A pair of robotic space probes circling the moon to reveal what is inside will make suicidal plunges to the lunar surface next week, a planned — albeit dramatic — finale to a mission that is giving scientists new insights into how the solar system evolved.
The Gravity Recovery and Interior Laboratory, or GRAIL, mission will come to an end at 5:28 p.m. EST Monday (Dec. 17) when the twin spacecraft crash into a mountain near the moon’s north pole after nearly a year in lunar orbit.
The duo have been formation-flying to map the moon’s gravity, an innovative technique that has revealed a lunar crust that is thinner and far more deeply fractured than scientists expected and an extensive underground system of lava-filled cracks, the first direct evidence that the moon expanded after it was formed.
The information applies not just to the moon, but to the other solid bodies in the inner solar system, including Earth and Mars.
Seeing the extent of the damage from impacting comets and asteroids, for example, makes it easier to visualize how water on the surface of ancient Mars might have made it way inside the planet, where it might still exist today.
“There’s a lot of questions about where did the water that
we think was on the surface of Mars go. Well, if a planetary crust is that
fractured these fractures provide a pathway deep inside the planet. It’s very
easy to envision now how a possible ocean on the surface could have found its
way deep into the crust of the planet,” GRAIL lead scientist Maria Zuber, with
the Massachusetts Institute of Technology, told reporters on a conference call
The GRAIL spacecraft, nicknamed Ebb and Flow, completed
their primary mapping mission in May, flying about 34 miles above the lunar
surface. By precisely and continuously measuring the distance between the two
probes, scientists were able to map the moon’s gravity, revealing its interior
structure. The distance changed slightly as the leading spacecraft and then the
following one sped up or slowed down as they flew over denser or less-dense
regions of the moon in response to the gravitational tugging.
This summer, the pair’s orbit was lowered to about 14 miles
above the surface for more detailed mapping. Now, about out of maneuvering
fuel, the spacecraft are down to about 7 miles while scientists make a last map
of the youngest crater on the moon.
“We have achieved everything that we could have possibly hoped for,” Zuber said. “In my wildest dreams, I could not have imagined that this mission would have gone any better than it has.”
The crash site was selected to avoid the possibility that the probes would crash into any artifacts left behind by the Apollo and other lunar missions.
The spacecraft will hit the surface at about 3,760 mph. No pictures are expected because the region will be in darkness at the time of impact.
“We are not expecting a big crash or a big explosion. These are two small spacecraft. They are apartment-sized washer- and dryer-sized spacecraft with empty fuel tanks. So we’re not expecting a flash that is visible from Earth,” Zuber said.
However, a sister spacecraft circling the moon, NASA’s Lunar Reconnaissance Orbiter, will attempt to make observations, Zuber added.
The GRAIL science instruments are scheduled to be turned off
Images: Top: Final flight over the moon for NASA’s GRAIL mapping probes will come on Monday. Insert: The spacecraft will follow each other into a mountain near the moon’s north pole. Credit: NASA