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Lunar orbiter yielding rich results at six-month mark

Posted: December 20, 2009

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A NASA probe circling the moon has found an unexpected lunar radiation source and detected the coldest known location in the solar system, scientists announced last week.

The Lunar Reconnaissance Orbiter arrived at the moon in June on a mission to the map the moon and search for suitable landing sites for future human expeditions. Seven science instruments on the spacecraft also provide priceless data to researchers.

The Apollo 11 landing site is seen in this image from the LRO camera. Credit: NASA/GSFC/Arizona State University
The probe's high-resolution camera, named LROC, has taken pictures of all six Apollo landing sites, revealing leftover equipment and astronauts' tracks on the surface.

The camera is also charged with observing 50 potential landing sites on the moon, evaluating the scientific value and topography of the locations for mission planners on the ground. Officials must carefully weigh science and engineering demands for any future lunar landing missions.

"What we're seeing in the LROC images not only have exploration value and scientific value, there's also a lot of natural beauty on the moon," said Mark Robinson, LROC principal investigator at Arizona State University.

LRO is managed for the first year of its mission by NASA's exploration division. The science mission directorate will take over operations next year, but the spacecraft is already returning a treasure trove for scientists.

"This is a real example of where exploration enables science, and science enables exploration," said Michael Wargo, chief lunar scientist at NASA headquarters.

The orbiter's radiation detector is characterizing the threat cosmic rays and energetic particles will pose to astronauts at the moon. The instrument evaluates how plastic analogs to human tissue react to radiation exposure, according to Harlan Spence, principal investigator for the CRaTER at Boston University.

Scientists are using spacecraft stationed throughout the solar system to measure the affects of the current solar minimum, an unprecedented and extended low level of activity in the sun's natural cycle.

The reduced solar wind has caused the heliosphere, a zone of the sun's influence, to shrink and allow more high-energy particles to reach the inner solar system from faraway sources in the galaxy.

There are 20 percent more cosmic rays at the moon than in previous solar minima, but annual radiation levels are still comparable to doses received by X-ray technicians or nuclear power plant workers on Earth, Spence said.

"Every square inch of your body would be receiving a particle that goes in one side and comes out the other every second," Spence said. "The good news that it's a tolerable level. It's the kind of thing we're used to in terms of exposures for people who are accustomed to working in occupations of ionizing radiation."

Artist's concept of LRO orbiting the moon. Credit: NASA
As sun activity increases, the impact of solar energetic particles will increase relative to galactic cosmic rays.

"We are in a period during this historic solar minimum where the radiation rates are elevated but are at tolerable levels in terms of radiation risk," Spence said.

But the CRaTER instrument also discovered unexpected levels of radiation closer to the lunar surface, where the moon should block cosmic rays.

Many scientists believed the moon's mass would shield astronauts working on the surface, but LRO's discovery shows something on the moon is producing a secondary source of radiation.

"The net affect could be that you don't get much help from the moon at all," Spence said. "In fact, some of the secondary particles might be even more hazardous in terms of their radiation dose."

LRO takes measurements from an altitude of about 31 miles, so researchers are not sure what dosages astronauts will see at the surface. Spence said surface radiation levels from the secondary lunar source may be as high as the dose from galactic cosmic rays.

A leading hypothesis for the lunar radiation source is the interaction between cosmic rays and soil. As energetic particles strike regolith, the collisions could liberate other particles and hurl them high above the moon.

Spence said the revelations would not prohibit human exploration of the moon, but researchers are eager to learn more about the surprising radiation fluxes.

As solar activity increases, LRO will gather information on radiation dangers posed by particles in the solar wind. LRO data, combined with results from a fleet of space weather monitors and solar observatories, will hopefully lead to more accurate forecasts of solar events emitting high radiation, according to Wargo.

Such predictions are the "holy grail" for lunar scientists, Wargo said.

"There are often precursors to the solar proton events, so you have some hope if you're outside a habitation or out of shielding to get back to some safer place where there's significant shielding," Spence said. "I think at the moon there are ways to mitigate and deal with both types of radiation risk."

Another instrument on LRO is collecting data for the most detailed thermal map of the moon ever assembled.

Thermal map from Diviner showing Hermite crater's cold trap at left. Credit: NASA/GSFC/UCLA
"The moon has one of the most extreme thermal environments of any body in the solar system," said David Paige, principal investigator for the Diviner payload at the University of California, Los Angeles.

In October, Diviner detected a cold trap in Hermite crater with temperatures of nearly -415 degrees Fahrenheit.

"To put that kind of temperature into perspective, nothing like this has ever yet been measured in the solar system," Paige said. "One would have to maybe travel far beyond the Kuiper Belt to find an object with a temperature this low."

Hermite is located near the moon's south pole, and surrounding highlands prevent sunlight from ever reaching the deepest reaches of the crater. Diviner measured the temperature at midnight in the middle of winter in the lunar northern hemisphere.

Temperatures as low as those in Hermite are cold enough to support water and other volatile compounds for extended periods, according to Paige.