|
On
the 4th of July, a NASA spacecraft will blast a hole in Comet
Tempel 1.
+ Play Audio |
+ Download Audio | +
Historia en Español | +
Email to a friend | + Join mailing list
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.
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.
 Right:
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.
 Right:
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.
SEND
THIS STORY TO A FRIEND
Authors: David
H. Levy and Dr. Tony Phillips | Editor:
Dr. Tony Phillips | Credit: Science@NASA
|
