美国两颗退役卫星成功坠毁于月球

 

The duo were commanded to slam into a 2km-high mountain in the far lunar north.

 

The deliberate ditching avoids the possibility of an uncontrolled descent on to locations of historic importance, such as the Apollo landing sites.

 

Nasa's deep-space radio-tracking system confirmed the loss of signal from the satellites just before 22:30 GMT.

 

Afterwards, it was announced the impact site would be named for Sally Ride, the first female American astronaut who died earlier this year. Ms Ride's educational programme had run the outreach（延伸） cameras on the spacecraft.

 

The satellite twins returned some remarkable² data during their operational mission, which got under way in March. Their maps of the subtle variations in gravity across the Moon's surface are expected to transform many areas of planetary science.

 

"Ebb and Flow have removed a veil from the Moon and removing this veil will enable discoveries about the way the Moon formed and evolved for many years to come," said principal investigator³ Prof Maria Zuber from the Massachusetts Institute of Technology, US.

 

Together known as Grail (Gravity Recovery and Internal Laboratory), the pair hit the flank of the lunar-nearside mountain about 3km and 30 seconds apart.

 

The peak - located at 75 degrees North latitude⁴ close to a crater⁵ named Goldschmidt - was in darkness at the time.

 

Being only the size of washer-driers, and having completely depleted⁶ their fuel tanks, the pair were not expected to produce any sort of impact flash visible to Earth observers.

 

That said, another of Nasa's missions at the Moon, its Lunar Reconnaissance Orbiter (LRO), was looking out for the crashes.

 

If it was lucky, LRO's ultraviolet imager might have seen some volatile⁷ materials being driven off the surface by the heat from the impacts. The orbiter will also image the site in a couple of weeks to see if it can discern any new craters⁸.

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