NASA's twin satellites were successfully launched in early September to map the Moon's gravitational field and internal structure. The Delta-II rocket carrying the space probes lifted from Cape Canaveral on the 10th of September, after a two-day delay. This was the last start of the legendary launch vehicle from the East Coast and most likely the penultimate overall. The Delta-II lifted a lot of GPS and Iiridum birds and numerous scientific satellites, space probes and telescopes, including all American Mars probes since the nineties. Only a single one of its 146 launches was a complete failure although that one produced very spectacular fireworks.

We've described the scientific objectives of the mission before, so now let's have a look at the technical details. GRAIL was born from the mix of two previous missions, actually: the architecture of the probes is based on the XSS-11 (eXperimental Satellite System) satellite, developed by the US Air Force while the the techniques to measure the Moon's gravitational field were adopted from the GRACE mission.

The task of XSS-11 was to demonstrate rendezvous and investigation of other satellites: it repeatedly approached its own Minotaur rocket stage as well as defunct American satellites, fully autonomously once the target was selected. It also sparked some controversy as rendezvous-capable satellites can be used not just for in-flight inspection and servicing but for evil purposes just as well. That could mean anything from simply spying to a kamikaze-style knockout of adversary satellites, depending on proximity. XSS-11 will hopefully have a greater career as a platform for smallsats – though, as of today, only GRAIL have been based on it.

The two GRACE satellites orbiting the Earth

GRAIL is going to follow the principle used by the GRACE mission when it mapped the Earth's gravitational „bumpiness” by measuring the distance between the two probes with great accuracy. When flying over a mass concentration, the local, somewhat stronger gravitational pull will cause the leading member to hurry forward, then the follower to lag behind a little bit compared to the other probe. The deviations from the Moon's average gravitational field are of course extremely small, requiring great precision. The heart of the distance-measuring system is an ultra-stable oscillator, basically a reference time-signal. It will be used by the two radio transmitters: the Microwave Assembly unit will send signals in the Ka-band (32 Ghz) to measure the distance directly, while the Time-Transfer Assembly unit will send GPS-like ranging codes between the two probes in the S-band (2 GHz). Additionally, the spacecrafts will transmit to the ground when flying over the Earth-facing hemisphere to verify the oscillator itself. All these measurements are required to determine the distances with micrometer precision. Finally, the combination of lots and lots of distance measurements will allow to determine the local gravitational field strength and will also help to determine the inner structure of the Moon, from crust to core.

The many ways the two spacecrafts and the ground will communicate. They will transmit in two bands towards the Earth too: one will do the actual communications, the other will be a one-way signal, to check the drift of the ultrastable oscillator's frequency.

László Molnár

Image sources:

1.) 3.) 4.) NASA public domain

2.) Ben Cooper, launchphotography.com