BOULDER, Colorado - Late Sunday and early Monday, skywatchers might be treated to celestial fireworks unlike
anything witnessed before.
Like some Space Age
equivalent of WrestleMania, NASA's Deep Impact spacecraft is
double-billed to tangle with Comet Tempel 1 on July 4. The mission is a two-part project:
A "Flyby" vehicle will unleash an "Impactor" probe
that will slam into the fast-moving comet.
The comet and Impactor will collide at about 23,000 mph.
Deep Impact is the first
mission to make contact with a comet's surface. The hope is to produce a crater
in the large comet and reveal what is underneath the surface.
The event could be visible from
backyards. But for some, the novel space shot conjures visions of comet chunks
careening into Earth. Is there any chance that Deep Impact will result in icy
lumps of the comet splitting off, placing our planet in danger?
Put your crash helmets back
in the closest and get some sleep.
Comet kamikaze mission
"We're sending a bug onto
the windshield of a train," said Monte Henderson, Deputy Director of Programs
in Civil Space Systems for Ball Aerospace & Technologies Corporation here.
Ball Aerospace, in
association with the University
of Maryland and NASA's
Jet Propulsion Laboratory (JPL), developed and integrated the Flyby spacecraft,
the Impactor spacecraft, and science instruments,
including three telescopes, three cameras and a spectrometer for analyzing the
interior of the comet - as revealed by the high-speed run-in.
Comet Tempel 1 is moving at a brisk 66,000 miles
per hour. The ejected Impactor is slated to be speeding
at roughly 43,000 miles per hour. Deep Impact won't have a head-on crash
with the comet, like the old anti-ballistic missile defense adage of
"hitting a bullet with a bullet".
"We're being overrun
from behind," Henderson
told SPACE.com. "The speeds are just mind-boggling."
Telescopes onboard the
Flyby vehicle will be trained on the Impactor as it
is overtaken by the comet. The intent is to see crater formation as it's
occurring, Henderson
said. Scientists expect to see a crater shaped that's a 100 yards long, and
between seven to twelve stories deep, "with nice vertical walls," he said.
In the event that the Impactor isn't successfully ejected by the Flyby
spacecraft, then the entire vehicle is on a true comet kamikaze mission. It is
to pile-drive itself into the comet, Henderson
said. "It's important to the science community that we create a crater."
"This is a one-time
exciting event," Henderson
explained.
Simulation of a natural event
The excavation of the comet by Deep
Impact may create a cloud of meteoroids, objects larger than the gas and dust
that NASA predicts. And that possibility might not be so unnatural, contends
Peter Jenniskens, an astronomer with the SETI
Institute in Mountain View,
California.
In a paper accepted for publication
in the September issue of the Astronomical Journal, Jenniskens
and co-author Esko Lyytinen,
an amateur astronomer and meter shower expert from Finland, report that as a result of
Tempel 1 breaking, a meteoroid stream will be created
"in much the same manner as the mechanism that causes most of our meteor
showers."
Jenniskens has been studying the formations of
meteoroid streams by the disintegration of comets. He points to the idea that
many meteoroid streams are caused by wholesale disintegration of comets, which
are loose assemblages of cometesimals and are known
to frequently break apart.
There are several possible causes of
such fragmentations. But the idea that collisions between comets and large
meteoroids spark such fragmentation is on the table, Jenniskens
said. Furthermore, the Deep Impact mission acts as a simulation of such a
natural event.
"Depending on how the
kinetic energy of the impact will be distributed, there is a real possibility
that sufficient internal gas pressure builds up to break the comet apart. At
that time, we can study the peculiarities of dust generation in the way that
led to our main meteor showers," Jenniskens and Lyytinen conclude. They also predict that the dust
production will be moderate.
Risk to Earth?
There is a "way outside
chance" that Deep Impact will promote an explosive release of built-up,
sub-surface gas pressure, said Donald Yeomans,
Supervisor of the Solar System Dynamics Group at the Jet Propulsion Laboratory
(JPL) in Pasadena, California.
