SYDNEY: Circling the Sun are billions of comets and asteroids. Both are small and primordial and frequently cross the path of the planets – sometimes with cataclysmic results. Comets are icy, fluffy, fragile masses that spiral into the inner Solar System from the Kuiper Belt or the Oort Cloud. Asteroids are remnants of rocky debris that failed to form a planet or met their doom in the distant past.
A harbinger of doom for the Anglo-Saxons at the Battle of Hastings in 1066, it still held surprises when spacecraft from flew past in 1986. The Europe’s Giotto got within 600 km, revealing the hidden nucleus in the glowing coma – generated as ice sublimates from solid to gas. Halley’s coma consists of 80% water, 10% carbon dioxide and traces of methane; its nucleus is surprisingly dark, encrusted with rubble or complex carbon compounds.
Size: 15 km Orbital period: 76 years
In July 1994, the 20-odd fragments of Comet Shoemaker-Levy 9 plummeted like a broken string of pearls into the gas giant. About 12 of the pieces were at least 2 km-wide and slammed into Jupiter at 60 km/sec, each releasing 10,000 times the energy of a large hydrogen bomb. The collision left a chain of scars on the transitory surface, and telltale spectral signatures, illuminating the nature of Jupiter’s gaseous layers.
Size: Multiple pieces, 12 more than 2 km (destroyed on impact)
The behemoth of the Asteroid Belt, between Mars and Jupiter, Ceres is the Solar System’s biggest asteroid, containing a third of all the belt’s mass. Hubble’s images of its smooth, spherical body have led astronomers to believe that – unlike other asteroids, which are largely homogenous rocky or metallic lumps – Ceres may harbour interior layers and ice.
Size: 940 km Orbital period: 4.61 years
A long-period comet, Hale-Bopp’s appearance in 1997 caused a brief spectacular show as it swung by. Though never nearer than 200 million km, it was still incredibly bright: 1,000 times brighter than Halley at the same distance. Comets have two visible tails: a dust tail of microscopic particles forced out tens of millions of kilometres behind the comet by solar radiation; and a tail of charged ions created by the solar wind. Studies of Hale-Bopp revealed a third tail of neutral sodium atoms.
Size: 30 – 40 km Orbital period: 2,000 – 4,000 years
In 2004, NASA’s Stardust probe opened a gel-filled grid in the dusty tail of Wild 2. Two years later, the sample landed in the Utah desert, and scientists found traces of microscopic particles: strange mix of ices from the edge of the Solar System and igneous minerals formed in the hot inner regions. This mosaic overturned the theory that comets formed solely in the outer Solar System. Experts also found an amino acid, evidence for the idea that comets may have brought ingredients for life to Earth.
Size: 5 km Orbital period: 6.39 years
Asteroid Itokawa is a metal- and silicate-rich rock that travels between Earth and Mars on an elliptical orbit. Because it crosses the planets, its orbit is chaotic; beyond 200 years, we cannot predict where it will end up, says JAXA, Japan’s space agency; though it will likely smash into an inner planet or the Sun. JAXA landed the Hayabusa probe on Itokawa in 2005, finding evidence for space weathering and vibrations that have altered its surface. In 2010, Hayabusa will return the first sample of an asteroid to Earth.
Size: 500 m Orbital period: 1.5 years
This short period comet should provide new insight into the origin of comets when Europe’s Rosetta spacecraft arrives in 2014. It will orbit the comet at a distance of a few kilometres and eject a lander onto its surface. These may witness eruptions of gas from vents in the surface, and analyse the speed and nature of particles that stream from the comet as it nears the Sun.
Size: 4 km Orbital period: 6.6 years
When first detected, asteroid Apophis was calculated as having a 2.7% chance of striking Earth in 2029. We now know that on Friday 13 April 2029, it will pass within 29,470 km – within the orbit of some satellites! The asteroid may pass through a gravitational keyhole in 2029, which would have an unpredictable effect: “If it comes through at the right distance, it will bring it back on a collision course with Earth seven years later,” says Rob McNaught from Siding Springs Observatory in Coonabarabran, New South Wales. “That tiny possibility of a collision has really got people’s attention.”
Size: 330 m Orbital period: uncertain
Follow Cosmos on Twitter! twitter.com/cosmosmagazine