A lunar orbiter being built by India may be hauling a U.S. radar experiment designed to help unravel whether the Moon's poles contain pockets of water ice.

The device is a small, lightweight, low-power imaging radar developed by the U.S. Department of Defense. It would be attached to Chandraayan-1, a lunar probe now being built by the Indian Space Research Organization (ISRO) and headed for a September 2007 liftoff.

The miniature-synthetic aperture radar - mini-SAR --- would be delivered to ISRO as a flight module by February 2007. The miniature imaging radar instrument has been approved by ISRO for inclusion onboard Chandraayan-1, but awaits a U.S. State Department go-ahead.

A similar, but more high-tech radar unit could be added to NASA's Lunar Reconnaissance Orbiter as a technological demonstration, although the outcome of on-going discussion about this prospect is uncertain.

If flown on the NASA lunar orbiter, the radar could work in tandem with Chandraayan-1 in surveying the Moon for evidence of water ice.

Export approval needed

Last month, NASA selected the Moon Mineralogy Mapper (M3) to fly as part of the scientific payload for the Chandraayan-1 mission. The M3 is designed to create a mineral-resource map of the moon. It will be flown as part of the Chandraayan-1 mission if it is selected by ISRO in an independent competition.

The principal investigator for M3 is Carle Pieters of Brown University in Providence, Rhode Island. The final confirmation of the M3 investigation, however, is subject to successful negotiation of an international agreement between NASA and ISRO.

The U.S. mini-SAR would chart the Moon's poles, radar-scanning for water ice thought to be present in permanently shadowed lunar craters. Finding such a resource would be useful for future expeditionary crews. Armed with the right processing gear, water ice could be transformed into rocket fuel and oxygen to help sustain a lunar base.

"We've been informed by the India Space Research Organization that our payload has been accepted, pending funding and export approval. We have the funding, but are still working on getting export approval from the State Department," said Paul Spudis, a space scientist at The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland. "We are on schedule to design, build, and deliver the flight instrument in early 2007."

Spudis is a co-principal investigator of the mini-SAR instrument/experiment, along with Chris Lichtenberg of Naval Air Warfare Center, Weapons Division at China Lake, California.

Focus on the poles

Spudis told SPACE.com that the goals of the mini-SAR are to map the lunar poles and their deposits. "We are focusing only on the poles, to keep our data and power impacts to the spacecraft at a minimum." 

The mini-SAR onboard Chandraayan-1 should be able to identify any materials with "unusual scattering properties". That information -- melded with data about comparable deposits on Mercury and icy satellites around other planets -- would identify locations of lunar ice, Spudis said, as well as lead to estimations of the extent and purity of that ice.

The mini-SAR was proposed for NASA's Lunar Reconnaissance Orbiter (LRO), Spudis said, but the device was not selected to be onboard.

LRO is the first spacecraft to be built as part of the Vision for Space Exploration, put into action last year by U.S. President George W. Bush. LRO is slated for a liftoff in the fall of 2008.

The LRO underpins NASA's interest in replanting human footprints on the Moon. President Bush has called for the space agency to conduct the first extended human expedition to the lunar surface as early as 2015, but no later than the year 2020.

Scatter plan

Work on a second mini-SAR is underway, hardware that would be similar to the unit on Chandraayan-1 - but features technology that is restricted by U.S. policy for shipment abroad.

If carried by LRO, the mini-SAR would be a NASA, Department of Defense technology demonstration, and not principal investigator led. If LRO does fly in 2008, it could be operating at the same time as Chandraayan-1's two-year mission.

At this moment, there has been no competitively-selected SAR imaging experiment designated to fly on the 2008 LRO mission, said Jim Garvin, NASA's chief scientist in Washington, D.C.

"The risks of accommodating any additional experiments, whether as SAR or radio frequency demonstrations or whatever, has been assessed by the Lunar Program
Office at NASA's Goddard Space Flight Center," Garvin told SPACE.com. 

"Should any additional experiment be imposed on LRO, it is the view of the project and leaders of NASA in systems engineering, that it would require the mission launch date to slip into 2009," Garvin added, a decision that would also trigger such things as adding additional risk due to mission complexity and more fuel loading.

Resolve the issue

Spudis said that, ideally, if two radar-carrying spacecraft were in lunar orbit, compatibly configured with radio frequency equipment, bistatic imaging could be done. Bistatic simply means that the transmitter is not co-located with the receiver. 

"Bistatic imaging has the potential to definitively resolve the water ice issue," Spudis said. The technique can be used to distinguish between surface (roughness) scattering and volume (ice) scattering, he said.

"Such a series of measurements would greatly add to our confidence that we are indeed detecting ice and produce detailed maps of its distribution," Spudis advised. "Of course, a bistatic radar mapping experiment could be done by any two satellites carrying the instrumentation...whether one of them is LRO or not."