Links to resources
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. 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. Radar reflecting off other worlds can reveal more than light in some cases. It can see through the clouds of Venus and resolve individual boulders on passing asteroids.
Hubble Space Telescope images of solar system targets
APOD. - Astronomy Picture of the Day
Flyby, orbit, landing, roving, sample return...
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. Luna 3 (1959) flew past the Moon and made the first images of its far side. 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) happened in the summer of 2015. A flyby is brief, so for detailed observations over a long period an orbiting mission is necessary. Mariner 9 was the first Mars orbiter (1971-1972). Galileo orbited Jupiter and Cassini orbited Saturn. The first Mercury orbiter, Messenger, finished studying the planet in 2015. Landing is more difficult again, but gives a very close look at the surface and lets us dig or drill into it and analyze it. Luna 9 (January 1966) was the first spacecraft to land on the Moon, Mars 3 and then Viking 1 landed on Mars. We could extend this sequence to include rovers (mobile vehicles). Lunokhod 1 was the first rover on the Moon, Sojourner on Mars. Another type of mobility would be balloons (Vega 1 and 2 at Venus) or other kinds of aircraft. Finally, robotic sample return missions bring back rock or soil to us for analysis. So far these have only flown to the Moon (Luna 16, 20, 24, Chang'e 5), a comet (Stardust), and asteroids (Hayabusa, Hayabusa 2, OSIRIS-REx). The ultimate stage would be human exploration, which is so far limited to the Moon.
Planetary exploration chronology - browse it!!
Jet Propulsion Laboratory - home of most planetary missions
Johns Hopkins University's Applied Physics Lab - home of several recent missions
The closest world to Earth, and the first target of solar system exploration. The Moon is covered with craters, the scars of millions of impacts with asteroids and comets. The Moon may have been born out of a gigantic collision between the early Earth and another large object. Some areas of the Moon are covered with dark lava which flooded onto the surface and filled depressions several billion years ago. Since then nothing much has happened except the occasional impact (much less frequent now than when the solar system was young) and perhaps a few last episodes of volcanic activity. The Moon is essentially dead now, but that makes it all the more interesting - it preserves a record of the ancient past which has been destroyed on our dynamic Earth. Water and frozen gases are now known to be trapped in cold shaded areas near the lunar poles and m ight provide useful resources for people on the Moon in the future.
The Moon's exploration history doesn't follow the flyby-orbit-landing sequence exactly. Luna 3 was a flyby which imaged the far side in 1959, but the first landings (Luna 9, Surveyor 1 in 1966) came before the first orbiters (Luna 10, Lunar Orbiter 1, later in 1966). The first phase of lunar exploration culminated with the Apollo astronaut landings in 1969-1972, and the Soviet Union's rovers and sample return missions. The Soviets focussed more on physical and compositional studies, and did not undertake systematic mapping of the Moon and most other worlds. We are now in a new international phase of lunar exploration. Europe, Japan, China and India have all flown orbital mapping missions to the Moon in recent years, China landed a spacecraft (Chang'E 3) with a rover (Yutu) in 2013 and plans a far side lander in 2018 and a sample return in 2019. Other nations are also planning landers in the next few years - India, Russia, Japan and a new entrant, South Korea.
The currently active lunar missions are NASA's Lunar Reconnaissance Orbiter, two Artemis orbiters mapping charged particles, the Chang'E 3 lander carrying a telescope, and a small Chinese orbiter called Longjiang 2. The recent GRAIL gravity-mapping spacecraft also carried cameras for a public outreach imaging campaign set up by NASA's first female astronaut (Sally Ride, who died in 2012). LADEE measured gas and dust near the surface. LCROSS smashed into a permanently shadowed polar crater on the Moon, excavating water and other volatiles including methane and carbon dioxide for analysis. More orbiters, rovers and sample returns are expected in the next decade. Several past attempts to fly private commercial lunar missions were abandoned in the 1990s, and the Google Lunar X-Prize (GLXP) - $20 million for the first non-government rover to be landed on the Moon - was cancelled in 2018 without a winner. Several of the GLXP teams (including Astrobotic, Team Indus, SpaceIL and PTScientists) are continuing their efforts and may land on the Moon in 2019 and 2020. Golden Spike was a company set up in 2012 which attempted to send people back to the Moon. These companies all suffer from the same problem, the extreme difficulty of attracting funding. Golden Spike was forced to give up, but the Moon is back in favour and similar companies may appear again. Will this trigger a new commercial space business?
