CRISISBOOM

indianexpress.com

Why would NASA really crash the probes on the Dark Side of the Moon of all places…

NASA’s latest moon mission ended last Monday—not with a whimper, but a splat.

Two splats, actually.

Ebb and Flow, two space probes the size of washing machines that have been orbiting the moon and measuring its gravity field, performed an orchestrated death plunge, crashing into the body’s dark side.

The exercise was not for the advance of science, but rather something of a garbage-disposal operation, to make sure that the probes—which are running out of fuel—do not come to rest in a historically significant place, like on Neil Armstrong’s footprints.

The moon has been affronted this way many times, especially during the space race of the 1960s, but NASA is now trying to dispose of its litter more carefully.

This time, the first impact came 40 seconds past 5:38 pm ET when Ebb slammed into a mountain near the moon’s north pole at 3,760 mph. The second, from Ebb’s twin, Flow, came 20 seconds later.

Unfortunately, since the action happened on the dark side of the moon, there was nothing for earthlings to see. That is by design as NASA wraps up its Grail Recovery and Interior Laboratory mission, or Grail, for short.

To map the gravity, the two spacecraft are in an orbit passing over the moon’s north and south poles. They pass over all parts of the lunar surface as the moon revolves below.

If the probes’ fuel ran out and their orbits decayed, they could crash anywhere on the moon, and there would be a slim chance—8 in 1 million—that one of them could obliterate those famous footprints or another historic site.

With the spacecraft guided into a mountain, the chances are zero.

Even in their demise, however, Ebb and Flow will be able to aid the cause of science. Another of NASA’s spacecraft, the Lunar Reconnaissance Orbiter, will pass over the crash sites, and scientists hope that they will be able to tell something about the mountain—which is a remnant of a crater rim – from the gouges created by Ebb and Flow.

Launched in September 2011, the two spacecraft slipped into lunar orbit at the beginning of this year. For the primary mission, the two spacecraft orbited at an altitude of 34 miles. Wobbles in the distance between them told of variations in the moon’s gravitational field. Because of Ebb and Flow, scientists now have more precise measurements of the moon’s gravity than of the Earth or any other planet.

After the primary mission was completed, the two spacecraft were nudged to a lower altitude of 14 miles and then, on December 6, even lower to 6.6 miles. Data from the Grail mission already has shown that the moon’s crust is thinner than had been thought and that it was pulverised by impacts during the early history of the solar system. The moon’s density, deduced from the gravity, places constraints on what it could be made of. For example, Zuber said, the moon appears to contain the same abundance of aluminum as the Earth, supporting the theory that the moon was formed out of a cataclysmic collision of a Mars-size body and the Earth.

The data so far does not support a hypothesis that the Earth once had two moons that later collided and combined into one.

“This actually decreases the costs of future exploration of the moon,” Zuber said.