A cosmic collision: Two satellites due to smash into surface of the moon… but don't worry, it was all planned by Nasa

NASA has deliberately guided the twin satellites into a cliff close to the moon’s northern pole

Two Nasa satellites will slam into the surface of the moon later today as they finish their 15-month lunar gravity measuring mission.

Nasa has deliberately guided the twin satellites, nicknamed Ebb and Flow, into a cliff close to the moon’s northern pole. The crash site has been chosen to avoid areas of historical significance, such previous moon landing locations.

Despite an estimated impact speed of 3,760mph, the impact is unlikely to be visible from earth, as the satellites are only about the size of a home washing machine and are carrying little or no fuel.

The crash marks the conclusion of the highly successful $496 million Grail mission, which launched in September 2011 to map the moon’s gravity field in unprecedented detail.

Maria Zuber, Grail’s principal investigator, told reporters: “Grail has produced the highest-resolution, highest-quality gravity field for any planet in the solar system, including Earth.”

According to Space.com, the resulting map revealed a pulverised lunar crust which suggests that the moon, Earth, Mars, Mercury and Venus have been pounded by impacts far more violently than previously thought.

With the two satellites running out of fuel, a collision with the moon’s surface was inevitable, so scientists opted for a controlled impact into an insignificant area.

On Friday morning the satellites’ control panels were switched off and scientists ordered the low-flying spacecraft to crash into the rim of a crater.

The impact is set to take place at 22.58 GMT, with the craft expected to create small holes in the moon surface but leave little other trace.

“They're going to be completely blown apart at the time,” said Grail mission manager David Lehman of Nasa's Jet Propulsion Laboratory in Pasadena, California.

Not content with the satellites’ successful gravitational mapping, scientists are hoping the impact itself will be something of a research mission.

There is hope that as the satellites slam into the moon’s surface, ice or other volatile material will be ejected into the atmosphere where it can be detected.

But the Grail team doesn't necessarily expect to see ice. The crater rim is in sunlight much of the time, and the craft aren't large enough to kick up much material.

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