FIRST LOOK INSIDE A COMET!
What's deep inside a comet?
Comets are time capsules that hold clues about the formation and evolution of the solar system. They are
composed of ice, gas and dust, which is the primitive debris from the solar system's earliest and coldest
formation period - 4.5 billion years ago. Deep Impact, a NASA Discovery Mission, is the first space mission
to probe beneath the surface of a comet and reveal the secrets of its interior.
On July 4, 2005, the Deep Impact spacecraft arrives at Comet Tempel 1 to impact it with a 370-kg (~820-lbs)
mass. On impact, a crater is produced expected to range in size from that of a house to that of a football stadium,
and two to fourteen stories deep. Ice and dust debris is ejected from the crater revealing fresh material
beneath. Sunlight reflecting off the ejected material provides a dramatic brightening that fades slowly as the
debris dissipates into space or falls back onto the comet. Images from cameras and a spectrometer are sent to
Earth covering the approach, the impact and its aftermath. The effects of the collision with the comet will also
be observable from certain locations on Earth and in some cases with smaller telescopes. The data is analyzed
and combined with that of other NASA and international comet missions. Results from these missions will lead
to a better understanding of both the solar system's formation and implications of comets colliding with Earth.
The Deep Impact mission lasts six years from start to finish. Planning and design for the mission took place
from November 1999 through May 2001. The mission team is proceeding with the building and testing of
the two-part spacecraft. The larger "flyby" spacecraft carries a smaller "impactor" spacecraft to Tempel
1 and releases it into the comet's path for a planned collision.
In January 2005, a Delta II rocket launches the combined Deep Impact spacecraft which leaves Earth's
orbit and is directed toward the comet. The combined spacecraft approaches Tempel 1 and collects images
of the comet before the impact. In early July 2005, 24 hours before impact, the flyby spacecraft points
high-precision tracking telescopes at the comet and releases the impactor on a course to hit the comet's
The impactor is a battery-powered spacecraft that operates independently of the flyby spacecraft for just
one day. It is called a "smart" impactor because, after its release, it takes over its own navigation and
maneuvers into the path of the comet. A camera on the impactor captures and relays images of the
comet's nucleus just seconds before collision. The impact is not forceful enough to make an appreciable
change in the comet's orbital path around the Sun.
After release of the impactor, the flyby spacecraft maneuvers to a new path that, at closest approach
passes 500 km (300 miles) from the comet. The flyby spacecraft observes and records data about the
impact, the ejected material blasted from the crater, and the structure and composition of the crater's
interior. After its shields protect it from the comet's dust tail passing overhead, the flyby spacecraft turns
to look at the comet again. The flyby spacecraft takes additional data from the other side of the nucleus
and observes changes in the comet's activity. While the flyby spacecraft and impactor do their jobs, pro-fessional
and amateur astronomers at large and small telescopes on Earth observe the impact and its
aftermath, and results are broadcast over the Internet.
Comet Tempel 1
Comet Tempel 1 was discovered in 1867 by Ernst Tempel. The comet has made many passages through
the inner solar system orbiting the Sun every 5.5 years. This makes Tempel 1 a good target to study evo-lutionary
change in the mantle, or upper crust. Comets are visible for two reasons. First, dust driven from
a comet's nucleus reflects sunlight as it travels through space. Second, certain gases in the comet's
coma, stimulated by the Sun, give off light like a fluorescent bulb. Over time, a comet may become less
active or even dormant. Scientists are eager to learn whether comets exhaust their supply of gas and dust
to space or seal it into their interiors. They would also like to learn about the structure of a comet's inte-rior
and how it is different from its surface. The controlled cratering experiment of this mission provides
answers to these questions.
The flyby spacecraft carries a set of instruments and the smart impactor. Two instruments on the flyby
spacecraft observe the impact, crater and debris with optical imaging and infrared spectral mapping. The
flyby spacecraft uses an X-band radio antenna (transmission at about eight gigahertz) to communicate to
Earth as it also listens to the impactor on a different frequency. For most of the mission, the flyby spacecraft
communicates through the 34-meter antennae of NASA's Deep Space Network. During the short
period of encounter and impact, when there is an increase in volume of data, overlapping antennas
around the world are used. Primary data is transmitted immediately and other data is transmitted over
the following week. The impactor spacecraft is composed mainly of copper, which is not expected to
appear in data from a comet's composition. For its short period of operation, the impactor uses simpler
versions of the flyby spacecraft's hardware and software - and fewer backup systems.
The Deep Impact mission is a partnership among the University of Maryland (UMD), the California Institute
of Technology's Jet Propulsion Laboratory (JPL) and Ball Aerospace and Technology Corp. The scientific
leadership of the mission is based at UMD. Engineers at Ball Aerospace design and build the spacecraft
under JPL's management. Engineers at JPL control the spacecraft after launch and relay data to scientists
for analysis. The entire team consists of more than 250 scientists, engineers, managers, and educators.
Deep Impact is a NASA Discovery Mission, eighth in a series of low-cost, highly focused space science
investigations. Deep Impact offers an extensive outreach program in partnership with other comet
and asteroid missions and institutions to benefit the public, educational and scientific communities.
National Aeronautics and Space Administration
Jet Propulsion Laboratory
California Institute of Technology
JPL 400-936 2/01