Comets and asteroids

The smaller worlds of the Solar System hold the answers to many of the questions about its origins - and since the 1980s they have been visited by a wide variety of spaceprobes.

Between and beyond the orbits of the eight major planets there are countless small objects, ranging in size from mere boulders to minor worlds comparable to our Moon. The largest of these are icy dwarf planets that lurk within the Kuiper Belt beyond the orbit of Neptune (see p.306). Closer to the Sun, the objects are generally rockier and are known as asteroids. Comets are occasional visitors from the outer Solar System - irregular chunks of rock and ice following long elliptical orbits, which only develop their familiar tails when they are heated by the Sun. Both comets and asteroids are thought to preserve pristine material from the dawn of the Solar System, unaffected by the geological and chemical processes that have helped to shape the larger planets and moons. This makes them important targets for unmanned spacecraft.

to investigate different aspects of the comet. NASA did not participate but laid a claim to the first comet probe by diverting ISEE-3, a satellite already studying the solar wind of particles blowing out from the Sun, to intercept orbit of typical.

near-Earth asteroid

Earth's orbit

>er 2005

Hayabusa es down on 143 Itokawa ts to collect m the surface.

Early investigations

The first of these small worlds to be targeted for investigation was the famous Halley's Comet

VARIED ORBITS

Most asteroids orbit the Sun in the main asteroid belt beyond the orbit of Mars. Near-Earth asteroids comet's orbit

Early investigations

The first of these small worlds to be targeted for investigation was the famous Halley's Comet

VARIED ORBITS

Most asteroids orbit the Sun in the main asteroid belt beyond the orbit of Mars. Near-Earth asteroids comet's orbit

>er 2005

Hayabusa es down on 143 Itokawa ts to collect m the surface.

GIOTTO TO HALLEY

The Giotto probe (above), launched in 1985 on an Ariane 1 rocket, was battered by high-speed dust particles on its final approach to the nucleus of Hal ley's Comet (top).

^ The armada of probes sent to Halley - the Soviet Vegas 1 and 2, the Japanese Sakigake and Susei, and Europe's Giotto - all flew past the comet in March 1986, when it was at its most active. Their data helped to build a comprehensive picture of the comet, but Giotto had the most spectacular success, sending back pictures of the comet's "dirty snowball" nucleus from just 600km (370 miles) away.

Orbiting an asteroid

The first close-up studies of asteroids were made by the Galileo probe as it passed through the main asteroid belt on its way to Jupiter. Pictures provided an intriguing glimpse of elongated Gaspra and the irregularly shaped Ida, which Galileo revealed was orbited by a tiny moon, Dactyl. But it was not £ until 1996 that a spaceprobe dedicated to asteroid research was launched. This was NEAR, the Near Earth Asteroid Rendezvous \ mission. After a long journey, it slipped into orbit around asteroid 433 Eros, arriving appropriately on Valentine's Day, 14 February 2000. Over the following months, NEAR mapped and probed Eros with a variety of instruments, before it was finally guided to a gentle touchdown on the surface one year to the day after its arrival.

Looking inside

Since NEAR, studies of comets and asteroids have accelerated, with a wide range of missions to study their properties. Several of these were developed

ASTEROID EROS

NEAR's images of Eros revealed a surprisingly smooth surface. Scientists believe that a major impact around a billion years ago sent shock waves along the entire 31-km (l9V4-mile) length of Eros, shaking up and evening out the surface.

under NASA's Discovery programme - a series of spaceprobes with very specific objectives that followed a new "faster, better, cheaper" philosophy. ^HL

Stardust was the first to be launched, in February 1999. Its mission was to rendezvous with Comet Wild 2, collecting a sample of particles from the comet's ; — |j coma on a lightweight material called an aerogel. Stardust then looped back to Earth •

in 2006, and ejected its precious cargo in a ___B

re-entry capsule.

Another impressive NASA mission was -

the appropriately named Deep Impact. This time, the objective was to fire a barrel-like 370kg (8141b) projectile into a comet and study the material that was flung into space. The data from the spectacular impact on Comet Tempel 1 in July 2005 caught many scientists by surprise and led to a rethink of previous comet models - Tempel 1 turned out to be far more dusty, and less icy, than expected.

Japan's JAXA launched its own ambitious Hayabusa probe in May 2003. Hayabusa aimed to touch down on the surface of the near-Earth asteroid 25143 Itokawa, collect samples of material, lift off, and return to Earth. Although the mechanism intended to blast material from the surface of Itokawa failed, there is some hope that the collection system may still have swept up some floating dust - we will know for sure only after Hayabusa releases its reentry capsule in June 2010.

Perhaps the most ambitious of all the recent comet probes, however, is ESA's Rosetta. Launched in 2004, it will rendezvous with Comet Cheryumov-Gerasimenko in 2014, deploy a small lander called Philae onto its surface, and then orbit the comet as it falls back towards the Sun, heats up, and becomes active.

Ml

COMET CHASER

Engineers at ESA's European Space Research and Technology Centre in the Netherlands prepare to test the Rosetta probe's tolerance to high and low temperatures. As it follows its target comet from beyond the orbit of Mors around the Sun, it will be exposed to both extremes.

MISSION DEEP IMPACT

(Left) The Deep Impact fly by spacecraft is stacked with the impactor prior to launch.

(Above) Fifty minutes after the impact on Comet Tempel 1, the flyby spacecraft captured this image of a plume of gas and dust rising above the limb of the comet - the colours represent the brightness of moterials in the comet.

MISSION DEEP IMPACT

(Left) The Deep Impact fly by spacecraft is stacked with the impactor prior to launch.

(Above) Fifty minutes after the impact on Comet Tempel 1, the flyby spacecraft captured this image of a plume of gas and dust rising above the limb of the comet - the colours represent the brightness of moterials in the comet.

4 July 1997

Mars Pathfinder and its Sojourner Rover touch down on Mars.

MARS GLOBAL SURVEYOR

The thrusters on MGS were able to tilt the probe 30° in any direction to photograph the Martian surface at oblique angles, and even look at objects close to Mars such as its two small moons and other orbiting spacecraft.

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Mars Pathfinder and its Sojourner Rover touch down on Mars.

12 September 1997

Mars Global Surveyor (MGS) enters orbit and begins aerobraking.

1 April 1999

MGS begins its primary mission.

23 September 1999

Mars Climate Orbiter crashes into the planet during orbit insertion.

3 December 1999

Mars Polar Lander is lost shortly before landing on the planet.

24 October 2001

Mars Odyssey enters orbit and begins aerobraking.

25 December 2003

Mars Express enters orbit. Beagle 2 drops into the Martian atmosphere but fails during landing.

4 January 2004

The Spirit Mars Exploration Rover lands in Gusev Crater.

25 January 2004

The Opportunity Mars Exploration Rover lands in Meridiani Planum.

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