The objectives of the Deep Impact mission are to rendezvous with comet
P/Tempel 1 and launch a projectile into the comet nucleus. Observations will then be made
of the ejecta, much of which will represent pristine material from the interior of the
comet, the crater formation process, the resulting crater, and any outgassing from the
nucleus, particularly the newly exposed surface. This project was selected as a Discovery
class mission in July, 1999.
The spacecraft consists of a 350 kg copper impactor attached to a flyby
bus with a combined launch mass of 1020 kg. The spacecraft is a
box-shaped honeycombed aluminum framework with a flat rectangular Whipple debris shield
mounted on one side to protect components during comet close approach. Mounted on the
framework are one high- and one medium-resolution instrument, each of which consists of an
imaging camera and an infrared spectrometer which will be used to observe this ejected ice
and dust, much of which will be exposed to space for the first time in over 4 billion
years. The medium resolution camera has a field of view (FOV) of 0.587 degrees and a
resolution of 7 m/pixel at 700 km distance and is used for navigation and context images.
The high resolution camera has a FOV of 0.118 degrees and a resolution of 1.4 m/pixel at
700 km. The infrared spectrometers cover the range from 1.05 to 4.8 micrometers with FOV
of 0.29 degrees (hi-res) and 1.45 degrees (lo-res). The total flyby bus instrument payload
has a mass of 90 kg.
The flyby spacecraft measures approximately 3.2 m x 1.7 m x 2.3 m, is three-axis
stabilized and uses a hydrazine propulsion system. Communications with the ground from the
flyby bus are via X-band (8.000 MHz) through a 1 meter diameter parabolic dish antenna
mounted on a 2-axis gimbal and via S-band between the impactor and flyby spacecraft. The
maximum data rate will be 400 kbps. Power of 620 W at the encounter is provided by a 7.5
square meter solar array and stored in a small NiH2 battery.
The projectile is made of copper so it is easily identifiable in the spectra after
the projectile is largely vaporized and mixed in with the comet ejecta on impact. The
impactor is equipped with an Impactor Targeting Sensor, an imager which provides knowledge
for autonomous control and targeting, and a cold-gas attitude control system. The total
budget for the mission is $240 million.
Deep Impact was launched on 12 January 2005 on a Delta-7925. After entering a heliocentric orbit
the spacecraft flew by Moon in January 2005. During this flyby the Moon was used to test
the targeting system performance without releasing the impactor. The spacecraft was about
880,000 km from comet P/Tempel 1 on 3 July 2005 moving at a velocity of 10.2 km/s relative
to the comet. The projectile was released at this point and shortly after release the
flyby spacecraft executed a maneuver to slow down relative to the impactor at 120 m/s. On
4 July the impactor struck the sunlit side of the comet nucleus 24 hours after release. At
the 10.2 km/s velocity, the impactor formed a crater roughly 25 meters deep and 100 meters
wide, ejecting material from the interior of the nucleus into space and vaporizing the
impactor and much of the ejecta. The flyby spacecraft was approximately 10,000 km away at
the time of impact and began imaging 60 seconds before impact. At 600 seconds after impact
the spacecraft was about 4000 km from the nucleus and observations of the crater began and
continued until closest approach to the nucleus at a distance of about 500 km. At 961
seconds after impact imaging ended when the spacecraft reorients to cross the inner coma.
At 1270 seconds the crossing of the inner coma was complete and the spacecraft oriented
itself to look back at the comet. At 3000 seconds the spacecraft began playback of data to
Earth. The comet and spacecraft were about 0.89 AU from Earth and 1.5 AU from the Sun
during the encounter. Real time return of selected impactor images and flyby images and
spectra were returned to Earth during the encounter. Primary data return was to be over
the first day after encounter, with a 28 day supplemental data return period. Earth-based
observatories also studied the impact. The spacecraft ranged over a distance of 0.93 to
1.56 AU from the Sun during the mission.
End of primary mission was in August 2005 and the spacecraft was put on a trajectory to
an earth flyby and a possible flyby with a comet is considered. Deep Space was put in safe
mode and waits for a possible reactivation.
Extended Mission
In July 2007 the extended mission called EPOXI (Extrasolar Planet Observations
/ eXtended Investigation of comets) was approved. This mission melds two
compelling science investigations - the Deep Impact Extended Investigation (DIXI) and the
Extrasolar Planet Observation and Characterization (EPOCh).
- DIXI (Deep Impact eXtended Investigation of comets) will involve a flyby of comet
Boethin, which has never been explored. Boethin is a small, short period comet, or one
that returns frequently to the inner solar system, from beyond Jupiter's orbit. This
investigation will allow the recovery of some of the science lost with the 2002 failure of
the COmet Nucleus Tour (CONTOUR) mission that was designed to
make comparative studies of multiple comets. DIXI was originally to be targeted to fly by
comet 85P/Boethin 5 December 2008, but as comet 85P/Boethin apparently is no longer
detectable, it has been rerouted to comet 103P/Hartley 2 for a flyby on November 4, 2010 with a flyby distance of ~700 km.
- The EPOCh (Extrasolar Planet Observations and Characterization) investigation also will
use the Deep Impact spacecraft to observe several nearby bright stars, watching as the
giant planets already known to be orbiting the stars pass in front of and then behind
them. The collected data will be used to characterize the giant planets and to determine
whether they possess rings, moons, or Earth-sized planetary companions. EPOCh's
sensitivity will exceed both current ground and space-based observatory capabilities.
EPOCh also will measure the mid-infrared spectrum of the Earth, providing comparative data
for future efforts to study the atmospheres of extrasolar planets. This search for
extrasolar planets will be made en route to comet 103P/Hartley 2.
The mission ended after flybys of two comets and 4.7 billion miles of interplanetary travel following a software glitch which likely put the space probe into a tumble in early August 2013. NASA was not able to reestablish communications with the spacecraft.
