Lunar Arrival 
LAUNCH CONTROL: 5, 4, 3, 2, 1, zero, and liftoff of the Delta II with GRAIL, a journey to the center of the Moon.
A pair of twins is preparing to greet the new year in a special way — by entering orbit around the Moon. The first will arrive tomorrow, with the other following on New Year’s Day.
The “twins” are a pair of spacecraft that form a mission known as GRAIL. The two were launched in September, and followed an unusual, looping path to the Moon — one that allowed them to reach the Moon with a fairly small booster rocket.
Their mission is to probe both the Moon’s interior and its surface. The findings will help scientists determine the structure of the Moon’s core, and figure out why the lunar farside is so different from the nearside — the hemisphere that faces Earth.
GRAIL will accomplish that by mapping the Moon’s gravity field. The field is lumpy, with some regions exerting a stronger pull than others. The two craft will fly in formation, measuring the distance between them down to the micron — about the width of a red blood cell. Such tiny changes in the distance will reveal differences in the Moon’s gravity.
The craft will spend a couple of months synchronizing their orbits before beginning their “nominal” mission. During that three-month mission, the craft will drop lower and lower to produce an ever-sharper map of lunar gravity — a map that will help scientists probe the interior and the history of the Moon.
Script by Damond Benningfield, Copyright 2011