Lunar Picture of the Day (LPOD).
Soviet Lunar images.
Soviet Lunar images.
ESA's SMART-1 lunar images.
Moon images from Kaguya.
Moon images from Chandrayaan.
Chandrayaan 2 lander and rover mission
Moon images and data from China's Chang'e missions.
- Chang'E 3 (Yutu) images
- Longjiang 2 images
- Chang'e 4 images
Moon images from Apollo.
Flickr gallery - Moon images from Apollo.
Moon images from Lunar Reconnaissance Orbiter.
- Moon images from Lunar Reconnaissance Orbiter.
- LROC image map - search for LROC images on a clickable map
- LROC WAC map - zoomable LROC wide angle mosaic
- LROC Quickmap - zoomable LROC global map
LCROSS Moon impact mission.
USGS lunar data page.
GRAIL gravity-mapping mission
GRAIL MoonKAM images.
- Intuitive Machines
- ispace (HAKUTO)
- SpaceIL (Beresheet)
Google Lunar X-Prize (ended).
Golden Spike - people to the Moon again? (cancelled)
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 extensive lava flows, long winding cliffs which suggest it has shrunk a bit, wrinkling the surface, and fewer fractures than on the Moon. It has a fairly strong magnetic field, while the Moon has almost lost its ancient field. Many volcanic vents suggest Mercury has been more active than the Moon and until more recently. Like the Moon, its polar regions contain permanently shaded craters with ice deposits inside. Radar from Earth, bounced off the planet, can detect the ice in those polar shadows.
Almost nothing was known about Mercury before the Space Age. Mariner 10 made three flybys of the planet in the 1970s, but only 45% of the surface was seen in its pictures. A new mission, Messenger, orbited the planet from 2011 until it crashed on the surface in April 2015. Its images covered all of the surface of Mercury at much better resolution than Mariner 10, and the planet's composition and magnetic field have been examined in much more detail. A European mission to Mercury called BepiColombo (named after an Italian scientist), combined with a Japanese orbiter, will explore Mercury in even more detail after launch in 2018.
Mariner 10 images
ESA's BepiColombo mission
Mercury data clickable map
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. Venus is a hostile desert with very high atmospheric temperature and pressure, so it has no water at all on the surface. The atmosphere is mostly carbon dioxide, with acidic clouds, a very unpleasant place. It is completely covered with volcanoes and lava flows, with a few highland areas that resemble Earth's continents. Despite similarities to Earth in size and (probably) active volcanoes, it shows no sign of plate tectonics (continental movements which create mountain ranges on Earth). Venus rotates backwards compared with every other planet. Why is Venus so different from Earth? Trying to find out why illustrates one of the ways planets can teach us something about our own world.
Several flyby missions have studied the planet's atmosphere, including Mariner 10 on its way to Mercury. Soviet and American orbiters have mapped the surface with radar, and landers have studied the surface up close. We have even flown balloons in the atmosphere (Soviet Union's Vega missions), the only place in the solar system other than Earth where that has been done. NASA's Magellan mission made the best orbital pictures using radar to look through the clouds, while the Soviet Union's Venera probes (Veneras 9, 10, 13, 14) provided the only surface photographs. Surface conditions are so extreme that no lander has survived more than two hours. The European spacecraft Venus Express orbited the planet from 2006 to 2015, primarily studying the atmosphere. A Japanese Venus orbiter called Akatsuki failed to enter Venus orbit in 2010, but it was able to loop around the Sun and enter orbit around Venus in 2015. It is now collecting information on the atmosphere. New missions to Venus including high resolution surface mapping from orbit and new landers have been proposed but tend to be out-competed for funding by missions to other targets. View Magellan and Venera images through the 'Venus missions' links.
