[meteorite-list] Deep Impact Crash Won't Destroy Comet or Earth

[Ron Baalke]

http://www.space.com/scienceastronomy/050628_deepimpact_effect.html

Cosmic Crash Won't Destroy Comet or Earth
By Leonard David 
space.com
28 June 2005

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 Temple 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."