Hubble Space Telescope to be Serviced by Columbia Shuttle Launch / Coolant Glitch

The space shuttle Columbia successfully launched mission STS 109 from Cape Canaveral, pad LC 39A, at 11:22 UTC (3:22 a.m. PST) on March 1. The shuttle is on a mission to service the Hubble Space Telescope (HST), with rendezvous planned early on March 3. The Hubble Space Telescope orbits Earth every 90 minutes at an altitude of 593 km (320 nmi). The seven member crew is made up of Commander Scott Altman, Pilot Duane Carey, Flight Engineer Nancy Currie and Payload Specialists John Grunsfeld, James Newman, Richard Linnehan and Michael Massimino. This is the 4^th service mission to the Hubble Space Telescope.

The shuttle's launch ascent was virtually flawless. However, just before the cargo bay doors were opened, data indicated the flow of coolant through one coolant loop radiator was sharply reduced and being bypassed. The Freon coolant loops are used to carry away heat generated by the shuttle's electronic systems. Once the cargo bay doors were opened and Freon-21 began flowing through a radiator in the left-side payload bay door, the cooling rate returned to normal. The reduced flow presumably will show back up when the cargo doors are closed for landing and the radiators are once again in bypass mode. From that point forward, the heat carried away by the Freon-21 must be dissipated by boiling water or, depending on the shuttle's altitude, ammonia. NASA flight rules require a minimum coolant flow rate of 211 pounds of Freon-21 per hour in each coolant loop. Just before the cargo bay doors were opened , engineers saw the flow rate in one loop drop to near the flight rule redline. The shuttle must have active cooling for re-entry. To protect against the possibility of a second failure, NASA's flight rules call for landing at the earliest U.S. opportunity, if one of the coolant loops is declared failed. That is not the case, but with a flow rate so close to the redline NASA's mission management team is meeting to discuss the problem and possible workarounds.

Using five back-to-back spacewalks by alternating two man teams, the astronauts will service the telescope’s electrical system, installing two smaller solar arrays, and a replacement 72 kg (160 lbm) Power Control Unit (PCU) for the main electrical distribution system. The new solar wings will generate 5.270 kW of electrical power, or about 20 to 25 percent more than the old panels. The additional power will enable scientists for the first time to operate all four of Hubble's science instruments simultaneously. The crew will also install a new reaction wheel assembly, the Advance Camera for Surveys and a “cryocooler” refrigerator for the dormant Near Infrared Camera and Multi-Object Spectrometer (NICMOS).

The Power Control Unit was not designed to be serviced by astronauts wearing spacesuit gloves. It is wired into Hubble's electrical system by 34 closely spaced, hard-to-reach cable connectors along its left side. Two more cables are connected at the base of the unit. The new PCU must be in place and power restored within 10 hours or so, or the low temperatures of space could damage the telescope’s sensitive electronics. The PCU replacement will not be attempted until the third spacewalk which is expected to last at least seven-and-a-half hours.

The NICMOS instrument is sensitive to infrared light and must be chilled to less than 100 degrees above absolute zero to work. The instrument was launched with a dewar of nitrogen ice coolant, but an internal “thermal short” caused the nitrogen to sublimate away faster than expected. The experimental cryocooler that will be installed uses neon gas and three small turbines spinning at 400,000 rpm to provide cooling to 75 degrees above absolute zero. From a spacewalk perspective, installation of the cryocooler is a complex task.

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