More Venus images
Soviet Venus images
Long a goal of dreamers, Mars stands alone as a potential habitat for future humans. But this cold desert planet with its thin atmosphere and very low temperatures is still less hospitable than any inhabited place on Earth. 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 polar caps shrink and grow with the seasons as on Earth, but most of the change is in thin carbon dioxide ice layers over thick permanently frozen water ice. Features that look like dry riverbeds and glaciers strongly suggest that water or ice, or both, were once active on Mars. Now the surface is dry and very cold, but very small amounts of water may flow down slopes on the surface at times, and radar has detected ice and liquid water below the surface. The water must be very salty to stay liquid at typical Mars temperatures. Mars has two little moons, Phobos and Deimos, both less than 30 km across. Because they are so small their gravity is too weak to force themselves into spherical objects like Earth, Mars etc., and they are quite irregular in shape. Phobos is lined with many groove-like valleys, Deimos has a very smooth surface as if covered with a thick layer of loose debris.
Early flyby missions (Mariners 4, 6, 7) were followed by orbiters (Mariner 9, Viking) and landers (Viking, Pathfinder, Phoenix). We have excellent images of Mars from orbit, including the current Mars Reconnaissance Orbiter (MRO) and Mars Odyssey missions (see links) and from the surface at seven landing sites. The Mars Exploration Rovers landed in 2004, and one of them (Opportunity) was still operating in 2018 (but may have been shut down by a bad dust storm - we are waiting to hear from it). The big Curiosity rover landed in August 2012. The Soviet Union sent missions to Mars as well, with many failures and limited data return, but they did succeed in making the first successful landing on Mars (Mars 3 in 1971). Unfortunately Mars 3 failed after only 20 seconds, but it did land successfully. The last Soviet mission tried to achieve a landing on the moon Phobos, but its two spacecraft both failed before they could land. The European Space Agency placed Mars Express in orbit in late 2003, where it is still operating, and tried to put a lander called Beagle 2 on Mars. It landed, apparently safely, on Mars but did not deploy properly and failed to send back data. MAVEN, a NASA orbiter, and an Indian orbiter (MOM, Mars Orbiter Mission) arrived at Mars in September 2014.
A European orbiter called the Trace Gas Orbiter and its lander Schiaparelli arrived in 2016. TGO entered orbit and is imaging the surface, but the lander crashed. A NASA lander called Insight, carrying a seismometer and other equipment will arrive late in 2018. An ESA (European Space Agency) ExoMars rover and another large NASA rover are scheduled to launch 2020 and land in 2021. China and the United Arab Emirates are also planning spacecraft in 2020, China a lander and rover, UAE an orbiter called Hope. The moons of Mars have been explored from orbit as well, especially by Viking and Mars Express. A Russian spacecraft was supposed to land on Phobos in 2012 and collect soil for return to Earth, but it failed after launch. That mission may be repeated in the 2020s. Phobos and Deimos may be targets for human exploration even before a Mars landing.
Mars exploration - current missions
1976 Viking missions.
- Viking retrospective
Mars Global Surveyor images.
2001 Mars Odyssey THEMIS website.
Mars Express images.
Mars Reconnaissance Orbiter.
- Mars Reconnaissance Orbiter (HiRISE) images.
1997 Mars Pathfinder mission
2004 Mars Exploration Rovers.
- MER dust devil movies.
- Opportunity colour images by Holger Isenberg.
- Mars rover images by the Pancam team.
Mars surface panoramas by Damia Bouic (in French).
2008 Phoenix mission.
The (failed) Phobos Sample Return mission.
Mars Science Laboratory (Curiosity).
- rover images - by Michael Howard, no longer updated.
- Curiosity images.
- Curiosity images.
