Nasa scientists have been outlining their preliminary results after crashing two unmanned spacecraft into the Moon in a bid to detect water-ice.

美国航空航天局的科学家大致描述了他们将两艘五人宇宙飞船坠落到月球以探测水冰的初步结果。

Both spacecraft hit the Moon after midday on Friday

A rocket stage slammed（砰然，猛然） into the Moon's south pole at 1231 BST (0731 EDT).

Another craft followed just behind, looking for signs of water in debris¹（碎片，残骇） kicked up by the first collision.

Instruments on the second spacecraft identified a flash from the initial impact as well as a crater²（弹坑，火山口）, but the expected debris cloud was not evident.

The $79m (£49m; 53m euro) US space agency mission is known as LCROSS (the Lunar Crater Observation and Sensing Satellite).

The first collision was expected to throw some 350 tonnes of debris up to altitudes of 10km (6.2 miles) or more.

No such dust plume³ was apparent in images sent back by the spacecraft however, proving a disappointment to some members of the public expecting a space spectacular（惊人的，壮观的）.

"We need to go back and carefully look at the data to see what it says," Dr Anthony Colaprete, principal investigator⁴ on the LCROSS mission told journalists at a post-impact news conference.

"Exploration has surprises. I'm glad we built our mission plan around all aspects of the impact… what's streamed on the video is not at the same fidelity⁵ as what we get fresh off the spacecraft. We need to look more closely before we conclude anything about an ejecta（喷出物，废物） cloud, or not."

Wait for water

Dr Colaprete, from Nasa's Ames Research Center in California, added: "I see something in the spectrometer（分光计） data - the spectrometers are more sensitive than the cameras. But I can't say anything more than that."

The team was also able to determine the temperature of the crater punched in the lunar surface.

Thus, questions about the persistence⁶ of water-ice on the Moon will have to wait.

But Dr Colaprete said: "We saw the impact, we saw the crater. We got good spectroscopic（分光镜的） measurements which is what we needed of the impact event."

"We have the data we need to address the questions we set out to address."

The identification of water-ice in the impact plume would be a major discovery, not least because a supply of water on the Moon would be a vital resource for future human exploration.

LCROSS was to have helped pave the way for US astronauts to return to the Moon by 2020.

If ice is present in shaded craters⁷, it could provide a valuable water source for future manned lunar bases.

But Nasa's lunar plans have been under scrutiny⁸（研究） since President Barack Obama ordered a sweeping⁹ review of Nasa's manned spaceflight programme.