The manned space shuttle program may recently have ended but unmanned space missions to explore the unknown are well and truly alive. Nasa latest space probe "The Gravity Recovery and Interior Laboratory" (GRAIL) is designed to map tiny variations of the Moon gravitational field and was launched from Cape Canaveral Complex 17B on the 10th September 2011.
Costing US$500 million the aim of GRAIL is to better understand the Moon's internal structure which should help to provide insight into many lunar mysteries. These include questions such as, why does the Moon's far side lack the great maria or "seas" that are prominent on the near side.
The spacecraft actually consists of two small identical probes, named Grail-A and Grail-B, weighing 132 kg. Unlike the Apollo manned missions that took about three days to reach the Moon, GRAIL will use a much longer (via Sun-Earth Lagrange point L1) but fuel-efficient journey requiring nearly four months to reach lunar orbit.
Once there, the two spacecraft will fly at a very low altitude of only 50km (31 miles) above the lunar surface. Grail-A will lead the charge around the Moon with Grail-B following about 200km behind. As the lead spacecraft flies through the uneven lunar gravity field, it will experience small accelerations or decelerations. These variations can then be determined with the help of the second spacecraft due to a resulting small change in the separation of the two probes, which can be very accurately measured.
The Primary mission objectives are:
- Map the structure of the lunar crust and lithosphere.
- Understand the asymmetric thermal evolution of the Moon.
- Determine the subsurface structure of impact basins and the origin of lunar mascons
- Ascertain the temporal evolution of crustal brecciation and magmatism.
- Constrain the deep interior structure of the Moon.
- Place limits on the size of the Moon's inner core.
The data collection phase of the mission will last 90 days, followed by 12 months of data analysis.