Tuesday, May 13, 2008

Returning to the Moon

New technologies on NASA's next lunar spacecraft will collect information to make human exploration safer.

By Brittany Sauser

NASA's next lunar orbiter will launch later this year, the first step in an ambitious plan to return humans to the Moon--and send them on to Mars. The spacecraft, called the Lunar Reconnaissance Orbiter (LRO), will use new technology to make precise maps of the Moon's surface, to search for resources such as ice, and to assess the threat that radiation in the environment could pose for humans.

LRO is the most advanced lunar satellite NASA has built, says Richard Vondrak, the project scientist for LRO, who adds that it will provide information that would have been impossible to collect a few decades ago. "We are surveying the Moon in more detail than any other celestial body for the benefit of all countries, including China, Japan, and India, who have said they have ambitions to put people on the Moon in the next 10 to 20 years," adds David Smith, a NASA scientist working on LRO.

LRO is part of NASA's Vision for Space Exploration, a program intended to, among other things, answer fundamental questions of physics, search for extraterrestrial life, and seek new resources, such as power sources, for Earth. The program calls for humans to return to the Moon. But before that happens, says Vondrak, it's necessary to understand much more about the Moon's surface radiation and topography.

"During Apollo, there were a number of near-fatal mistakes," says Smith. "We did not land on a flat surface, and there were boulders everywhere, which could have damaged the vehicle and prevented a return to Earth. Safety standards today would not have allowed Apollo."

The Apollo manned-spacecraft program shut down in 1975, and it was not until the 1990s that the United States sent more satellites to orbit the Moon--Clementine and the Lunar Prospector, which spent months orbiting the Moon and sending back data. Clementine was a joint project between the U.S. Department of Defense and NASA that also tested new ballistic technologies; the U.S. has launched no other lunar probes since.

LRO will collect more data with greater precision so that scientists can find safe and resource-rich landing sites and design systems appropriate for the lunar environment, says Vondrak.

Moonbound: The Lunar Reconnaissance Orbiter (above) will orbit and survey the Moon, providing greater detail about its surface and environment than any previous satellite has. One of two new instruments onboard the spacecraft will be the lunar orbiter laser altimeter (bottom), which will send out five laser beams 28 times per second to map the surface of the moon. Very short pulses of laser light are emitted through the narrow silver cone attached to the instrument’s optical assembly (gold-colored box). The large cone collects the laser light that is reflected back from the lunar surface.
Credit: NASA

LRO will collect more data with greater precision so that scientists can find safe and resource-rich landing sites and design systems appropriate for the lunar environment, says Vondrak.

LRO will orbit the Moon for one year at an altitude of 50 kilometers. Previous U.S. satellites maintained an altitude of approximately 100 to 200 kilometers, as have those sent by other countries, like China's Chang'e 1 and Japan's Kaguya, both launched in 2007. Orbiting at a lower altitude allows the spacecraft to get a closer view of the Moon, enabling the craft to obtain higher-resolution images, very detailed maps, and more-accurate temperature measurements, says Vondrak.

The lunar orbiter is equipped with six novel instruments, two of which will be making their space debuts: a cosmic-ray telescope, which will measure the effects that lunar radiation would have on humans, and a laser altimeter, which will make maps of the surface of the Moon.

The cosmic-ray telescope, called Crater, is a new kind of sensor developed by MIT, Boston University, the University of Tennessee at Knoxville, and the Aerospace Corporation. It can measure the radiation environment, not just in space, but also as it would be experienced by astronauts on the surface on a day-to-day basis. "By characterizing the radiation, we can build better shielding on spacecraft so that astronauts can survive long trips to the Moon and Mars," says Justin Kasper, a staff astrophysicist at the Harvard-Smithsonian Center for Astrophysics and the project scientist for Crater.

The human body responds to radiation in different ways, depending on the intensity, duration, and composition of the radioactive particles. The two things that scientists are most worried about are acute radiation poisoning from, for example, a solar flare, and long-term exposure to galactic cosmic rays, which can increase the risk of cancer. "In all cases, the danger is that ionizing radiation [high-energy, charged particles] can break the atomic bonds in DNA and damage cells and tissue," says Kasper.

The radiation detector consists of a series of silicon semiconductors, each about 35 millimeters in diameter and one millimeter tall. In between the pieces of silicon, the scientists have inserted large blocks of material called tissue-equivalent plastic. "The blocks are waxy and look like giant black crayons but have the same chemical composition as human tissue," says Kasper.

So while the silicon gauges the energy and composition of particles as they come flying through the detector (a proven technique for measuring radiation), the plastic is used to measure their biological effects. Previously, data from radiation detectors was sent back to Earth, where scientists attempted to calculate the effect that the measured radiation would have on humans. The plastic material provides a direct and more-accurate measurement of what radiation is like at different depths of human tissue, says Kasper.

The second instrument making its first spaceflight is the lunar orbiter laser altimeter (LOLA), developed by engineers at NASA Goddard Space Flight Center. It uses laser light to measure the distance between the spacecraft and the surface of the Moon. "It is going to measure that distance very precisely, to about 10 centimeters, and it will make measurements 28 times per second," says NASA's Smith, who is also the principal investigator for LOLA. Unlike current instruments, which send out a single laser beam at low repetitions, the new altimeter sends out five focused beams of laser light that are reflected back and received by five separate detectors, for a total of 140 measurements per second.

This allows scientists to make a high-density, very precise map of the shape of the lunar surface. "We can determine the altitude and slope of different spots on the Moon as well as the roughness of the terrain," says Smith. "We can also learn about the properties of the surface--for example, the shape of craters and their depth and size." The end goal is to identify the best spot, preferably flat, for a large lander to touch down and for astronauts to make a base.

Crater and LOLA will be accompanied on the lunar orbiter by four other instruments that will be imaging and mapping the Moon, measuring surface temperatures to identify potential ice deposits, and searching for hydrogen in the lunar polar regions. All the data will be continuously sent back to Earth for analysis.

"LRO is the first of our exploration missions for a return to the Moon and will have a significant impact on future human spaceflight," says Vondrak