- rover images, panoramas and route.
Mars Orbiter Mission (MOM). - India's Mars orbiter
- Mars Orbiter Mission images. - India's Mars orbiter
- Mars Orbiter Mission images. - Year 3 data
MAVEN. - NASA orbiter studying the atmosphere
ExoMars 2016 - ESA Trace Gas Orbiter, Schiaparelli lander
NASA Insight lander, 2018
Hope - United Arab Emirates orbiter, 2020
- Perseverance rover
- - Jezero map from ESA
- China's Tianwen-1
- ExoMars 2022, Rosalind Franklin rover
Mars images from several missions.
Mars global datasets.
Mars global maps from JMars.
Mars zoomable map.
Asteroids are the rocky leftover building blocks of the planets. Most of them orbit together in a broad zone between Mars and Jupiter, the 'asteroid belt' or 'main belt'. Some of them have been thrown out of the asteroid belt by collisions or gravitational interactions with Jupiter, and they end up on unstable orbits crossing the paths of other planets. Ultimately, some of them will hit a planet or moon and make a crater. They are generally too small to observe in detail from Earth, but radar has shown details of some and spacecraft have photographed others. 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).
Galileo, a Jupiter orbiter, flew past two asteroids (Gaspra, Ida) to reveal their surfaces in 1991 and 1993, the first times we ever saw asteroids close-up. Ida was found to have a small moon, and many moons of asteroids have been discovered since then. 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 asteroid Itokawa in 2005, landed briefly, collected a sample of the surface material and returned to Earth with this asteroid dust. The Dawn spacecraft orbited one of the largest asteroids, Vesta, in 2011-2012 and began orbiting the biggest asteroid Ceres in 2015. Rosetta (see Comets section) passed by and observed two asteroids, Steins and Lutetia. Stardust and Deep Space 1 both examined small asteroids on the way to other targets. A Chinese lunar orbiter, Chang'E-2, left the Moon to fly past an asteroid called Toutatis in December 2012. Japan's spacecraft Hayabusa 2 is now (2018) exploring an asteroid called Ryugu, dropping small landers on it in 2018 and collecting samples in 2019 to return to Earth later. A NASA spacecraft called OSIRIS-REX will arrive at its asteroid, Bennu, late in 2018 and will also collect samples for return to Earth. Another NASA mission called Psyche will visit a metallic asteroid of the same name in 2026, and a separate mission called Lucy will fly past several trojan asteroids (held in Jupiter's Lagrange points) from 2027 to 2033. It will also fly past a main belt asteroid, Donaldjohanson, in 2025 on its way to the Trojans. One now-cancelled plan (Asteroid Redirect Mission, ARM) involved putting a small asteroid, or a big boulder plucked from an asteroid, in orbit around the Moon to be visited by people before they venture all the way to a distant asteroid.
radar images of many asteroids
Galileo mission - asteroid Gaspra
Galileo mission - asteroid Ida
NEAR mission - asteroid Mathilde
NEAR mission - asteroid Eros
- NEAR - Eros images
- NEAR - Eros images
Hayabusa mission - asteroid Itokawa
- Hayabusa images of Itokawa (currently unavailable)
Stardust mission - asteroid Annefrank
Deep Space 1 mission - asteroid Braille
Rosetta mission - asteroid Steins
Rosetta mission - asteroid Lutetia
- Rosetta mission - Lutetia raw data
Dawn mission - asteroids Ceres and Vesta
- Dawn mission - Ceres images
- Dawn mission - Vesta images
- Vesta interactive map
Chang'E 2 images of Toutatis
- Japan's Hayabusa 2 at Ryugu
- NASA's OSIRIS-REX mission to Bennu
- NASA's Psyche mission
- NASA's Lucy mission
ARM - Asteroid Redirect Mission
Asteroid maps and feature names
Asteroid maps in NASA's Planetary Data System
To astronomers, comets are fuzzy clouds of gas and dust in the night sky, but at the heart of a comet is an ice-rich asteroid, called the nucleus of the comet. As the ice evaporates (strictly we should say it sublimes, turns from solid directly to gas) with the warmth of sunlight, the 'coma' or cloud of gas is formed, carrying dust with it into space and gradually stretching out to form a long 'tail'. The 'ice' is not just frozen water, but a mixture of ices, frozen carbon dioxide, methane and other substances. As all that volatile material escapes into space the comet's surface erodes and also becomes covered with the non-volatile material (dust, carbon compounds etc.) so these 'icy' objects may be as black as coal, but icy under the surface. Old comets which have passed the sun many times may dry out, or the dark surface may prevent further evaporation, causing the comet to become dormant, or extinct, and to behave like an asteroid. Similarly, something we call an asteroid may become comet-like if it gets warmer than usual or an impact exposes fresh ice. There is no obvious way to distinguish between asteroids and comets unless we see a coma and tail.
