• Introduction
    • GRAIL


      Mission Website


      The Gravity Recovery And Interior Laboratory, or GRAIL, will fly twin spacecraft in tandem around the Moon to precisely measure and map variations in the Moon's gravitational field. The mission will provide the most accurate global gravity field to date for any planet, including Earth. This detailed information will reveal differences in the density of the Moon's crust and mantle and will help answer fundamental questions about the Moon's internal structure, thermal evolution, and history of collisions with asteroids. The aim is to map the Moon's gravity field so completely that future Moon vehicles can safely navigate anywhere on the Moon's surface.


      Mission Management

      Dr. Maria T. Zuberl
      Dr. Maria T. Zuber
      Principal Investigator

      The GRAIL mission is managed for NASA by the Jet Propulsion Laboratory, Pasadena, CA. The Principal Investigator is Dr. Maria T. Zuber of the Massachusetts Institute of Technology, Cambridge, MA. The two GRAIL spacecraft will be provided by Lockheed Martin Space Systems, Denver, CO

  • Science Objectives
    • GRAIL scientists will use the gravity field information to determine the structure of the lunar interior, from crust to core and to advance our understanding of the thermal evolution of the Moon.


      Additional objectives include:


      Extending the knowledge gained on the internal structure and thermal evolution of the Moon to other terrestrial planets, leading to a better understanding of how other rocky planets formed

      Reducing the risk to future lunar robotic or human science and exploration missions by providing high resolution, global gravity field maps that can be used to help target desirable landing sites

      A view of the Moon taken by the Galileo spacecraft in 1992.
      A view of the Moon taken by the Galileo spacecraft in 1992.
  • Details

      The twin GRAIL spacecraft successfully lifted off from Cape Canaveral Air Force Station at 9:08 a.m. ET on Saturday, September 10, 2011, aboard a Delta II rocket.  Mission controllers received timely communications from both GRAIL-A and -B, indicating that they successfully separated from the upper stage of the rocket and that the solar arrays deployed.

      The two spacecraft flew similar but separate trajectories to the Moon. GRAIL-A successfully completed a main engine burn on December 31, 2011, and entered into orbit around the Moon an hour later. GRAIL-B followed suit and began its lunar orbit 24 hours later. Initially the craft had an orbital period of about 11.5 hours. When the science phase begins in March, the twin spacecraft will fly in a near-polar, near-circular orbit at an altitude of about 34 miles above the surface with a two-hour orbital period. They will fly in formation for the 82-day science phase, performing high-resolution, high-precision global, regional, and local gravity field measurements.

      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. GRAIL's gravitational maps will reveal areas of greater or lesser density, allowing researchers to decipher the structure of the Moon's interior and provide clues about the beginnings of the Earth and other rocky planets, which are thought to have formed in a similar way.

      Scientists have long known that the Moon's gravity field is strangely uneven and pulls on approaching spacecraft in complex and menacing ways. Twelve US, Soviet, and Japanese spacecraft crashed into the Moon's surface between 1959 and 1993.

      The source of the gravitational quirkiness is a number of huge mascons (short for "mass concentrations") buried under the surfaces of lunar maria or "seas." Formed by colossal asteroid impacts billions of years ago, mascons make the Moon the most gravitationally lumpy major body in the solar system. To minimize the effects of mascons, orbits must be carefully chosen. GRAIL's gravity maps will help mission planners make those critical decisions for future robotic and human missions. GRAIL will provide the most accurate global gravity field to date for any planet, including Earth, so future missions can navigate anywhere on the Moon.


        Launch Photo
        The Delta II heavy rocket carrying GRAIL lifts off.

      Credit: Thom Baur, United Launch Alliance


      A gravity map of the Moon
      A gravity map of the Moon made by the Lunar Prospector spacecraft in 1999. Mascons, shown in red, are concentrations of denser material below the surface that cause an increase in gravitational pull. The five largest mascons correspond to the largest lava-filled craters on the near side of the Moon.
  • Noteworthy
    • The twin GRAIL spacecraft no longer have their boring monikers, A and B. Thanks to the 28 students in Nina DiMauro's fourth grade class at Emily Dickinson Elementary School in Bozeman, Montana, the cool new names for the lunar twins are Ebb and Flow. 

      The announcement was streamed live on NASA-TV as the excited students revealed their winning entry.
      The winning names were suggested by fourth-graders at Emily Dickinson Elementary School in Bozeman, Montana.

      To involve K-12 classrooms in the spirit and excitement of lunar exploration, NASA held a nationwide contest to name the two GRAIL spacecraft. Nearly 900 classrooms with more than 11,000 students from 45 states, Puerto Rico and the District of Columbia participated in the contest.

      The announcement was streamed live on NASA-TV as the excited students revealed their winning entry.
      The announcement was streamed live on NASA-TV as the excited students revealed their winning entry.

      The winning names of Ebb and Flow were announced on January 17, 2012. Maria Zuber, GRAIL principal investigator, said, "We were really impressed that the students drew their inspiration by researching GRAIL and its goal of measuring gravity. Ebb and Flow truly capture the spirit and excitement of our mission." As they study our lunar neighbor, Ebb and Flow will undergo nearly the same motion as the tides we feel here on Earth that are controlled by the Moon.

      GRAIL is NASA's first planetary mission carrying instruments fully dedicated to education and public outreach. Each spacecraft carries a small camera called GRAIL MoonKAM (Moon Knowledge Acquired by Middle school students). Thousands of students in grades five through eight will select target areas on the lunar surface and send requests for study to the GRAIL MoonKAM Mission Operations Center in San Diego.

      The winning prize for the Dickinson students is to choose the first camera images. Dickinson is one of nearly 2,100 schools registered for the MoonKAM program, which is led by Sally Ride and her team at Sally Ride Science in collaboration with undergraduate students at the University of California in San Diego.