[meteorite-list] Prepare for Deep Impact

[Ron Baalke]

http://science.nasa.gov/headlines/y2005/28jun_deepimpact.htm

Deep Impact
NASA Science News
June 28, 2005

On the 4th of July, a NASA spacecraft will blast a hole in Comet Tempel 1.


June 28, 2005: For the last five 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.

The weapon: a NASA spacecraft named Deep Impact.

The target: a 10-mile wide comet named Tempel 1.

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
<http://deepimpact.umd.edu/amateur/> and here
<http://skyandtelescope.com/observing/highlights/article_1522_1.asp> for
observing tips.

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;
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.

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.