The first spacecraft missions to a comet visited Comet Halley in 1986, providing important data on composition and other characteristics but only low resolution images. ESA's Giotto spacecraft took images but was damaged by dust in the coma during a very high speed flyby. The Soviet Union's VEGA spacecraft (after dropping landers and balloons into the atmosphere of Venus) flew past comet Halley and took more images, and of course other data were collected as well. 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 pictures of its rugged icy nucleus. The dust samples were returned safely to Earth, the first material ever brought back to Earth from any other world beyond the Moon. 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 same Deep Impact spacecraft flew past Comet Hartley-2 in November 2010, and Stardust flew past Tempel-1 for a second look in February 2011. The European Rosetta mission orbited the nucleus of comet Churyumov-Gerasimenko and dropped a lander onto its surface in November 2014, the most intensive study of a comet ever undertaken. In October 2014 comet Siding Spring passed close to Mars and the various orbiters and rovers all studied it from a distance.
Giotto images of Halley's comet
VEGA images of Halley's comet
Deep Space 1 images of Comet Borrelly
Stardust images of Comet Wild-2
Stardust-NExT images of Comet Tempel-1
- Deep Impact images of Comet Tempel-1
Deep Impact images of Comet Hartley-2
Rosetta mission to Comet 67 P/Churyumov-Gerasimenko
- Rosetta blog
- Rosetta images
Comet Siding Spring at Mars
Jupiter is a vast gas giant planet - an envelope of gas more than 300 times as massive as Earth with a rocky core several times bigger than Earth, far away in the remote outer solar system. There is no solid surface, even the rocky core must be molten, and all we see is the ever-changing clouds. Jupiter gives off more heat than it receives from the Sun, and its vast magnetic field traps strong radiation belts and emits powerful radio waves. Jupiter is surrounded by a faint ring of dust and by nearly 80 moons, four of them almost planetary in size.
Io and Europa are about the size of our Moon, around 3000 km across. Io is rocky like our Moon and is covered with active volcanoes. Europa has a rocky body a bit smaller than our Moon surrounded by a 100 km thick ocean. The top few tens of kilometres of the ocean are frozen, and that icy crust is covered with fractures and places where ocean water may have risen to the surface and frozen. Possible plumes of freezing water vapour have been seen by the Hubble Space Telescope, suggesting active venting. Ganymede and Callisto are about the size of Mercury, over 5000 km across, and are about half rock, half ice. Callisto is dark, covered with craters as if nothing but impacts have ever happened there, and ice in its surface may be gradually evaporating (subliming) out of the surface like a giant comet. Ganymede, the biggest satellite in the solar system and bigger than Mercury, probably started out like that, but it has been heated up so much of its surface is covered with fractures. It has its own magnetic field and may have an ocean deep within it. The heat for internal activity in Io, Europa and Ganymede is generated by tides caused by Jupiter, strongest at Io which is closest to the planet.
