moon, Mars 3 and then Viking 1 on Mars. We could extend this sequence to include robotic sample return missions - so far only ever flown to the Moon (Luna 16, 20, 24) and a comet (Stardust). A Japanese asteroid sample return mission (Hayabusa) has left its asteroid on its way home. The ultimate stage would be human exploration, so far limited to the Moon.
Planetary exploration chronology - browse it!!

The Moon

The closest world to Earth, and the first target of solar system exploration. The Moon is covered with craters, the scars of its formation out of millions of separate fragments. As each one hit to build up the Moon, it made a crater. Some areas are covered with dark lava which flooded onto the surface and filled depressions several billion years ago. Its exploration history doesn't follow the flyby-orbit-landing sequence exactly. The first landings (Luna 9, Surveyor 1) came before the first orbiters (Luna 10, Lunar Orbiter 1). Lunar exploration culminated with the Apollo landings of astronauts in 1969-1972, and the Soviet Union's rovers and sample returm missions. The Soviets focussed more on physical and compositional studies, and did not undertake systematic mapping, for the Moon and other worlds. Recent missions have been orbiters with surface composition and mapping instruments (Clementine, Lunar Prospector, SMART-1, Kaguya, Chang-e 1). Read about these missions in the links below... And we are now in a new phase of lunar exploration! - Europe's SMART-1 ended its mission and crashed on the Moon in 2006. Now, Japan's Kaguya and China's Chang-e 1 are orbiting the Moon and returning data, and both India and the U. S. will launch lunar probes this year. More orbiters, rovers and sample returns are expected in the next few years. Several recent attempts to fly private commercial lunar missions have been abandoned. But now we have the Google Lunar X-Prize - $20 million for the first non-government rover to be landed on the Moon. Will it trigger a new commercial space business?
Moon missions.
Moon missions.
The first private Moon mission, probably cancelled now.
Moon images.
Google Moon.
Google Lunar X-Prize.


Venus can come closer to us than any other world but its thick cloudy atmosphere hides its surface completely. Before the space age we didn't know if its surface was an ocean, a desert or a swampy forest. Flyby missions probed the atmosphere, orbiters mapped the surface, and landers studied the surface. We have even flown balloons in the atmosphere, uniquely in the solar system. Venus is a hostile desert with very high atmospheric temperature and pressure. The US mission Magellan made the best orbital pictures, while the Soviet Union's Venera probes (Veneras 9 10, 13, 14) provided the only surface photographs. But why is Venus so different from Earth? This comparison illustrates one of the ways planets teach us about our own world. The European spacecraft Venus Express is currently orbiting the planet. View Magellan and Venera images through the 'Venus missions' links.
Venus missions
Venus Express


Mercury is so close to the sun that, apart from being hot, it is hard to see. (I have never seen it). It is a cratered world like the Moon, but not exactly the same, with long winding cliffs (thrust faults?) and fewer fractures than on the Moon. It has a fairly strong magnetic field, while the Moon has almost lost its ancient field. Only 45% of it was seen in pictures from Mariner 10 in 1973-1974. Now a new mission, Messenger, is on its way to orbit Mercury.
Mariner 10
Mariner 10 images


Long a goal of dreamers, Mars stands alone as a potential habitat for future humans. Did it support life? We don't know yet. Mars is a hybrid world, half moonlike, half earthlike. Its southern hemisphere is mostly cratered like much of the Moon, its northern half is low-lying, flat and might have held seas or a small ocean at one time. Mars has giant features, the biggest volcanoes and canyons in the solar system. Its carbon dioxide polar caps shrink and grow with the seasons as on Earth. But this cold desert planet with its thin atmosphere and very low temperatures is still less hospitable than any inhabited place on Earth. Early flyby missions (Mariners 4, 6, 7) were followed by orbiters (Mariner 9, Viking) and landers (Viking, Pathfinder). We have excellent images of Mars from orbit, including the current Mars Reconnaissance Orbiter (MRO) and Odyssey missions (see links) and from the surface including the current rovers. Look for high resolution orbital images in 2007 and future landers in 2008 and 2010.
Mars missions.
2004 rover missions.
Mars Global Surveyor images.
Mars Odyssey images.
Mars Express images.
MRO images.


