Leroy Chiao is a former NASA astronaut. During his 15-year career, he flew four missions into space, three times on space shuttles and once as the co-pilot of a Russian Soyuz spacecraft to the International Space Station. On that flight, he served as the commander of Expedition 10, a 6½ month mission. He has performed six spacewalks, in both US and Russian spacesuits, and has logged nearly 230 days in space. The opinions expressed in this commentary are solely those of the author.

(CNN) Donald Trump's administration is floating a proposal to return to the moon -- and to shut down the International Space Station to help pay for it.

Leroy Chiao

The first part of this idea is good. The second is horrible. If enacted, it could well spell the end of NASA's human spaceflight for the foreseeable future.

There is a wild card here, too: I refer to the commercial spaceflight efforts of companies like Elon Musk's SpaceX and Jeff Bezos' Blue Origin. For the first time, visionary leaders of commercial companies are striving to build space infrastructure and exploration programs funded by commercial activities. Yes, there is the possibility of NASA partnering with them, but that is not the pressing question in my view now; the continuation of ISS is.

Consider, to begin, that the International Space Station, or ISS, is an essential piece of the National Aeronautics and Space Administration's program for space exploration. The biggest technical challenge to sending astronauts on farther and longer missions is biomedical: How do we keep them healthy? We need the ISS to help us find out.

We need it as an engineering test bed, so we can learn how to build robust life-support and other systems for voyages to destinations like Mars.

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