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. They took low resolution images of Jupiter's clouds, only a bit better than we can see from Earth, but showed the trip was possible. 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 Io, one of its four big moons, as well as several new 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. Galileo made very close observations of the big moons, but its discoveries were limited by communication problems. Future missions will probably explore the moon Europa, and maybe Ganymede as well. On December 30, 2000 Cassini flew past Jupiter on its way to Saturn and took new images. On February 28 2007 the New Horizons spacecraft flew past Jupiter and made important observations on its way to Pluto. These flyby opportunities and ongoing ground-based studies allow us to monitor the constant volcanic activity on Io, and atmospheric activity and even asteroid or comet impacts on Jupiter itself. Juno, a Jupiter orbiter which will study its gravity and interior, is orbiting to the planet now and returning spectacular images of the clouds. New orbiters to survey the Jupiter system and especially Europa are being developed by NASA and ESA, and a lander for Europa may be developed later.
Pioneer missions to Jupiter.
Jupiter images including some from Voyager
Jupiter images from Galileo
Impact on Jupiter
Another impact on Jupiter
Jupiter images from Cassini
New Horizons at Jupiter
Juno mission to Jupiter
Juno images - link to RESULTS FROM JUNO.
NASA Europa Clipper mission
ESA Jupiter Icy Moon Explorer
Saturn is another gas giant planet, a bit smaller than Jupiter. It has an enormous ring system made of billions of icy chunks, probably fragments of moons destroyed by impacts. Tidal forces from Saturn and its larger moons prevent the chunks from coming together to build new moons. There is only one planet-sized moon, Titan, as big as Mercury, and about 60 smaller satellites ranging from a few kilometres across to 1500 km across. Titan has a thick hazy nitrogen atmosphere over an icy surface, and clouds of methane droplets which can fall as rain to fill rivers and lakes on the surface. Most of the lakes are at the moon's poles. This is the only place in the solar system except Earth (and to a very small extent Mars) where we can see liquids in action. The other moons show varying degrees of activity, from almost none on the crater-covered moons Rhea and Mimas to heavily fractured surfaces (Dione) and the bizarre small icy moon Enceladus whose old cratered areas are almost completely erased by intense fracturing and fresh ice deposits. Enceladus is hot inside and can be seen squirting jets of water vapour (quickly freezing in space) from deep cracks near its south pole. It seems to have a liquid water ocean under its icy shell, even though it is only 500 km in diameter and should have cooled and frozen long ago. Saturn's many small moons include several near the edge of the rings or in gaps inside the rings, often with bizarre shapes and surface features. Pan and Atlas have equator-girdling ridges, probably built of ring dust settling onto the moons. All of these moons are mixtures of rock and ice, with mainly icy surfaces, but on Iapetus dark soil covers half of the Moon while the other half consists of bright white ice. Iapetus also has an equatorial ridge of unknown origin. Jupiter's moons are more active near the planet, the inner planets like Earth are more active if they are bigger, but there is no obvious pattern to activity among the moons of Saturn.
The first spacecraft visitor was Pioneer 11, which flew past Jupiter in 1974 and arrived at Saturn in 1979 to pave the way for Voyager, testing a safe passage through the ring plane. 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. The first orbital mission, Cassini, arrived at Saturn in July 2004 and explored this complex system until 2017 when it was dropped into Saturn's atmosphere and destroyed. Cassini's cameras have taken images of many of the moons, and if the use infrared wavelengths they can see the surface of Titan through the haze. Otherwise the surface is visible only with radar, as on Venus. In January 2005 Cassini's probe 'Huygens' dropped into the thick hazy atmosphere of Titan, returning spectacular images of muddy plains and channels in hilly areas nearby. Some people are thinking about exploring Titan's lakes or the enigmatic Enceladus in the future.
Pioneer 11 images of Saturn.
