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UPDATE, July 4, 2005: Deep Impact successfully hit Comet Tempel 1. Get the full story, with pictures, at the Deep Impact home page.

June 28, 2005: For the last 4.5 billion years of our planet's violent history, Earth has been walloped by comets. These small bodies and their asteroid cousins whacked Earth often in its early years, knocking the stuffing out of our young world. As the solar system matured, impacts happened less often—but they have never ceased. Earth bears its scars in the form of weathered craters and extinct species.

This 4th of July is payback time. For the first time in history, Earth gets to strike back.

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Above: An artist's rendering of Deep Impact. Credit: Pat Rawlings. [More]

A NASA spacecraft named Deep Impact is going to shoot an 820-pound projectile into the rocky, icy nucleus of Comet Tempel 1. The 23,000 mph collision will form a big crater, and Deep impact will observe the stages of its development, how deep it gets and how wide it becomes. Researchers expect a plume of gas and dust to spray out of the crater. Deep Impact will measure its composition and record what the billowing plume does to the comet's atmosphere. In all, Deep Impact should be able to peer into the new crater for almost 15 minutes before the craft speeds away, continuing, like its cometary quarry, to orbit forever around the Sun.

Back on Earth, amateur astronomers will be watching, too. The comet glows like a 10th magnitude star and can be seen through backyard telescopes. It should brighten considerably when Deep Impact strikes. The impact plume will reflect sunlight, boosting the visibility of the comet to 5th or 6th magnitude, making it a faint naked-eye object. The Pacific side of Earth will be facing the comet at the moment of impact (0552 UT on July 4th; 10:52 pm PDT on July 3rd); observers in Hawaii, Mexico and the US southwest are favored. Click here and here for observing tips.

see captionRight: Look for Comet Tempel 1 near the bright star Spica on July 3rd and 4th. [Larger sky map]

Other spacecraft have flown by comets. Most recently, in 2004, Stardust approached Comet Wild 2 close enough to gather dust particles from the comet's atmosphere for return to Earth in 2006. Deep Space 1 visited Comet Borrelly in 2001; Giotto and others visited Comet Halley in 1986. We've seen what a comet's dark crusty nucleus looks like from the outside. Deep Impact, for the first time, will poke a hole through the crust and let us look inside.

Why Comet Tempel 1? We know of more than a thousand comets. Choosing this particular one has a lot to do with timing: Mission planners needed a comet that would be relatively easy to reach at about the time the spacecraft was ready for launch. According to principal investigator Mike A'Hearn of the University of Maryland, Tempel 1 has a big nucleus, about 5 miles wide; the impact should form a crater, not obliterate the comet. Tempel 1 also has an orbit that would allow the spacecraft to reach the comet with a high velocity and on the sunward side, so that the impact would be sunlit and visible from Earth. "Tempel 1 was in the right place at the right time," says mission co-investigator Lucy McFadden.

Blasting a comet is satisfying on many levels.

For one thing, it could save our planet. Comet Tempel 1 is not on a collision course with Earth, but suppose, one day, astronomers find a comet that is. What kind of projectile or bomb should we use to deflect or destroy it? If comets break into dangerously-big pieces when they're "touched," thus multiplying the hazard, shooting at them might not be a good idea. Consider Deep Impact to be a first experiment in planetary protection.

For another, it reveals the true value of comets. Is there "stuff" in there we can use? NASA is planning to send people back to the Moon by 2020, followed by trips to Mars and beyond; this is NASA's Vision for Space Exploration. Eventually comets could serve as interplanetary filling stations, providing explorers with raw materials they need, particularly water, which can be broken apart into hydrogen (for rocket fuel) and oxygen (to breathe) or simply melted and drank. Deep Impact is going to help planners understand exactly what materials comets contain and how difficult they might be to extract.

see captionRight: Comet Tempel 1, seen through a 4-inch refracting telescope. Credit: Mike Holloway of Van Buren, Arkansas. [Larger image]

Finally, Deep Impact takes us back to the beginning--of everything. Impacts are the process by which the planets in our solar system formed. One big example: Some 4.5 billion years ago a planetesimal body the size of Mars sideswiped the Earth, ripping off its crust and creating a huge ring of debris which lasted a year or so. Much of that debris later collided with itself to form a new world, which we now see as the Moon.

About half a billion years later, the Earth and Moon were hit by an act of cosmic terror unseen since then. A brief "period of late heavy bombardment" left the Earth and Moon a wasteland, and as we look up at the Moon, we see the results as the huge impact basins that form the eyes and face of the Man in the Moon. Even after that period subsided, comets kept on smashing into Earth, bringing with them the building blocks of life--carbon, hydrogen, oxygen, and nitrogen. Had there been no impacts, some scientists believe, there would be no life on Earth.

And once life formed, the process continued. At least one of the great extinctions of the past can be traced to the impact of a comet or an asteroid some 65 million years ago, resulting in the extinction of 70% of all species of life. It also led to great change in the history of life; instead of an age of reptiles, there was an age of mammals. And out of all this life there arose one small group of humans, who were inspired to send a return package.

Our planet comes full circle: prepare for Deep Impact.

Authors: David H. Levy and Dr. Tony Phillips | Editor: Dr. Tony Phillips | Credit: Science@NASA

More Information

Editor's Note: Some readers have asked if Deep Impact could alter Comet Tempel 1's orbit and send it toward Earth. No. The impactor is too small. Comet Tempel 1 poses no threat to Earth now and Deep Impact won't change that.

Deep Impact -- mission home page

Observing Tips -- Amateur astronomers can watch what happens when Deep Impact strikes.

Watching Comet Tempel 1 -- an observer's guide from Sky and Telescope

NASA Announces Spectacular Day of the Comet -- press release

Deep Impact animations

NASA's Vision for Space Exploration