If all goes to plan, tomorrow at 1:21 p.m. PST, NASA's GRAIL-A spacecraft will enter orbit about the Moon. GRAIL-A is one of two unmanned orbiters launched in September, with its sibling GRAIL-B due to enter orbit the following day (New Year's Day) at 2:05 p.m. PST. Together the two craft constitute the GRAIL mission (Gravity Recovery And Interior Laboratory), charged with mapping the Moon's gravitation field.

At the time of writing, GRAILs A and B are on final approach, knocking off the last of their 250,000 miles (400,000 km). Two days ago, NASA put their respective speeds at 745 mph (1,200 km/h) and 763 mph (1,228 km/h). Both orbiters will approach from the south, on a course directly over the Moon's south pole.

The tricky process of orbital insertion will see both orbiters shed approximately 430 mph (691 km/h) of excess velocity over a period of 40 minutes, settling the spacecraft into a near pole-to-pole orbit within a period of 11.5 hours. Further burns between then and March will reduce the orbital period to just two hours, at an altitude of 34 miles (55 km). From there the orbiters will finally be able to do some science.

When monitoring begins, the speed of the orbiters will be affected by the gravitational eddies caused by lunar features such as mountains, craters and sub-surface masses. As the first orbiter passes over such a feature, its relative distance from the second will alter. Onboard instruments will monitor these changes, building a database translatable into a map of the Moon's gravitational field.

But for now at least, the eyes of all at NASA's GRAIL team are on orbital insertion. "Our team may not get to partake in a traditional New Year's celebration," said GRAIL project manager David Lehman, "but I expect seeing our two spacecraft safely in lunar orbit should give us all the excitement and feeling of euphoria anyone in this line of work would ever need."

View gallery - 4 images