Gravity Recovery and Interior Laboratory
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The Gravity Recovery and Interior Laboratory (GRAIL) is an American lunar science mission in NASA's Discovery Program, which will use high-quality gravitational field mapping of the Moon to determine its interior structure. The two small spacecraft GRAIL A (Ebb) and GRAIL B (Flow)[4] were launched on 10 September 2011 aboard a single launch vehicle: the most-powerful configuration of a Delta II, the 7920H-10.[2][5][6] GRAIL A separated from the rocket about nine minutes after launch, GRAIL B followed about eight minutes later. They arrived at their orbits around the Moon 24 hours apart.[7] The first probe entered orbit on 31 December 2011 and the second followed on 1 January 2012.[8]
Overview
Unlike the Apollo program missions, which took three days to reach the Moon, GRAIL made use of a three- to four-month low-energy trans-lunar cruise via the Sun-Earth Lagrange point L1 to reduce fuel requirements, protect instruments and reduce the velocity of the two spacecraft at lunar arrival to help achieve the extremely low 50 km (31 mi) orbits with separation between the spacecraft (arriving 24 hours apart) of 175 to 225 km (109 to 140 mi).[9][10] The very tight tolerances in the flight plan left little room for error correction leading to a launch window lasting one second and providing only two launch opportunities per day.[11]
The science phase of GRAIL will last for 90 days. Following the science phase (or extended mission phase), a five-day decommissioning period is planned, after which the spacecraft will impact the lunar surface in about 40 days.[12] The gravity mapping technique is similar to that used by Gravity Recovery and Climate Experiment (GRACE), and the spacecraft design is based on XSS-11.[13]
The orbital insertion dates were December 31, 2011 (for GRAIL-A) and January 1, 2012 (for GRAIL-B).[14]
Maria Zuber of the Massachusetts Institute of Technology is GRAIL's principal investigator. The mission's team of expert scientists and engineers also includes former NASA astronaut Sally Ride, who will lead the mission's public outreach efforts. NASA's Jet Propulsion Laboratory manages the project. As of August 5, 2011, the program has cost US$496 million.[15] Upon launch the spacecraft were named GRAIL A and GRAIL B and a contest was opened to school children to select names. Nearly 900 classrooms from 45 states, Puerto Rico and the District of Columbia, participated in the contest. The winning names, Ebb and Flow, were suggested by 4th grade students at Emily Dickinson Elementary School in Bozeman, Montana.[4]
Objectives
Each spacecraft transmits and receives telemetry from the other spacecraft and Earth-based facilities. By measuring the change in distance between the two spacecraft, the gravity field and geological structure of the Moon can be obtained. The gravitational field of the Moon will be mapped in unprecedented detail.[16]
Primary objectives
- 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, and will be followed by 12 months of data analysis.[16] Results will begin to become available about 30 days after the collection begins.[9] The knowledge acquired will aid understanding of the evolutionary history of the terrestrial planets.[9]
Instruments
- Ka band Lunar Gravity Ranging System (LGRS), derived from the Gravity Recovery and Climate Experiment instrument.[1] 90% of the GRACE software was reused for GRAIL.[17]
- Radio science beacon (RSB)
- Moon Knowledge Acquired by Middle school students (MoonKAM).[18] Each MoonKAM system (one per spacecraft) consists of a digital video controller and four camera heads.[19]
Propulsion
Thrusters aboard each spacecraft are capable of producing 22 newtons (4.9 lbf).[1] Each spacecraft was fueled with 103.5 kilograms (228 lb) of hydrazine to be used by the thrusters and main engine to enable the spacecraft to enter lunar orbit and transition to the science phase of its mission. The propulsion subsystem consists of a main fuel tank and a He-repressurization system which will be activated shortly after lunar orbit insertion.[20]
Science phase
The spacecraft will be operated over a 90 day mission, with possibility for extension. That mission is divided into 3 27.3 day long nadir-pointed mapping cycles including of 2 daily 8-hour passes in view of the Deep Space Network for transmission of science and "E/PO MoonKam" data.[21]
At the end of the science phase and any mission extensions, the spacecraft will be powered down and decommissioned over a five day period. Each will impact the lunar surface after approximately 40 days.[21]
Launch attempts
All times are in EDT (UTC-4).
| 1 |
8 Sep 2011, 8:37:06 am |
scrubbed[14] |
--- |
high level winds |
8 Sep 2011, 8:30 am |
40% |
A weather balloon was released minutes before the decision point to take the latest readings of upper level winds and Air Force weather reconnaissance aircraft were aloft beginning at 7 am. |
| 2 |
8 Sep 2011, 9:16:12 am |
scrubbed[14] |
0 days, 0 hours, 39 minutes |
high level winds |
8 Sep 2011, 9:07 am |
40% [11] |
Range was reconfigured for omni antennae instead of tracked ones to support 99 degree azimuth. |
| 3 |
9 Sep 2011, 8:33:25 am |
abandoned[14] |
0 days, 23 hours, 17 minutes |
rocket propulsion |
|
40% |
An issue with the rocket's propulsion system was detected while the Delta 2 rocket was drained of fuel. |
| 4 |
10 Sep 2011, 8:29:45 am |
scrubbed[14] |
0 days, 23 hours, 56 minutes |
high level winds |
10 Sep 2011, 8:21 am |
60% |
|
| 5 |
10 Sep 2011, 9:08:52 am |
Success[14] |
0 days, 0 hours, 39 minutes |
|
|
|
|
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Spacecraft technicians close the payload fairing around GRAIL before launch.
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References
External links
http://en.wikipedia.org/wiki/GRAIL
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