Lecture 6

First useful image transmitted from the surface of Mars, Viking 1, 20 July 1976 (JPL/NASA).

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.
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) 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 now studying the planet. 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 rovers (mobile vehicles). Lunokhod 1 was the first on the Moon, Sojourner on Mars. Another type of mobility would be balloons (Vega 1 and 2 at Venus) or other 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), a comet (Stardust), and an asteroid (Hayabusa). 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 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). 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, for the Moon and other worlds. All recent missions have been orbiters with surface composition and mapping instruments. We are now in a new international phase of lunar exploration. Europe, Japan, China and India have all flown orbital mapping missions in recent years, and all of them are now planning landers for the next few years. The only currently active missions are NASA's Lunar Reconaissance Orbiter and two Artemis spacecraft Two more, calleed GRAIL, are on their way to the Moon to map its gravity. LCROSS recently hit the Moon, discovering water and other volatiles (methane, carbon dioxide etc.) in a polar shaded area. 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, 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.
Lunar Picture of the Day (LPOD).
Soviet Lunar images.
Soviet Lunar images.
Moon images.
ESA's SMART-1 lunar images.
Moon images from Kaguya.
Moon images from Chandrayaan.
Moon images from Chang-e 2.
Moon images from Apollo.
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
LROC Wide Angle image gallery
LCROSS Moon impact mission.
USGS lunar data page.
Google Moon. (a much better version of this is available in Google Earth)
Google Lunar X-Prize.

Mercury

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?) suggesting 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. Only 45% of Mercury was seen in pictures from Mariner 10 in 1973-1974, but now a new mission, Messenger, has started to orbit the planet in 2011. Messenger images have now covered almost all of the surface of Mercury, and have revealed evidence of volcanic activity in and between the craters. Mercury was more active than the Moon and probably was active for longer.
Mariner 10
Mariner 10 images
Mercury maps
Messenger
Mercury data clickable map

Venus

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 studied the atmosphere, orbiters mapped the surface, and landers 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. Venus is a hostile desert with very high atmospheric temperature and pressure. But 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. The European spacecraft Venus Express is currently orbiting the planet but a Japanese Venus orbiter failed to enter Venus orbit in 2010. View Magellan and Venera images through the 'Venus missions' links.
Venus missions
Venus images
More Venus images
Venus maps
Venus Express

Mars

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 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. 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, 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 six landing sites. The Mars Exploration Rovers landed in 2004, and one of them is still operating in 2011. The big Curiosity rover is on its way to Mars, arriving in 2012. Look for continuing high resolution orbital images in the next few years and future rovers (maybe) in 2018. Mars has two little moons, Phobos and Deimos. A Russian spacecraft was supposed to land on Phobos in 2012 and collect soil for return to Earth, but it failed after launch.
Mars missions.
Mars exploration - current missions
Mars Global Surveyor images.
Mars Express images.
Mars Reconnaissance Orbiter images.
1997 Mars Pathfinder mission
2004 Mars Exploration Rovers.
MER dust devil movies.
Mars surface panoramas by James Canvin.
Mars surface colour images by Daniel Crotty.
Mars rover images by the Pancam team.
2008 Phoenix mission.
2011 (launch) Phobos Sample Return mission.
Mars maps.
Mars images from several missions.
Mars global datasets.

Asteroids

Asteroids are the rocky leftover building blocks of the planets. They 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 asteroid Itokawa in 2005, landed briefly, collected a sample of the surface material and returned to Earth with this asteroid dust. The Dawn spacecraft is now in orbit around one of the largest asteroids, Vesta, and will visit the biggest asteroid Ceres in 2014. Rosetta (see Comets section) has looked at two asteroids, Steins and Lutetia. Stardust and Deep Space 1 both examined small asteroids on the way to other targets. 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
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
Stardust mission - asteroid Annefrank
Deep Space 1 mission - asteroid Braille
Rosetta mission - asteroid Steins
Rosetta mission - asteroid Lutetia
Dawn mission - asteroids Ceres and Vesta
Asteroid maps and feature names

Comets

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 with the warmth of sunlight, the 'coma' or cloud of gas is formed, carrying dust with it into space. 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. 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 will orbit and land on a comet nucleus in 2014.
Giotto 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

Jupiter

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. 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 Io, one of its four big moons. The moon Ganymede is bigger than Mercury and is the largest moon in the solar system. 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. Future missions will probably explore the moon Europa, which has a liquid water ocean under its icy surface. Io, the volcanic moon, is rocky, but Ganymede and Callisto are half rock and half ice. 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 on its way to the planet now. A new orbiter to survey the Jupiter system and orbit Europa is being planned, if its cost can be made reasonable.
Pioneer missions to Jupiter.
Voyager
Jupiter images including some from Voyager
Jupiter images from Galileo
Impact on Jupiter
Another impact on Jupiter
Yet more impacts on Jupiter...
Jupiter images from Cassini
New Horizons at Jupiter

Saturn

Saturn is another gas giant with an enormous ring system. 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. Now we are lucky to be able to watch, as it happens, the first orbital mission: Cassini arrived at Saturn in July 2004 and is still exploring this complex system. Saturn has many moons with very complex and varied geology, and its giant moon Titan is nearly as big as Mercury, covered with a thick atmosphere, and has lakes of methane at its poles. All these moons are mixtures of rock and ice, with mainly icy surfaces. 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 muddy plains and channels in hilly areas nearby.
Pioneer 11 images of Saturn.
Voyager at Saturn
Cassini - latest news
Huygens on Titan
Huygens on Titan
Radar images of Titan
Radar images of Titan

Uranus

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 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. An orbital mission is being planned.
Discovery of Uranus.
Voyager 2 at Uranus.

Neptune

Neptune was discovered in 1846. In 1989 the methane-rich gas giant 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 gas jets erupting from its surface into its atmosphere. There are no current plans to return for a closer look, but possible missions are being designed.
Discovery of Neptune
Voyager 2 at Neptune

Pluto

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 three 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, 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, but cost overruns threaten its future.
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).