On 4 July -- U.S. Independence Day -- "Deep Impact" may help them come closer to understanding the origin of our solar system. Lucy-Ann McFadden (eds: female) is an astronomer at the University of Maryland on the U.S. East Coast, and a member of the "Deep Impact" team.
"We're very excited and looking at our data and preparing for the big day that's coming up. We have set the impact for 0558 universal time [0758 Prague time] -- that's the time at Greenwich, England, on 4 July. And we still have two trajectory maneuvers planned, so we are sure we'll hit on the sunlit side of the comet. But everything is on schedule," McFadden said.
The target of the mission, the 14-kilometer-wide Tempel 1 comet, circles the sun every 5.5 years. Much of its composition dates back to its creation during the formation of the planets some 4.5 billion years ago.
Comets have puzzled mankind since ancient times. The bright stars with long arching tails, seemingly appearing out of nowhere and moving slowly across the sky, have been a mystery to astronomers and laymen alike. Several comets have passed close to Earth, but even so many questions remain about them.
Professor Monica Grady is a planetary sciences specialist at Britain's Open University.
"We know a lot about comets, but we don't know as much as we'd like to. We have a view of a comet as a dirty snowball, and the thought is that it might be a rocky nucleus surrounded by ice. But we don't know whether that's true," Grady said.
Grady says the comets could also comprise fluffy snow with a hard, icy hydrocarbon crust. As they approach the sun they begin to evaporate, releasing gases and dust, and their tails appear -- sometimes growing to millions of kilometers in length.
McFadden says the aim of the "Deep Impact" mission is to find out more about what's inside a comet.
"We'll create a hole in the comet, and gases will rush out, and ice will evaporate. And it will just get sucked out into the vacuum of space. So we are anticipating it being a newly activated jet in the comet. And we hope we'll be able to watch it and see the effects of it here on Earth," McFadden said.
The meter-wide probe will separate from the mother ship 24 hours before impact. It carries a camera that takes pictures of the approaching comet up until two seconds before they collide.
The collision will release energy equivalent to some 4.8 tons of TNT, and cause a crater tens of meters deep. The main "Deep Impact" craft will pass by at a distance of some 500 kilometers, taking pictures and analyzing flying debris.
Grady says that if all goes according to plan, the age-old mystery of the comet may be revealed -- and provide a fireworks show in space on the same day many Americans will be enjoying them on Earth.
"It's going to allow us to see below the surface for the first time, which is going to be really exciting. We'll be able to see how much ice is there, how much gas is there, how much dust is there, how much carbon is there in the dust. It's looking back in time," Grady said.
McFadden says the impact will be best visible from the U.S. island state of Hawaii. The orbiting Hubble space telescope will capture images of the collision.
Amateur space enthusiasts may be able to enjoy the show as well. If the impact is forceful enough, the comet will sprout a new tail and grow significantly brighter. If that happens, the comet will be visible for days through binoculars or even to the naked eye.