GRAIL twins successfully arrive in Lunar Orbit

NASA’s two Gravity Recovery And Interior Laboratory (GRAIL) spacecraft have successfully completed their planned main engine burn and have arrived in lunar orbit. GRAIL-A and GRAIL-B will study the moon to yield a better understanding of the internal structure and thermal evolution. This will allow scientists to formulate a model of the Moon’s formation which can also be applied to terrestrial planets.

GRAIL Arrival:

GRAIL-B achieved lunar orbit at 2:43 p.m. PST (5:43 p.m. EST) on Sunday, which completed the insertion of the spacecraft, after GRAIL-A successfully completed its burn at 2 p.m. PST (5 p.m. EST) the previous day.

The insertion maneuvers placed the spacecraft into a near-polar, elliptical orbit with an orbital period of approximately 11.5 hours.

Over the coming weeks, the GRAIL team will execute a series of burns with each spacecraft to reduce their orbital period to just under two hours. At the start of the science phase in March 2012, the two GRAILs will be in a near-polar, near-circular orbit with an altitude of about 34 miles (55 kilometers).

Their successful arrival followed a smooth launch via the 150th flight of the Delta II launch vehicle, which lifted off from SLC-17B at 09:08 Eastern on September 10, 2011.

Shortly after launch, the GRAIL spacecraft deployed their solar arrays as they passed into sunlight for the first time since separating from their carrier rocket. The transit for the GRAIL to the Moon involved a low-energy trajectory, via the Sun-Earth Lagrange 1 point (L1).

The spacecraft then had the key event of firing their main engines to enter a selenocentric, or lunar, orbit – as was carried out without issue. The spacecraft will subsequently manoeuvre into lower orbits, before they are moved into formation to begin collecting scientific data.

At the start of the scientific phase of the mission, the spacecraft will be in circular orbits at an altitude of 55 kilometres. Scientific operations are expected to commence on 8 March next year, and last for 82 days. Decommissioning of the spacecraft will begin on 29 May, and the spacecraft are expected to impact the lunar surface in June.

“NASA greets the new year with a new mission of exploration,” said NASA Administrator Charles Bolden. “The twin GRAIL spacecraft will vastly expand our knowledge of our moon and the evolution of our own planet. We begin this year reminding people around the world that NASA does big, bold things in order to reach for new heights and reveal the unknown.”

The GRAIL mission being flown as part of NASA’s Discovery program, which was started in 1992. Discovery is a medium-class programme intended to study the Solar system. many of NASA’s recent planetary missions have been conducted as part of it.

The principal scientific objectives of the GRAIL mission are to produce a map of the Moon’s lithosphere, to allow scientists to understand the Moon’s thermal evolution, and the evolution of breccia within the Moon’s crust, and to determine more details of the interior, particularly the size of the core, and the structure beneath impact basins.

The two spacecraft are identical, apart from the positioning of star trackers and instruments to allow the spacecraft to fly with their antennae pointing towards each other. They were built by Lockheed Martin, based around a bus developed for the USA-165, or XSS-11, satellite; a technology demonstration spacecraft operated by NASA and the United States Air Force, which was launched in 2005. Each GRAIL spacecraft has a mass of 307 kilograms, including 106 kilograms of hydrazine fuel.

The spacecraft are each equipped with two 1.9 square metre, 520-cell, solar arrays, which will generate at least 700 watts of power. The solar arrays will charge a 30 amp-hour lithium ion battery in each spacecraft, which will be used to store power for when the spacecraft are not in sunlight. Propulsion of each spacecraft will be provided by an MR-106L monopropellant engine, capable of generating 22 newtons of thrust.

The spacecraft are three-axis stabilised, with reaction wheels and eight warm gas thrusters, each capable of producing 0.9 newtons of thrust, being used aboard each spacecraft for attitude control. Sun and star trackers and inertial measurement units will allow the spacecraft to determine their orientation. The spacecraft carry avionics systems which are derived from those developed for the Mars Reconnaissance Orbiter, which was launched in 2005.

Each spacecraft carries two transponders operating in the IEEE S band (NATO E band), which will be used to relay data to the ground and to upload commands to the spacecraft. A further S band transponder, the Time-Transfer Assembly, will be used to transmit signals between the spacecraft to synchronise their onboard chronometers.

Two IEEE X band (NATO I or J band) transponders, the Radio Science Beacon, will be used to transmit signals to Earth for Doppler ranging. Finally an IEEE Ka band (NATO K band) transponder, the Microwave Assembly, will be used to find the distance between the two spacecraft, and track their relative motion.

The Ka band transponder forms part of the Lunar Gravity Ranging System or LGRS, which is GRAIL’s primary instrument. LGRS consists of four elements; the Ultra-Stable Oscillator, or USO, will be used to generate an oscillating signal to synchronise the instruments. This signal will then be transmitted through both the Microwave Assembly (MWA) and Time-Transfer Assembly (TTA) antennae.

TTA broadcasts the signal as a ranging code, similar to those transmitted by Global Positioning Satellites. Finally, the data is collected by the Gravity Recovery Processor Assembly, or GPA, which processes it for transmission back to Earth.

LGRS is derived from the K-Band Ranging (KBR) instrument aboard the Gravity Recovery And Climate Experiment, or GRACE, spacecraft, which were launched in March 2002. GRACE, like GRAIL, consists of two spacecraft using radio signals to map the gravitational field, however it is studying Earth’s gravitational field instead of the Moon’s.

The two spacecraft also carry the Moon Knowledge Acquired by Middle school students, or MoonKAM, student outreach payload. This will be used to image areas of the Moon at the request of schoolchildren.

A similar programme for Earth imagery, EarthKAM, has been operated aboard the International Space Station since 2001 and also flown on Space Shuttle missions STS-89 and STS-99. A prototype, KidSat, was also flown on STS-76, STS-81 and STS-86.

“I can’t think of a better way to ring in the New Year than to place two spacecraft into orbit around the moon. Our team is celebrating today,” said Stu Spath, GRAIL program manager at Lockheed Martin Space Systems Company. “We used the moon’s gravity to capture the two GRAIL spacecraft into orbit, and now the science team is going to analyze that same gravitational field to an extraordinary level.”

(Images via NASA and ULA).

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