- Twin spacecraft called GRAIL
- Will map the Moon’s gravitational field
- Aim is to study the Moon from core to crust
NASA’S TWIN SPACECRAFT to study the Moon from crust to core are nearing their main-engine burns to place the duo into lunar orbit.
Named the Gravity Recovery And Interior Laboratory (GRAIL), the spacecraft are scheduled to be placed in orbit beginning at 8:21am Sydney time for GRAIL-A on January 1, 2012, and 9:05am for GRAIL-B the following day.
The distance from Earth to the Moon is approximately 402,000 kilometres. NASA’s Apollo crews took about three days to travel to the Moon. Launched from Cape Canaveral Air Force Station September 10, 2011, the GRAIL spacecraft are taking about 30 times that long and covering more than 4 million kilometres 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. That allowed it to reach a stable operating temperature long before science measurements from lunar orbit are to begin.
“This mission will rewrite the textbooks on the evolution of the Moon,” said Maria Zuber, GRAIL principal investigator from the Massachusetts Institute of Technology. “Our two spacecraft are operating so well during their journey that we have performed a full test of our science instrument and confirmed the performance required to meet our science objectives”.
A complex arrival
As of December 28, GRAIL-A was 106,000 kilometres from the Moon and closing at a speed of 1,200 kilometres per hour. GRAIL-B was 128,000 kilometres from the Moon and closing at a speed of 1,228 kilometres per hour.
During their final approaches to the Moon, both orbiters will move toward it from the south, flying nearly directly 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 688 kilometres per hour.
GRAIL-B’s insertion burn 25 hours later will last about 39 minutes and is expected to change the probe’s velocity by 691 kilometres per hour.
The insertion manoeuvres 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 55 kilometres.
Mapping the Moon’s gravity
When science collection begins, the spacecraft will transmit radio signals to each other as they orbit the Moon, enabling scientists to precisely define the distance between them.
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 neighbours in the inner Solar System developed into the diverse worlds we see today.
Adapted from information issued by NASA / JPL-Caltech / LMSS / KSC.
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