NASA's twin Grail spacecraft (short for Gravity Recovery And
Interior Laboratory) are slated to start circling the moon one
day apart, with Grail-A arriving on Saturday and Grail-B
following on Sunday --both to study our lunar satellite from
crust to core. The two probes will orbit the moon in tandem,
mapping the lunar gravity field in unprecedented detail and
helping scientists better understand how the moon formed and
evolved. Grail-A and Grail-B will spend two months circling
lower and lower, eventually settling into orbits just 34 miles
(55 kilometers) above the lunar surface, and begin taking
measurements in March. Regional differences in the moon's
gravitational field will cause the two spacecraft to constantly
adjust their speed --slowing down slightly, changing the
distance between them, using microwave signals that they bounce
back and forth to be able to determine how far apart they are
from each other to within a few microns — less than the width of
a human red blood cell, researchers have said. Th