NASA's GRAIL A and GRAIL B spacecraft successfully completed a crucial phase of their mission on December 31st and January 1st. Launched in September, the craft traveled on an energy saving Trans Lunar Trajectory which took them to the Sun-Earth LaGrange point L1. Unlike the Apollo missions to the Moon which took nearly 2 to 3 days, this path is less direct. It allows time for corrections to the path, checks of on-board electronics and systems, as well as time for the spacecraft to release any small quantities of trapped gases. It also allows the craft to approach the Moon with a slower relative velocity and hence need less fuel to adjust their orbits at lunar orbit insertion.
The Gravity Recovery And Interior Laboratory (GRAIL) mission will create the most accurate gravitational map of the Moon to date, improving our knowledge of near-side gravity by 100 times and of far-side gravity by 1000 times. The high-resolution gravitational field, especially when combined with a comparable-resolution topographical field, will enable scientists to deduce the Moon's interior structure and composition, and to gain insights into its thermal evolution--that is, the history of the Moon's heating and cooling, which opens the door to understanding its origin and development.
GRAIL is similar to the GRACE mission launched in 2002 which lasted for 5 yrs and mapped the gravity field of Earth. GRACE involved two identical spacecraft flying about 130 miles apart in a polar orbit 300 miles above the Earth.
More details about the upcoming phases of the GRAIL mission are below the squiggle.
The following 2 minute video briefly shows the initial phase right after launch with the two spacecraft on the rocket and their sequential releases. It also presents some of the portions of the orbit insertion phase from December 31
st and January 1
st. Some additional details are included below the video.
The timing of the path taken by the two spacecraft was crucial. They left Earth on the same launch vehicle in September. GRAIL A was released followed by B about 8 minutes later. They each deployed solar panels and oriented themselves. Contact with stations on the ground occurred about 1 minute after deployment. This NASA graphic illustrates the sequence of events.
For the next 3 months, the 2 craft coasted away from Earth and toward the Sun, slowing down gradually. They slowly turned and started to head back toward the Earth-Moon system. Course adjustments directed each toward rendezvous with the Moon. Arriving over the lunar South Pole December 31st, GRAIL A fired its engine for 38 minutes to slow its velocity by 427 miles per hour. GRAIL B made an identical maneuver 25 hrs later.
The spacecraft are now in two slightly different orbits about the Moon with periods of about 11 hours. These periods will be decreased over the next 4 weeks to about 2 hours. In doing so, the altitudes of the orbits will end up only about 35 miles. During February, the two spacecraft will be carefully placed into a tandem arrangement with B leading A in a polar orbit of the Moon. They will be approximately 120 miles apart. The actual Science Phase of the mission begins March 8 and lasts until the end of May 2012. At that time, the angle of the Sun on the solar panels might not provide enough energy to power the electronics.
Mapping Method and Special Challenges |
The spacecraft begin the mapping phase separated between 60 and 120 miles. This larger distance allows mapping of deeper and larger concentrations of mass within the Moon. It is hoped GRAIL will be able to add information to answer whether there is a 'liquid core' and if so, what is its extent. Later in the mapping phase, the craft will be brought closer together to about 40 miles. This will allow the mapping of smaller mass concentrations nearer the surface.
The range of distance between A and B will be measured with a GPS-like technology to accuracy of millionths of a meter. Smoke particles and blood cells are about that size. The 3 minute video below illustrates the dynamic nature of these measurements. As the leading spacecraft approaches a mass concentration, it will pull slightly ahead of the trailing spacecraft. As they pass on by, they will end up back to their initial distance apart. The timing and rate of these distance changes will be the key data allowing scientists to map the locations and densities of massive regions below the Moon's surface.
Since GRAIL is measuring the distance between the spacecraft to such a great precision of microns, there are several very small effects which can interfere with those distance measurements. The four primary effects are illustrated in the 2 minute video below.
- Force of the bright sunlight reflecting off the solar panels
- Force of the escaping infrared radiation from the spacecraft
- Thermal expansion and contraction of the dimensions of the craft
- Out gassing of small amounts of trapped gas in spacecraft parts
Links To More Information |
Science Objectives and Techniques
Spacecraft and Payload Information
Middle school students will be able to request images from GRAIL through a program called MoonKam. The project, headed by
Sally Ride, includes registration and classroom materials.
Student Involvement With MoonKam
Stay tuned for updates in March as the science phase begins.