Comet Tempel 1 is by far the most closely studied comet in history. We saw it up close for the first time in July 2005, when NASA’s Deep Impact mission flew past it, pelting in with a projectile on the way. On 14 February 2011, another NASA mission, Stardust-Next visited Tempel 1. What did it find?
Circling the Sun every 5.5 years, Tempel 1’s orbit lies between the orbits of Mars and Jupiter. Its surface is darker than coal, reflecting less than four percent of the sunlight that falls on it, although there are rare outcrops of ice. Here and there, the surface is pock-marked with circular features, ranging from 40 to 400 m (131 to 1312 ft) in diameter, these may be impact craters or pits left as ice deposits boiled away into the vacuum. Enhanced images show at least five faint jets where this process continues today. Other odd smooth areas which had looked like flows of material on the surface in 2005 seem to have experienced some erosion since the last visitation. These and other changes to the comet’s surface are caused by subsurface ice evaporating in the rays of the distant Sun, the escaping water vapour blasts into space, carrying away dust and gravel as it goes.
Since it was launched in 1999, the Stardust spacecraft has travelled about 5.7 billion km (3.5 billion miles) on its odyssey from Earth to comet Wild 2, back to Earth then on to comet Tempel 1. It hurtled within 180 km of Tempel 1 at 10.9 km per second (24 300 mph) and both comet and spacecraft were 336 million km (209 million miles) from Earth at the time of the encounter.