[meteorite-list] Cassini Expected to Fly By Phoebe

[Ron Baalke]

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.