GRAIL Is Go For Lunar Orbit
NASA Twin Spacecraft on Final Approach for Moon Orbit
“NASA’s twin spacecraft to study the moon from crust to core are nearing their New Year’s Eve and New Year’s Day main-engine burns to place the duo in lunar orbit. Named Gravity Recovery And Interior Laboratory (GRAIL), the spacecraft are scheduled to be placed in orbit beginning at 1:21 p.m. PST (4:21 p.m. EST) for GRAIL-A on Dec. 31, and 2:05 p.m. PST (5:05 p.m. EST) for GRAIL-B the next day.”
Is it just me, or does the acromym seem forced? Also, what was the unusually long trajectory? A gradually expanding earth orbit? When did the vehicle transition to lunar orbit? Just curious.
The spacecraft used a “weak stability boundary” transfer trajectory. This involves going out to the Earth-Moon L1 point. The advantages of this approach are that none of the needed TCMs are “critical” (one can be missed and then made up), very little delta-V is needed for the transfer from the Earth to the Moon, and the 3.5 month cruise gave the ultra-stable oscillators (USOs) on the two spacecraft plenty of time to stabilize after the shaking, etc. from launch. The USOs are essential to the functioning of the mission. Lunar Orbit Insertion is on December 31 for GRAIL-A and January 1 for GRAIL-B. The LOIs are separated to allow the ops team (and the DSN) to turn their full attention on each event.
The GRAIL spacecraft are using a “weak stability boundary” (low energy) trajectory that involves going through the Earth-Moon L1. This approach has several benefits: it’s lower in energy (delta-V) than the transfer orbit Apollo used, the GRAIL team was able to break up all the required TCMs into very little maneuvers so no TCM is “critical” — if one is missed it can easily be made up, and, the 3.5 month cruise has given the ultra-stable oscillators (USOs) on the craft to stabilize after the shock of launch. The USOs are a critical component of the single instrument of the mission — the Wide-Band Radio Ranging System.
GRAIL-A should enter lunar orbit on 31 December and GRAIL-B follows on 1 January. The events are separated in time so that the ops team (and the DSN) can turn their full attention on each event.
About the acronym: many mission acronyms seem forced to me. GRAIL’s seems less forced to me than OSIRIS-Rex or MESSENGER…
Almost right. The weak stability boundary (WSB) strategy for the Moon actually has the spacecraft going out to an Earth-Sun Lagrange point. In this case, L1. That’s about four times the lunar distance. The Earth-Moon Lagrange point is much closer, and it would be pretty hard to take three months to get there!
This strategy is an excellent one because the insertion burn into lunar orbit is much smaller than it would otherwise be. If GRAIL was going to an Earth-Moon Lagrange point, the WSB trajectory would actually be such that an insertion burn wouldn’t be needed at all, and about 1 km/s of delta-V would be saved.
You’re right, it’s the Earth-Sun L1. Sorry.
AnonymousFourEyedCoward gave a good explanation. Also, the NASA press kit has a diagram of the orbit on page 14; they’re on the “Window Open” path.
http://www.nasa.gov/pdf/582…