Gravity Recovery and Interior Laboratory
The Gravity Recovery and Interior Laboratory (GRAIL) is an American lunar science mission in NASA's Discovery Program
Discovery Program
NASA's Discovery Program is a series of lower-cost, highly-focused American scientific space missions that are exploring the Solar System. It was founded in 1992 to implement then-NASA Administrator Daniel S. Goldin's vision of "faster, better, cheaper" planetary missions...

, which will use high-quality gravitational field
Gravitational field
The gravitational field is a model used in physics to explain the existence of gravity. In its original concept, gravity was a force between point masses...

 mapping of the Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 to determine its interior structure. The two small spacecraft GRAIL A and GRAIL B were launched on 10 September 2011 aboard a single launch vehicle: the most-powerful configuration of a Delta II
Delta II
Delta II was an American space launch system, originally designed and built by McDonnell Douglas. Delta II is part of the Delta rocket family and was in service from 1989 until November 1, 2011...

, the 7920H-10. GRAIL A separated from the rocket about nine minutes after launch, GRAIL B followed about eight minutes later. They will arrive at their orbits around the Moon 24 hours apart.

The science phase of the mission 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. The gravity mapping technique is similar to that used by Gravity Recovery and Climate Experiment
Gravity Recovery and Climate Experiment
The Gravity Recovery And Climate Experiment , a joint mission of NASA and the German Space Agency, has been making detailed measurements of Earth's gravity field since its launch in March 2002....

 (GRACE), and the spacecraft design is based on XSS-11.

Unlike the Apollo program missions, which took three days to reach the Moon, GRAIL will make 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 (31.1 mi) orbits with separation between the spacecraft (arriving 24 hours apart) of 175 kilometre. The very tight tolerances in the flight plan leaves little room for error correction leading to a launch window lasting one second and providing only two launch opportunities per day.

Maria Zuber
Maria Zuber
Maria T. Zuber is the E. A. Griswold Professor of Geophysics at the Massachusetts Institute of Technology where she also leads the Department of Earth, Atmospheric, and Planetary Sciences. Zuber has been involved in more than half a dozen NASA planetary missions aimed at mapping the Moon, Mars,...

 of the Massachusetts Institute of Technology
Massachusetts Institute of Technology
The Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological education and research.Founded in 1861 in...

 is GRAIL's principal investigator. The mission's team of expert scientists and engineers also includes former NASA astronaut Sally Ride
Sally Ride
Sally Kristen Ride is an American physicist and a former NASA astronaut. Ride joined NASA in 1978, and in 1983 became the first American woman—and then-youngest American, at 32—to enter space...

, who will lead the mission's public outreach efforts. NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

's Jet Propulsion Laboratory
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...

 manages the project. As of August 5, 2011, the program has cost US$496 million.


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.

Primary objectives

  • Map the structure of the lunar crust
    Crust may refer to:* Crust * The Crust, television seriesPhysical sciences:* Crust , at least continent-wide structure* Soil crust, local biology-sensitive structureFood:* Crust, dense surface layer of bread...

     and lithosphere
    The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's 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
    The formation of igneous rocks from magma is known as Magmatism.Magmatism is a process responsible for mountain formation. The process of magmatism produces an additional mass and volume to the Earth’s surface. For example the formation of volcanoes or island arcs at convergent plate boundaries...

  • Constrain the deep interior structure of the Moon
  • Place limits on the size of the Moon's inner core
    Inner core
    The inner core of the Earth, its innermost hottest part as detected by seismological studies, is a primarily solid ball about in radius, or about 70% that of the Moon...

The data collection phase of the mission will last 90 days, and will be followed by 12 months of data analysis. Results will begin to become available about 30 days after the collection begins. The knowledge acquired will aid understanding of the evolutionary history of the terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...



  • Ka band
    Ka band
    The Ka band covers the frequencies of 26.5–40 GHz. The Ka band is part of the K band of the microwave band of the electromagnetic spectrum. This symbol refers to "K-above" — in other words, the band directly above the K-band...

     ranging assembly (KBR)
  • Radio science beacon (RSB)
  • Moon Knowledge Acquired by Middle school students (MoonKam). Each MoonKAM system (one per spacecraft) consists of a digital video controller and four camera heads.

Launch attempts

All times are in EDT
Eastern Time Zone
The Eastern Time Zone of the United States and Canada is a time zone that falls mostly along the east coast of North America. Its UTC time offset is −5 hrs during standard time and −4 hrs during daylight saving time...


Mission status

The two small spacecraft which compose GRAIL were launched on 10 September 2011. GRAIL will make 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 (31.1 mi) orbits with separation between the spacecraft (arriving 24 hours apart) of 175 kilometre. The planned orbital insertion dates are (for GRAIL-A) and (for GRAIL-B).

External links

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