Asteroids are generally too small to observe in detail from Earth, but radar has shown details of some and spacecraft have photographed others. Galileo, a Jupiter orbiter, flew past two asteroids (Gaspra, Ida) to reveal their surfaces for the first time in 1991 and 1993. The NEAR (Near Earth Asteroid Rendezvous) mission flew past Mathilde (1997) and then orbited asteroid Eros for a year (2000-2001) before successfully landing on its surface. A Japanese sample return mission (Hayabusa) studied an asteroid in 2005, and 2007 will see the launch of a long mission (Dawn) to orbit two asteroids and fly past more in 2008-2012. Rosetta (see Comets section) will also pass by two asteroids. Asteroids are both a threat (they might hit us; if so I will be marketing an asteroid repellant), and a potential benefit (they might provide resources for future space industries).
radar images
Gaspra images
Ida images
Eros images
Mathilde images
NEAR mission
Hayabusa mission


Comets are the least explored components of the solar system. The first comet missions were to Comet Halley in 1986. Important data on composition and other characteristics were obtained, but only low resolution images were taken. In 2001 Comet Borrelly was seen in more detail by Deep Space 1, and in 2004 the Stardust mission collected dust from Comet Wild-2 and took the best pictures to date. The Deep Impact mission gave a detailed view of Comet Tempel-1 in July 2005, and used a heavy projectile to dig a crater in its surface and study the resulting debris. The European Rosetta mission will orbit and land on a comet nucleus in 2014.
Halley's comet
Comet Borrelly
Stardust comet nucleus images
Deep Impact comet nucleus images


Jupiter is far away from Earth in the remote outer solar system. The Pioneer 10 and 11 spacecraft first tested the route through the asteroid belt and survived the radiation belts of Jupiter, in 1973 and 1974. The spectacular Voyager missions flew past Jupiter in 1979 as the first step in their epic reconnaissance of the outer solar system, surely the greatest voyage of exploration ever undertaken. The two Voyagers discovered a thin ring about Jupiter and active volcanoes on one of its moons. These discoveries were followed up by the Galileo mission from 1995 to 2003, which orbited the planet and dropped a probe into its atmosphere. Future missions will explore the moon Europa, which may have an ocean under its icy surface. On February 28 2007 the New Horizons spacecraft flew past Jupiter and made some observations on its way to Pluto.
Pioneer missions to Jupiter.
Jupiter images including some from Voyager
Jupiter images from Galileo
New Horizons at Jupiter


Saturn is in the news again after a 20 year hiatus between spacecraft visitors. Planetary scientists have to learn patience... The first visitor was Pioneer 11, which flew past Jupiter in 1974 and arrived at Saturn in 1979 to pave the way for Voyager. The two Voyager spacecraft flew past Saturn in 1980 and 1981, providing the first detailed studies of the planet and its many moons and vast rings. Now we are lucky to be able to see, as it happens, the first orbital mission: Cassini arrived at Saturn in July 2004 and will spend at least four years exploring this complex system. Check Cassini news every week. In January 2005 Cassini's probe 'Huygens' dropped into the thick atmosphere of Saturn's big moon Titan, returning spectacular images of chanel-eroded hills and muddy plains.
Pioneer 11 images of Saturn.
Voyager at Saturn
Cassini - latest news
Huygens on Titan
Huygens on Titan


Uranus was the first planet to be discovered (in 1781), all those closer to the sun having been known since ancient times. It has only been visited once by a spacecraft, Voyager 2, in 1986. It is a world with thin dark rings and geologically interesting moons, tilted on its side compared with its orbit and most other planets. There are no specific plans for a return visit, but discussions are now taking place.
Discovery of Uranus.
Voyager 2 at Uranus.


Neptune was discovered in 1846. In 1989 the planet and its rings and moons were briefly visited by Voyager 2. Most of what we know of the planet comes from this one flyby. Neptune's large moon Triton has a thin atmosphere and active geyser-like eruptions. There are no current plans to return for a closer look.
Discovery of Neptune
Neptune images


Pluto was only found in 1930. It is a small icy world similar to many outer planet moons, and people often debate whether it is big enough to be considered a planet at all. In 2006 it was officially reclassified as a 'dwarf planet'. Today it is regarded as one of the largest members of the Kuiper Belt, an icy asteroid-like belt beyond Neptune. A spacecraft (New Horizons) is now on its way to fly past Pluto, its large moon Charon and two newly-discovered moons, and then go on to visit one of the other Kuiper-belt objects.
Pluto summary
'New Horizons' Pluto mission

Space Telescopes

Much important space exploration work is done by telescopes in orbit, where they are not affected by Earth's atmosphere. The best known is the Hubble Space Telescope. Hubble, launched in 1990 on the Space Shuttle, must be serviced by the Shuttle. One more servicing mission is scheduled in about 2010. A replacement, the James Webb Telescope, is being designed.
Space Telescope Science Institute.
Hubble Space Telescope images of solar system targets