The FINANCIAL -- NASA's twin spacecraft to study the moon from crust
to core are nearing their New Year's Eve and New Year's Day main-engine
burns to place the duo in lunar orbit, according to NASA.
Named Gravity Recovery And Interior Laboratory, the spacecraft are scheduled to be placed in orbit beginning at 1:21 p.m. PST for GRAIL-A on Dec. 31, and 2:05 p.m. PST on Jan. 1 for GRAIL-B. The distance from Earth to the moon is approximately 250,000 miles. NASA's Apollo crews took about three days to travel to the moon. Launched from Cape Canaveral Air Force Station Sept. 10, 2011, the GRAIL spacecraft are taking about 30 times that long and covering more than 2.5 million miles to get there.
This low-energy, long-duration trajectory has given mission planners and controllers more time to assess the spacecraft's health. The path also allowed a vital component of the spacecraft's single science instrument, the Ultra Stable Oscillator, to be continuously powered for several months. This will allow it to reach a stable operating temperature long before it begins making science measurements in lunar orbit. As of Dec. 28, GRAIL-A is 65,860 miles from the moon and closing at a speed of 745 mph . GRAIL-B is 79,540 miles from the moon and closing at a speed of 763 mph.
During their final approaches to the moon, both orbiters move toward it from the south, flying nearly over the lunar south pole. The lunar orbit insertion burn for GRAIL-A will take approximately 40 minutes and change the spacecraft's velocity by about 427 mph. GRAIL-B's insertion burn 25 hours later will last about 39 minutes and is expected to change the probe's velocity by 430 mph.
The insertion maneuvers will place each orbiter into a near-polar, elliptical orbit with a period of 11.5 hours. Over the following weeks, the GRAIL team will execute a series of burns with each spacecraft to reduce their orbital period from 11.5 hours down to just under two hours. At the start of the science phase in March 2012, the two GRAILs will be in a near-polar, near-circular orbit with an altitude of about 34 miles.
When science collection begins, the spacecraft will transmit radio signals precisely defining the distance between them as they orbit the moon. As they fly over areas of greater and lesser gravity, caused both by visible features such as mountains and craters and by masses hidden beneath the lunar surface. they will move slightly toward and away from each other.
An instrument aboard each spacecraft will measure the changes in their relative velocity very precisely, and scientists will translate this information into a high-resolution map of the Moon's gravitational field. The data will allow mission scientists to understand what goes on below the surface. This information will increase our knowledge of how Earth and its rocky neighbors in the inner solar system developed into the diverse worlds we see today.
JPL manages the GRAIL mission. MIT is home to the mission's principal investigator, Maria Zuber. The GRAIL mission is part of the Discovery Program managed at NASA's Marshall Space Flight Center in Huntsville, Ala. Lockheed Martin Space Systems in Denver built the spacecraft.
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