[meteorite-list] How Deep Impact Works

[Ron Baalke]

http://science.howstuffworks.com/deep-impact.htm

How Deep Impact Works
by Carolyn Snare 
How Stuff Works

Comets are traveling balls of astronomic history. Their origins go 
back to the formation of the solar system, approximately 4.6 billion 
years ago. When the sun was formed, it caused gases
and dust to be dispelled into space. Some of these materials later
formed planets, while quantities of these gases and dust settled into
orbits around but far from the sun.

Comets are thought to be consolidated balls of these materials,
containing ice, dust, organic matter and possibly rock, formed
approximately 4 billion years ago. As they travel through the solar
system, they pick up additional debris. In this way, comets are windows
into the history of the solar system. But with diameters of up to 60
miles (100 km), you can't just reach up and snag one in a big net in
order to study it.

Still, scientists are finding a way to get at the information: On
January 12, 2005, NASA's Discovery Mission Deep Impact launched with the
intent to probe beneath the surface of a comet. On July 4, 2005, Deep
Impact will encounter Comet Tempel 1.

The Basics

Comet Tempel 1 will be in its most solid stage, consisting of a nucleus
approximate 3.7 miles (6 km) in diameter, when it encounters the Deep
Impact spacecraft in July 2005. (For information on comets, including
their structure and composition, check out How Comets Work
<http://science.howstuffworks.com/comet.htm>.) The primary goal behind
the Deep Impact mission is to study the interior and the exterior of the
same comet.

The Deep Impact spacecraft consists of two parts: a flyby and an
impactor. When the spacecraft is close to the comet, the two parts will
separate. The impactor will put itself in the comet's path, causing a
collision between the two bodies.

If all goes well, the impactor will create a crater in the comet that
goes well below the surface and exposes the protected material below --
the "pristine material" that was formed during the birth of the solar
system. By studying both the material that comes out of the crater upon
impact and the characteristics of the comet that the crater exposes,
scientists will have an unprecedented view of the solar system in its
infancy. To learn more about impact craters, see Deep Impact: Cratering
<http://deepimpact.jpl.nasa.gov/science/cratering.html>.

The Science Behind the Mission

When scientists were developing the Deep Impact mission, they set forth
the following objectives:

    * Observe how the crater forms
    * Measure the crater's depth and diameter
    * Measure the composition of the interior of the crater and the
      material that is ejected upon its creation
    * Determine the changes in natural outgassing produced by the impact

They hope that the information they gather from these objectives will
help them answer three primary questions about comets:

    * Where is the pristine material in comets?
    * Do comets lose their ice or seal it in?
    * What do we know about crater formation?

Scientists believe the nucleus of a comet consists of two layers: an
external layer called the mantle and an internal layer considered to be
pristine. As a comet moves through the solar system, its mantle changes.
As it approaches the sun, some of the external ice sublimates and is
dispelled. It may also encounter and pick up additional debris. The
protected, pristine interior of the comet, however, is thought to be
unaffected by the comet's travels and could be as it was when the comet
was formed. Scientists believe that a study of the differences between
the two layers will tell them a great deal about the nature of the solar
system, both its formation and its evolution through the years.

Another major question scientists have about comets is whether or not
they go dormant or extinct due to the heat of the sun. A dormant comet
is one in which the mantle has sealed off the pristine interior layer,
and no gases pass from this interior layer to the exterior layer and out
of the comet. An extinct comet has no more gases in its nucleus at all,
and as such will never change. Results from the Deep Impact mission will
give scientists a better view of the nature of the mantle and enable
them to determine if Tempel 1 is active, dormant or extinct.

When the impactor makes the crater on Tempel 1, the results will provide
lots of information about the nature of comets. The actual formation of
the crater, how fast it is formed and its final dimensions will tell
scientists how porous the mantle and the pristine layers are. A study of
how the material is ejected from the crater site will show both its
porosity and density and potentially the mass of the comet as well.
Information from the entire cratering process
may give some indication of what kind of material actually makes up the
comet, which will help scientists understand how the comet formed and
how it has evolved over time.

The Muscle and Mind Behind the Mission

The Deep Impact spacecraft consists of two parts, the flyby spacecraft
and the impactor, and is about the size of a sport utility vehicle. The
flyby carries a High Resolution Instrument (HRI) and a Medium Resolution
Instrument (MRI) for imaging, infrared spectroscopy and optical
navigation. It uses a fixed solar array and an NiH2 battery
to power itself. The impactor remains attached to the flyby until 24 hours 
before it impacts Tempel 1.

Once released, the impactor guides itself into the path of the comet
using a high-precision star-tracker (which navigates by looking at the
stars), the Impactor Target Sensor (ITS) and auto-navigation algorithms
specially developed for this mission. The impactor also contains a small
hydrazine propulsion system for more precise trajectory and attitude
control. The HRI, MRI and ITS work together to guide the flyby
spacecraft to the comet and record scientific data before, during and
after the impact.

The complete flight system was launched as a payload on a Boeing Delta
II rocket (see How Rocket Engines Work
<http://science.howstuffworks.com/rocket.htm>) in January 2005. It is
due to encounter Tempel 1 in early July 2005. Twenty-four hours before
impact, the impactor will detach itself from the flyby spacecraft. At
this point, the flyby will slow down and position itself to observe the
impact as it passes by the comet.

Once the impactor leaves the flyby spacecraft, it will position itself
to impact the comet on the sunlit side, allowing for better-quality
images. Scientists estimate that the impactor will deliver 19 gigajoules
of kinetic energy (equivalent to 4.8 tons of TNT
<http://science.howstuffworks.com/question397.htm>) to the surface of
the comet, creating a massive crater. The energy will come from a
combination of the mass of the impactor, 816 pounds (370 kg), and its
velocity at the time of impact, 6.34 miles per second (10.2 km per
second). Ideally, the crater will be at least 325 feet (100 m) in
diameter and more than 80 feet (25 m) deep. 
These dimensions depend entirely upon the nature and type of material
that makes up Tempel 1. It is expected that the impactor will be
destroyed when it collides with the comet.

The flyby's imaging equipment will observe the nucleus for more than 10
minutes after the impact, imaging the impact, the crater development and
the crater interior. The flyby will also acquire spectrometry of the
nucleus and the crater site. It will send all of the images and
spectrometry back to the Deep Space Network on the ground.

How Deep Impact Came About

Deep Impact began when Alan Delamere and Mike Belton were working on a
collaboration to study Comet Halley. "We got Halley data and
investigated it and found the comet was far blacker than we had
imagined, blacker than coal. So we asked ourselves: How could this
happen?" Delamere said. "We became increasingly curious as to just how
this black layer accumulated." In 1996, Belton and Delamere, now joined
by Mike A'Hearn, submitted a proposal to NASA. They wanted to explore
another comet, this time a dead one named Phaethon. They had decided to
use an impactor to hit the comet and then observe the results. But NASA
was not convinced they could hit the comet. NASA wasn't even convinced
that Phaethon was a comet.

Delamere, Belton, and A'Hearn continued to think about the project and
try to figure out better ways to do it. In 1998, A'Hearn had taken over
leadership of the team, and they made a second proposal. This time, they
were going to impact an active comet, Tempel 1. They had also added a
guidance system to the impactor, increasing the odds that they would be
able to control the spacecraft well enough to hit their target. NASA
accepted the new proposal and agreed to fund the project. The Deep
Impact mission was born.

Deep Impact is a partnership between the University of Maryland, The
California Institute of Technology's Jet Propulsion Laboratory and the
Ball Aerospace and Technology Corporation.