CAPE CANAVERAL, Fla. — High altitude winds scrubbed today’s launch of the GRAIL lunar probes. Due to the precise orbit the probes need to take, each attempt only has a 1 second window for liftoff. NASA will try again tomorrow, but in the meantime read more about the mission and what it can tell us about the moon and our home planet.
GRAIL is an acronym for Gravity Recovery and Interior Laboratory. The twins, GRAIL A and GRAIL B, will create the most accurate gravitational map of the moon to date – improving knowledge of lunar gravity by as much as 1000 times. A high-resolution map developed over the three month mission of the moon’s gravitational field will help scientists deduce the moon’s interior structure, better understand the moon’s origin and development, and be invaluable in future lunar missions. On a broader scale, GRAIL will also help understand the evolutionary histories of the other rocky planets in our solar system – and possibly others.
GRAIL’s ride into space:
The first stage of the Delta II uses a Rocketdyne RS-27A engine, which burns a mixture of thermally-stabilized kerosene and liquid oxygen for 263 seconds before being jettisoned into the Atlantic Ocean. Assisting the first stage will be 9 external solid rocket motors. The graphite-epoxy solid motors are 46 inches in diameter, each fueled with 37,500 pounds of hydroxyl-terminated polybutadiene propellant. Six of the nine solid rocket motors will ignite at the time of lift off. The remaining three are ignited shortly after the initial six burn out. Each motor will burn for approximately 80 seconds. After each solid rocket is burned, it will be jettisoned from the spacecraft into the ocean.
The second stage of the rocket is an Aerojet AJ10-118K engine, burning a combination of Aerozine 50 and nitrogen tetroxide. The second stage is a restartable stage, and will burn multiple times separated by periods of coasting to precisely provide the velocity change needed to inject the spacecraft into the desired low-energy flight path toward the moon.
The GRAIL probes
The Grail spacecraft are built on the Lockheed Martin Experimental Small Satellite bus – a rectangular composite structure which serves as the frame of the probe. Fully fueled and ready for flight, each GRAIL has a mass of 677 pounds.
In addition to the science package, packed in this frame is a main hydrazine engine for large maneuvers and orbital corrections, 9 small attitude correction thrusters, a sun sensor, star tracker, internal measurement unit, command and data handling subsystems. Power is provided by a pair of 6.2 square foot solar panels, and a 30 amp-hour lithium ion battery for when the solar panels are unable to generate enough. The spacecraft communicate with Earth using an S-band transponder, and a pair of low gain antennas. GRAIL A and GRAIL B are identical spacecraft, with the exception of the orientation of the instruments and science packages – one is essentially a mirror image of the other, so each spacecraft can maintain proper orientation with the Earth and the Moon.
The science package on each spacecraft consists of a Lunar Gravity Ranging System, and the public outreach MoonKam system. MoonKam is a digital imaging array, used as part of the educational and outreach activities for GRAL. Each MoonKam consists of four cameras – one pointed slightly forward of the spacecraft, two directly down, and one facing slightly aft. The system can be used to take images and video of the lunar surface at up to 30 frames per second. MultiKAM is operated by undergraduate students at the University of California at Sand Diego under supervision of the faculty and in coordination with Sally Ride science. Middle school students from around the country will have the opportunity to become involced with MoonKam imaging.