Yeomans said in the event that any
meteoroid material is released, it will be modest and come no where near the
Earth. (Meteor showers on Earth develop when Earth passes through a stream
of comet debris.)
"So a modest meteoroid
stream might be created that would follow the parent comet but no visible
meteors would result of course," Yeomans said.
"Perhaps an enhanced meteoroid stream following the comet might be discernable
in pre- and post-impact infrared images. It would be unlikely, but not out
of the question," he explained.
But given that the Impactor and Tempel 1 will bang
into each other at a force of four-and-a-half tons of TNT, why wouldn't that
explosive energy toss out huge bits of comet...splinters that could head toward
Earth?
"The bottom line is that we
have an object the size of a washing machine colliding with a comet the size of
Manhattan Island. No contest," Yeomans
said.
If Comet Tempel 1 happened to be smaller, say roughly 500 feet (150
meters) across as opposed to its true average diameter of 4 miles (6 kilometers),
the Deep Impact crash would be sufficient to adjust the comet's course by an
amount equal to Earth's radius ten years after the spacecraft impact, Yeomans told SPACE.com.
Mitigation techniques
Yeomans and his colleagues have done some
calculations, just to help put worries about the risk to Earth to rest. They
assumed a 98-foot (30-meter) chunk was broken off as a result of the impact.
That size was selected as the smallest chunk that might cause some ground
damage at Earth.
But for that chunk to be
propelled into a new Earth-crossing trajectory, Deep Impact would need to
impart a wallop that must be, well, simply astronomical. Furthermore, for the
same impact velocity as Deep Impact, the impacting spacecraft mass would then
have to about 5,900 times the mass of the actual Impactor.
"On a number of levels, it
would be impossible for Deep Impact to deliver a comet chunk to Earth," Yeomans concluded.
While the primary science
goals for Deep Impact do not include a demonstration of possible mitigation
techniques, Yeomans said that the rather
sophisticated autonomous navigation process used to impact the comet would be
similar to those necessary to mitigate a threatening near-Earth object in the
years to come.
Comets: devilishly
difficult to pin down
But while Deep Impact is an
impressive science mission, it won't be all that helpful in learning how to
fend off troublemaking asteroids.
That's the view of former
NASA Apollo astronaut Russell Schweickart. He is
chairman of the B612 Foundation, a group of experts dedicated to significantly
alter the orbit of an asteroid in a controlled manner by 2015.
"It will not tell us very much about
the issue of asteroid deflection," Schweickart said,
"even though kinetic impact is a possible option for a small sub-set of
asteroids that we might need to deflect." For one, the surface structural
characteristics are quite likely very different given the high water component
of comets, he added.
"The relative velocity in the Deep
Impact case is considerably higher than we would be able to achieve with most
asteroids," Schweickart noted. "And finally, the
change in the comet's trajectory will be so small that measurement and
attribution will be very challenging."
Comets are "devilishly difficult to
pin down accurately due to their outgassing," Schweickart suggested. After the impact it will be
difficult -- if not impossible -- to separate the effects of the impact itself
from any changes brought about by post-impact outgassing
- expected to rise dramatically due to the impact, he said.
"Still, it is an interesting
experiment to watch. The cratering process, the ejecta pattern and other observations will surely give us
some clues," Schweickart observed. Those involved
with Deep Impact "are clever folk and there are doubtless some lessons to be
learned."
Balance of the solar
system
The Deep Impact mission is
designed to alter the comet so that scientists can see beneath the surface,
said Lucy McFadden, a member of Deep Impact's science team. She is an Associate
Research Scientist for the University
of Maryland's Astronomy Department in College Park, Maryland.
As for altering the comet's path,
"the answer is no, not measurably," McFadden told SPACE.com. "This event
is happening in space, and space is huge, even when we are operating within our
solar system."
McFadden notes that objects moving
in space are not like billiard balls on a pool table.
"We won't knock the comet off its
orbit and into a collision course with Earth. The comet has tremendous orbital energy
that is orders of magnitude greater than that of the spacecraft," McFadden
explained.
The experiment mimics impacts in the
Solar System that have occurred since its formation, McFadden added. "It will
not upset the balance of the solar system."
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