GRAIL mission phases:
Put together by the GRAIL mission design engineer, this chart shows the path of the two GRAIL spacecraft as they leave the Earth after launch and head out towards the Sun-Earth Lagrange Point 1 (or “Earth Libration Point 1”) during the trans-lunar cruise (TLC) phase. Along the way the spacecraft will make trajectory correction maneuvers (TCMs) that will cause the spacecraft to swing back towards the Earth-Moon system. This path was chosen because it requires much less energy compared with the direct-to-the-Moon path, so the GRAIL spacecraft don’t need to carry much fuel for their journey (this has many advantages, perhaps the biggest being they are much cheaper to launch into space when lighter).
On December 31st, 2011, GRAIL-A will get captured by the Moon’s gravitational field and will begin to orbit the Moon—this is the lunar orbit insertion, or LOI, phase. On January 1st, 2012, GRAIL-B will experience LOI. Both spacecraft will approach the Moon from its south pole and will enter an elliptical polar orbit.
Once the two spacecraft are in orbit, they will enter the orbit period reduction (OPR) phase. During this five-week phase the spacecraft will repeatedly fire their main engines to make their orbits shorter and more circular. At the start of the OPR phase it will take the spacecraft 11.5 hours to orbit the Moon; at the end of the OPR, they will orbit in just under two hours and they will be less than 50 miles above the surface of the Moon.
Next up is the transition to science formation phase (TSF), which will take about a month. Up until this point the two spacecraft will have been operated completely independently, but during the TSF phase the orbits of the two spacecraft will be adjusted so they begin to fly in formation. This will be the first time two spacecraft will fly in formation around a body other than the Earth. An important (but difficult to meet) goal of this phase: Don’t let GRAIL-A and GRAIL-B ram into each other.
The next phase is the one that, hopefully, will be mentioned in textbooks: The science phase. This phase is 82-days long and will allow GRAIL to map the Moon’s gravity signature three separate times. The two spacecraft start out about 60 miles apart from each other, but that distance will be adjusted depending on the goal of the measurement. Data taken at the closer distances will tell us about the density of the surface-level features of the moon, and then data taken at the farther distances (a max of about 140 miles) will help detect (and characterize) the Moon’s core. GRAIL’s altitude will also be changing as the spacecraft get closer and closer to the surface. By the end of the science phase the two spacecraft will be about 10 miles above the surface of the Moon.
The decommissioning phase will be about a week long. The last bits of science will be collected, and shortly after the end of this phase will be the ultimate end of the mission, as GRAIL-A and GRAIL-B will impact the lunar surface. The possibility of an extended mission is not realistic because a partial lunar eclipse will starve the spacecraft of power (they will no longer be able to recharge their batteries via the solar arrays). The batteries will drain and the spacecraft will power down as their orbits continue to deteriorate, eventually resulting in two more lunar craters.
I intend to write up a few notes about how GRAIL actually works, since the engineering behind it is very interesting, but we’ll see if I can work it into my schedule in the next couple weeks.