[meteorite-list] Cassini Expected to Fly By Phoebe

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Jun 7 01:16:05 2004
Message-ID: <200406070516.WAA13083_at_zagami.jpl.nasa.gov>

Aviation Week & Space Technology
June 7, 2004

NASA's Cassini spacecraft is expected to fly by Saturn's moon Phoebe
this week, and has successfully tested its communications and
propulsion systems in preparation for capture into Saturn orbit on
June 30.

Trajectory correction maneuver No. 20 (TCM-20) on May 27 targeted
the Jet Propulsion Laboratory spacecraft to a 2,000-km. (1,250-mi.)
flyby of Phoebe, and was the first time the primary main engine had
been fully exercised in 5.5 years.

Engineers were relieved at the good results because the helium
regulator that pressurizes the propellant tanks has had a slow leak
since it was first activated inflight, but telemetry showed it
worked well. The leak is across the regulator and has the potential
to overpressurize the tanks. An electric latch valve normally
isolates the regulator from high-pressure helium. It was opened 100
sec. before firing and closed shortly after the 362-sec. firing. The
leak was slow enough that pressure remained acceptable over this
period, but that was something that engineers wanted to ensure.

TCM-20 also tested the communications scheme planned for the 96-min.
Saturn orbit injection firing. That firing attitude will result in
Cassini pointing its high-gain antenna away from Earth. Following
several spacecraft losses at critical events during which there was
no planned telemetry, NASA officials wanted some sort of
communication during orbit injection for diagnosis in case it went
awry.

The answer is to transmit from low-gain antenna No. 1 (LGA1).
However, to receive the signal real-time on Earth, it cannot carry
any data so it can focus all the energy into the carrier. But LGA1
does provide a precise Doppler signal to tell whether the rocket
engine is giving the proper acceleration and supplies the time of
any catastrophe if the signal were to disappear.

In TCM-20 the side-pointing LGA2 transmitted a carrier-only signal
of about the same strength expected during orbit injection.
Controllers were able to monitor the Doppler shift in real time and
observe the change in velocity.

Phoebe is a 140-mi.-dia. moon in a retrograde chaotic orbit. It is
dark and thought to be a captured object from the Kuiper Belt. The
June 11 Cassini flyby could provide clues about primordial matter
left over from the formation of planets 4.5 billion years ago.
Received on Mon 07 Jun 2004 01:15:54 AM PDT