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Grail

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First Launch: 2011-09-10. Last Launch: 2011-09-10. Number: 1 . Gross mass: 202 kg (445 lb).

The two Grail satellites perfored high-precision range-rate measurements between them using a Ka-band payload in a 50-km circular lunar orbit. Earth-based data analysis of range-rate data provided a direct measure of the lunar gravity. The 270-day mission included a 90-day gravity mapping Science Phase.

The spacecraft were derived from the Lockheed XSS-11 design. Each spacecraft bus had a rectangular composite structure with two fixed solar arrays. Maneuver was was via a 22N thruster system and orientation via three reaction wheels. Additional attitude sensing components include an IMU, sun sensor and star tracker. To measure the inter-spacecraft range-rate, each spacecraft had a Ka-band Lunar Gravity Ranging System (LGRS) derived from the GRACE instrument.

NASA NSSDC Master Catalog Description

The Gravity Recovery And Interior Laboratory (GRAIL) mission is a dual spacecraft effort designed to determine the structure of the lunar interior and to advance understanding of the thermal evolution of the Moon. The primary science objectives are to: map the structure of the crust and lithosphere; understand the Moon's asymmetric thermal evolution; determine the subsurface structure of impact basins and the origin of mascons, ascertain the temporal evolution of crustal brecciation and magmatism; constrain deep interior structure from tides; and place limits on the size of the possible inner core. The GRAIL mission was selected through the NASA Discovery Program.

Spacecraft and Subsystems

GRAIL comprises twin spacecraft built on the Lockheed Martin Experimental Small Satellite (XSS-11) bus with a science payload derived from the GRACE terrestrial gravity mission. The bus is a rectangular prism with a composite structure and a dry mass of 132.6 kg, fully fueled mass is 202.4 kg. Two solar panels extend from the upper platform which also holds a 22 N thruster for propulsion. Fine control is provided by a warm gas system and 3 reaction wheels. A sun tracker, star tracker, and IMU are used for guidance and attitude knowledge. Power from the solar panels is stored in a lithium-ion battery. Communication with Earth is via S-band. A Ka-band payload, the Lunar Gravity Ranging System (LGRS) will be used to allow high precision range-rate measurements between the two spacecraft. The Ka antenna is mounted in a thermal enclosure and in line with the two-spacecraft center-of-mass. The spacecraft will also each be equipped with up to 5 MoonKam cameras which will be used for education and public outreach.

Mission Profile

Both GRAIL spacecraft (GRAIL-A and GRAIL-B) were launched from Cape Canaveral Air Force Station on a single Delta II 2920-10 on 10 September 2011 at 13:08:52 UT. They were launched into 3.5 month low energy trans-lunar cruises via the Sun-Earth Lagrange point 1 for checkout and outgassing. The spacecraft will fly under the south pole of the Moon and execute separate 60-minute lunar orbit insertion maneuvers roughly 25 hours apart, GRAIL-A on 31 December 2011 and GRAIL-B on 01 January 2012. They will end up in eight-hour elliptical polar orbits, followed by four manuevers to lower them into tandem 50 km, 113 minute near-circular lunar polar orbits separated by 175-225 km. From these orbits GRAIL will enter a 90 day Science Phase to perform gravity mapping of the Moon. This phase will be divided into three 27.3 day cycles. Data downlink will occur in two DSN tracking passes. Total planned mission duration is 270 days, ending in a five day decommisioning phase and planned lunar surface impacts approximately 40 days later. The mission will result in a 30 x 30 km resolution global gravity field with better than 10 mGal accuracy. The estimated cost of the mission is $375 million.