Top 10: Comets
Comets and Asteroids – Learn more about the threat to human civilisation in our special report.
Halley’s Comet is the most famous of all comets. British astronomer Edmund Halley was the first to realise that comets are periodic, after observing it in 1682 and tallying it to records of two previous comet appearances. He correctly predicted it would return in 1757. The comet is also though to be depicted in the 1066 Bayeux Tapestry.
Halley’s Comet, which is 8 kilometres (5 miles) wide and 16 km (10 miles) long, travels around the Sun every 75 to 76 years in an elongated orbit. It last passed close to Earth in February 1986.
Comet Shoemaker Levy-9 distinguished itself by breaking into 21 pieces under the stresses of Jupiter’s gravity in 1992 and then slamming in succession into the giant planet in 1994. The spectacular show was watched by telescopes across Earth, in orbit and aboard the space probe Galileo.
The impact of one fragment – around 3 km across – is said to have yielded an explosion and fireball equivalent to 6 million megatonnes of TNT. The plume reached 22,000 km (13,700 miles) above the cloud tops.
An icy-blue blob with a faint gas tail created the most spectacular comet display for 20 years as Comet Hyakutake passed just 15 million kilometres (9.3 million miles) from Earth in March 1996. It was the closest the comet had come to the Sun in 9000 years. The comet left astronomers puzzled as it produced X-rays 100 times more intense than predicted.
The spacecraft Ulysses unintentionally passed through Hyakutake’s tail in May 1996, showing that it as at least 570 million km (350 million miles) long – twice as long as that of any other known comet.
Comet Hale Bopp made its closest approach to Earth for 4000 years in January 1997. The last time the cosmic wanderer was seen near Earth was during the Bronze age in 2000 BC. Hale Bopp is much larger and more spectacular than Halley’s comet. It has a nucleus up to 40 km (24 miles) in diameter and could be viewed from Earth with the naked eye. Hale-Bopp is so bright that it was visible from Earth as early as 1995, when it was still outside the orbit of Jupiter.
The advent of Hale Bopp led to a bizarre and tragic human event – 39 members of the Heaven’s Gate cult in San Diego, US, marked the arrival by committing suicide.
After Halley’s Comet, Comet Borrelly was only the second to be spied close-up by a spacecraft. NASA’s Deep Space 1 paid a visit in 2001 and gave researchers a detailed glimpse of the comet’s pitch black core. Its snapshots revealed that the rocky nucleus is shaped like a giant 8-kilometre-long bowling pin, and the entire comet is curiously lopsided.
Unlike Halley’s Comet, which formed in the Oort Cloud at the outer edges of the Solar System, Borrelly is believed to originate in an icy cloud of rocks beyond Neptune called the Kuiper Belt.
Comet Encke was the second comet discovered to be periodic, by German Astronomer Johann Franz Encke in 1819. The comet is also the parent body of the annual Taurid meteor shower in October and November. It is a relatively old comet that now gives off little gas.
NASA’s CONTOUR comet chasing spacecraft was due to rendezvous with Encke in November 2003, providing valuable insights into the formation of the solar system. However, the $159 million spacecraft is thought to have broken in two after its engines fired to propel the spacecraft out of Earth’s orbit in August 2002.
Temple Tuttle is the progenitor of the annual Leonid meteor shower. Thousands of shooting stars streak across the night sky every November, as the Earth passes through the dust particles and rocky meteoroids haphazardly shed by the comet.
Very bright meteor showers were seen in 2002 as Earth passed through debris trails left in 1767 and 1866. But astronomers have predicted that these may have been the last major Leonid storms for up to 30 years. This is because the comet melts and sheds matter unevenly on its journey through the solar system, and we may not pass through another dense cloud of debris for some time.
Comet Wild 2 was visited by NASA’s Stardust in January 2004. The spaceprobe flew within 236 kilometres (147 miles) of the nucleus, taking some of the best pictures yet.
It also collected the first ever sample of dust particles to be taken from a comet’s wake. Stardust returned to Earth with its precious cargo in January 2006. This will provide insight into the conditions under which Wild 2 – and the solar system – formed, 4.5 billion years in the Kuiper belt.
Wild 2 is roughly 5 km in diameter and riddled with depressions, craters and cliffs. These may have been formed by jets of gas exploding out from beneath the surface.
On 4 July 2005, NASA’s Deep Impact spacecraft fired a washing-machine-sized impactor into the path of comet Tempel 1. The impactor hit the surface at 37,000 km/h (23,000 mph), creating a huge plume of dust and blasting out a crater the size of a football stadium.
NASA aimed to punch a hole in Tempel 1’s crust to reveal details about the interior of comets. However, that may be impossible as the dust cloud was bigger than expected, and NASA can not correct the obscured images taken by the space probe.
Tempel 1 is 6 km in size and hurtles along at 10 km (6 miles) per second. Its orbit has been changed by the gravity of Jupiter since it was discovered in 1867, and it now orbits the Sun every 5 to 6 years.
Launched in 2004, the European Space Agency’s Rosetta space probe is due to touch down on Comet 67P/Churyumov-Gerasimenko – or Chury – in 2014. The comet is thought to be around five kilometres across and currently orbits the sun about every 6.6 years. Its orbit used to be much larger, but interactions with Jupiter’s gravity since 1840 have knocked it into a much smaller orbit.
After a few months in orbit around Chury, Rosetta will release a small cube-shaped lander called Philae on to the comet’s icy nucleus. The orbiter will then spend nearly two years circling Chury as the comet heads back towards the Sun. Rosetta will study the comet’s composition to help us better understand the formation of our solar system.
Top 10: Comets
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