Voyager at Saturn
Cassini - latest news
Huygens on Titan
Huygens on Titan
Radar images of Titan
Uranus was the first planet to be discovered, accidentally, by William Herschel, in 1781, all of the planets closer to the sun having been known since ancient times. Herschel thought it was a comet at first, but its orbit was found to be like a planet. It is a mid-sized methane-rich gas giant world with thin dark rings and geologically interesting moons, tilted on its side compared with its orbit and most other planets, perhaps because of a giant impact long ago. The moons show varying degrees of geological activity, generally more active close to the planet (Miranda and Ariel), less active further out (Umbriel, Titania and Oberon), but so far we don't know of any present-day activity.
Uranus has only been visited once by a spacecraft, Voyager 2, in 1986. Voyager 1 flew past Jupiter and Saturn, and then out into deep space without passing any more planets, But Voyager 2 flew past all four of the outer planets in sequence. An orbital mission to Uranus is being considered, but the planet is a low priority when compared with places like Europa, Titan and Enceladus. We can monitor clouds in the planet's atmosphere using large telescopes.
Discovery of Uranus.
Voyager 2 at Uranus.
Neptune was discovered in 1846 as a result of mathematical predictions, the only time this has ever happened. People observing Uranus noticed that it seemed to move faster than expected for a while, and then slower than expected, as if it was being affected by some other object's gravity. Mathematicians tried to predict where the new object might be, and eventually it was found. Neptune is a methane-rich gas giant planet, similar to Uranus, and it also has a system of rings and moons. Most moons are small irregular objects, but the biggest, Triton, is about 2400 km across and has a very thin atmosphere. Its complex surface has very few craters suggesting that its geological activity might still be active. In two places, thin plumes of dust carried high by rising air currents (or something similar) seem to suggest ongoing activity of some kind, not yet really understood. Triton is the only large moon to orbit backwards compared to its planet's rotation, and it has been suggested that it was a separate Kuiper Belt object that came too close to Neptune and was captured into orbit. It might have collided with pre-existing moons of Neptune, and tides in its solid body raised by Neptune might have melted it to drive the observed geological activity.
In 1989 Neptune and its rings and moons were briefly visited by Voyager 2 during its last planetary encounter. Most of what we know of the planet comes from this one flyby. The planet is still monitored by telescopes. There are no current plans to return for a closer look, but possible missions are being designed. They might orbit Neptune with frequent flybys of Triton, or drop an instrumented probe into the planet's atmosphere during a flyby of Neptune on the way to a more distant object.
Discovery of Neptune
Voyager 2 at Neptune
Pluto was only found in 1930, accidentally, during attempts to repeat the mathematical prediction of Neptune with a large planet even further from the sun. 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. It is the same size as Neptune's moon Triton, and it has a thin atmosphere with many layers of haze as well as one big satellite, Charon, and four small moons. Like Uranus, its rotation axis is tilted almost into its orbit plane. Pluto was examined by the New Horizons spacecraft in 2015 and found to be a very complex world with mountains, craters, fractures and featureless plains. The surface is made of ice (water ice mostly) but covered with deposits of nitrogen ice, carbon monoxide ice and many other substances including dark red carbon compounds. The smooth plains may be nitrogen glaciers. The big moon Charon is half the diameter of Pluto itself, too small to have an atmosphere, but its surface is fractured and cratered, and its north pole is covered with a dark red deposit similar to parts of Pluto. The four little moons have irregular shapes and several craters.
New Horizons is now on its way to visit one of the other Kuiper-belt objects, a small (40 km) object called 2014 MU69. It has a nickname, Ultima Thule, and New Horizons will fly past it on January 1st 2019.
'New Horizons' Pluto mission
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, was serviced (repairs, new instruments and components) several times by Space Shuttle crews, but now the shuttle has been retired and Hubble will eventually stop working. But not yet - it's still going strong. A replacement, the James Webb Space Telescope, is being built for launch in about 2018. It will be placed at the Sun-Earth Lagrange Point 2 and protected from the warmth of the sun by a massive sunshade so its instruments can be kept very cold, because it will observe in infrared wavelengths.
Space Telescope Science Institute.
Hubble Space Telescope images of solar system targets
First useful image transmitted from the surface of Mars, Viking 1, 20 July 1976 (JPL/NASA).