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First Look Inside a Comet
20 June, 2005
A Really Close Encounter
The Canberra Deep Space
Communication Complex
(CDSCC), as part of NASA's Deep Space Network (DSN) has a key
role in the Deep Impact rendezvous with Comet Tempel 1 on the
4th of July - that of maintaining the link between the spacecraft
and the Earth as its impactor payload collides with this mysterious
celestial traveller.
The DSN will be using specially
assigned radio frequencies for this link that are about a hundred
times higher than would be used to listen to an FM radio station.
The telecommunications link provides three main interactions
that are vital to Deep Impact - tracking, command, and telemetry.
• The tracking data measures
the distance from the spacecraft to the Earth; the navigators
use these measurements to guide Deep Impact toward Comet Tempel
1.
• The command link provides a
channel for control of the spacecraft and its computers and subsystems
by the mission controllers at the Jet Propulsion Laboratory (JPL).
• The telemetry link gives engineers
a view into the operation of the spacecraft to ensure that all
on-board tasks are being properly conducted. The telemetry link
also brings the scientific measurements of the on-board telescopes
and spectrometer to the science investigators.
For the critical events such
as important command sessions, spacecraft maneouvres, and observations
of the comet the DSN provides redundant coverage from two stations.
The DSN conducts ground operations
at its control center at JPL in the Space Flight Operations Facility
and via the Deep Space Communication Complexes, which include
the 70-metre antenna in Canberra, Australia, as well as those
in California, USA, and Madrid, Spain. The DSN is the major link
that deep space missions have with their engineering and science
teams.
The Deep Impact event is somewhat
unique in that it is observable by both Earth-based and space-based
telescopes. Coordinating the encounter across a number of observing
instruments adds to the complexity and to the richness of the
results.
With the help of its sister
stations in the DSN, the Canberra Complex will provide continuous
link support as Deep Impact approaches the comet. Because of
the volume of data and its importance at that exact time, overlapping
coverage is considered necessary from two of the DSN's three
70-metre structures around the world.
For the impact event itself,
the 70-metre antennas in Canberra and California will both have
the spacecraft in view - this is a fairly standard procedure
for crucial deep space encounters. The real-time telemetry channel
will be operating at 200 kilobits per second and every effort
will be made to ensure that this unique data is captured regardless
of weather or technical problems on the ground. After the close
passage by the comet, a period during which the flyby spacecraft
will play back the observations stored on board will complete
ther encounter coverage.
What will happen at the moment
of impact? What will scientists learn about these so-called "dirty
snowballs" in deep space? Only time will tell, and the DSN
will be ready to capture the vital signals and data that will
provide the answers.
For further information on
the Deep Impact Mission, visit http://deepimpact.jpl.nasa.gov
Article
compiled from a report on DSN activities for the Deep Impact
Mission. |
