Wednesday, September 2, 2009

Chandrayaan's orientation problematic during joint Mini-SAR survey of Erlanger with LRO

Chandrayaan-1 initial Mini-SAR portrait of a permanently shadowed portion of south pole crater Haworth, as released by NASA in January.

In the week prior to the August 28 premature shut-down of IRSO's Chandrayaan-1 it's NASA-built Mini-SAR side-glancing radar mapper continued to frustrate scientists correcting for earlier problems with India's first lunar orbiter.

Chandrayaan remained improperly aligned with its Mini-RF counterpart on-board NASA's Lunar Reconnaissance Orbiter (LRO).

On August 20 researchers worked to retrieve data from Chandrayaan-1 Mini-SAR using the LRO still undergoing commissioning, as its own near twin Mini-RF radar simultaneously gathered similar data from a slighly different angle of the same target, the permanently shadowed floor of 10 km Erlanger crater, near the Moon's north pole.

The maneuver brought Chandrayaan-1 and it's more recently arrived companion for the United State, LRO, "within kilometers" of one another in polar orbit of the Moon.

New Scientist reports "each spacecraft made it to its planned position, but programming problems on the US-built Mini-SAR instrument aboard Chandrayaan prevented the device from sending a radio pulse," according to NASA's Jason Crusan in Washington.

"Later analysis showed that even if Mini-SAR had released the pulse, the signal would not have reached its target because Chandrayaan-1's orientation was drifting more rapidly than anticipated," Crusan said.

Chandrayaan-1 has been orienting itself using spinning gyroscopes and the sun since its star-tracking system failed earlier this year.

To correct for the problem of the failed star-tracker, "the team found a way to correct Mini-SAR's programming and better estimate Chandrayaan-1's drift. Then ground controllers abruptly lost radio contact with Chandrayaan-1 on 28 August – a death knell for the mission."

"We were actually in the middle of planning to conduct the experiment at a future time," Crusan told New Scientist.

"Obviously with the termination of the Chandrayaan mission, that will not happen."

Stewart Nozette of the Lunar and Planetary Science Institute in Houston, who lead work on Mini-SAR's sister-instrument on LRO, called Mini-RF, said the joint experiment would have produced a very clear signal of water ice if it were present," a strategic goal of most of the recent series of lunar orbiters, and a top priority for NASA, beginning with LRO together with the LCROSS mission, still set for a shepherded kinetic explosion within one of six shadowed craters near the Moon's south pole on October 9.

"I think that it would have been the icing on the cake if we had been able to do that," Nozette told New Scientist. "It would have been the best thing you can do from the remote sensing perspective."

Even so, radar collected by LRO and Chandrayaan-1 individually could still reveal water ice – though not as clearly, Nozette said. Water ice, it is believed, should appear brighter at RF frequencies than rock viewed from above with star-lit equipment.

If such a signal appears only in areas that are permanently shaded – where free volatiles should stay protected from solar energy, it could suggest the presence of water.

It may not be long before new information on the ice question is revealed. Chandrayaan-1 flew over "a lot of little craters that looked like they had ice" and mapped 95 per cent of the polar regions before its mission ended," Nozette said.

The radar results are currently in review.

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