GRAIL. Gravity Recovery And Interior Laboratory.
NASA LAUNCHES MISSION TO STUDY MOON FROM CRUST TO CORE CAPE CANAVERAL, Fla. -- NASA's twin lunar Gravity Recovery and Interior Laboratory (GRAIL) spacecraft lifted off from Cape Canaveral Air Force Station in Florida at 9:08 a.m. EDT Saturday to study the moon in unprecedented detail. GRAIL-A is scheduled to reach the moon on New Year's Eve 2011, while GRAIL-B will arrive New Year's Day 2012. The two solar-powered spacecraft will fly in tandem orbits around the moon to measure its gravity field. GRAIL will answer longstanding questions about the moon and give scientists a better understanding of how Earth and other rocky planets in the solar system formed. "If there was ever any doubt that Florida's Space Coast would continue to be open for business, that thought was drowned out by the roar of today's GRAIL launch," said NASA Administrator Charles Bolden. "GRAIL and many other exciting upcoming missions make clear that NASA is taking its next big leap into deep space exploration, and the space industry continues to provide the jobs and workers needed to support this critical effort." The spacecraft were launched aboard a United Launch Alliance Delta II rocket. GRAIL mission controllers acquired a signal from GRAIL-A at 10:29 a.m. GRAIL-B's signal was eight minutes later. The telemetry downlinked from both spacecraft indicates they have deployed their solar panels and are operating as expected. "Our GRAIL twins have Earth in their rearview mirrors and the moon in their sights," said David Lehman, GRAIL project manager at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. "The mission team is ready to test, analyze and fine-tune our spacecraft over the next three-and-a-half months on our journey to lunar orbit." The straight-line distance from Earth to the moon is approximately 250,000 miles (402,336 kilometers). NASA's Apollo moon crews needed approximately three days to cover that distance. However, each spacecraft will take approximately 3.5 months and cover more than 2.5 million miles (4 million kilometers) to arrive. This low-energy trajectory results in the longer travel time. The size of the launch vehicle allows more time for spacecraft checkout and time to update plans for lunar operations. The science collection phase for GRAIL is expected to last 82 days. "Since the earliest humans looked skyward, they have been fascinated by the moon," said GRAIL principal investigator Maria Zuber from the Massachusetts Institute of Technology in Cambridge. "GRAIL will take lunar exploration to a new level, providing an unprecedented characterization of the moon's interior that will advance understanding of how the moon formed and evolved." JPL manages the GRAIL mission. It is part of the Discovery Program managed at NASA's Marshall Space Flight Center in Huntsville, Ala. Lockheed Martin Space Systems in Denver built the spacecraft. Launch management for the mission is the responsibility of NASA's Launch Services Program at the Kennedy Space Center in Florida. The Mission The GRAIL mission will place two spacecraft into the same orbit around the Moon. 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. This gravity-measuring technique is essentially the same as that of the Gravity Recovery And Climate Experiment (GRACE), which has been mapping Earth's gravity since 2002. Objectives GRAIL's engineering objectives are to enable the science objectives of mapping lunar gravity and using that information to increase understanding of the Moon's interior and thermal history. Getting the two spacecraft where they need to be, when they need to be there, requires an extremely challenging set of maneuvers never before carried out in solar system exploration missions. Mission Design The two GRAIL spacecraft will be launched together and then will fly similar but separate trajectories to the Moon after separation from the launch vehicle, taking about 3 to 4 months to get there. They will spend about 2 months reshaping and merging their orbits until one spacecraft is following the other in the same low-altitude, near-circular, near-polar orbit, and they begin formation-flying. The next 82 days will constitute the science phase, during which the spacecraft will map the Moon's gravitational field. Spacecraft Each of the two GRAIL spacecraft, GRAIL-A and GRAIL-B, is about the size of a washing machine and has about 200 kg of mass. They are nearly identical, but the need to point antennas on each at one another requires differences in the MoonKAM mounting and in the angles of the star trackers used for attitude control and the antennas through which the orbiters measure the changing distances between them. These orientations also require that GRAIL-B precede GRAIL-A in lunar orbit. The spacecraft design is based the Experimental Small Satellite-11 technology demonstration mission for the United States Air Force and the avionics are derived from NASA's Mars Reconnaissance Orbiter. The Attitude Control subsystem, which provides three-axis stabilized control, consists of a sun sensor, star tracker, reaction wheels, and inertial measurement unit. The electrical power subsystem includes two solar arrays and a lithium ion battery. Each solar array is capable of producing 700 watts at the end of the mission. They are deployed shortly after separation from the launch vehicle and remain fixed throughout the mission. Each battery has a capacity of 30 amp-hours, and is used to provide energy when the spacecraft orbits take them through the Moon's shadow. The propulsion system includes a hydrazine catalytic thruster for lunar-orbit insertion and trajectory changes, and a warm-gas system with 8 thruster valves for attitude control and other small maneuvers. The telecom subsystem includes the following: - 2 S-band transponder antennas to communicate with Earth - 2 X-band beacon antennas for Doppler ranging measurements from Earth of the Moon's near side - S-band time-transfer system antenna, which sends a time-synchronization code back and forth between the spacecraft - Ka-band ranging antenna for precision distance measurement between the spacecraft Each of the first two pairs of antennas has one antenna mounted on the sunny side of the spacecraft and one on the dark side. The sunny-side antennas point to Earth during the full moon and the dark-side antennas point to Earth during new moons. This system avoids the need to mechanically rotate the antennas during the mission, which would alter the spacecraft's center of mass and disturb the science measurements. Science Instrument There are two payload elements on each GRAIL orbiter: the Lunar Gravity Ranging System (LGRS) which is the science instrument, and the MoonKAM lunar-imaging system which is used for Education and Public Outreach. The LGRS is based on the instrument used for the Gravity Recovery and Climate Experiment (GRACE) mission which has been mapping Earth's gravity since 2002. The LGRS is responsible for sending and receiving the signals needed to accurately and precisely measure the changes in range between the two orbiters. The LGRS consists of an Ultra-Stable Oscillator (USO), Microwave Assembly (MWA), a Time-Transfer Assembly (TTA), and the Gravity Recovery Processor Assembly (GPA). The USO provides a steady reference signal that is used by all of the instrument subsystems. Within the LGRS, the USO provides the reference frequency for the MWA and the TTA. The MWA converts the USO reference signal to the Ka-band frequency, which is transmitted to the other orbiter. The function of the TTA is to provide a two-way time-transfer link between the spacecraft to both synchronize and measure the clock offset between the two LGRS clocks. The TTA generates an S-band signal from the USO reference frequency and sends a GPS-like ranging code to the other spacecraft. The GPA combines all the inputs received from the MWA and TTA to produce the radiometric data that is downlinked to the ground. In addition to acquiring the inter-spacecraft measurements, the LGRS also provides a one-way signal to the ground based on the USO, and is transmitted via the X-band Radio Science Beacon (RSB). The steady-state drift of the USO is measured via the one-way Doppler data provided by the RSB.