The phone call came like a bolt out of the blue, so to speak, in January 2011. On the other end of the line was someone from the National Reconnaissance Office, which operates the nation’s fleet of spy satellites. They had some spare, unused “hardware” to get rid of. Was NASA interested?

So when John Grunsfeld, the physicist and former astronaut, walked into his office a year later to start his new job as NASA’s associate administrator for space science, he discovered that his potential armada was a bit bigger than he knew.

Sitting in a room in upstate New York were two telescopes the same size as the famed Hubble Space Telescope, but built to point down at the Earth, instead of up at the heavens.

NASA, struggling to get human space exploration moving again, had spent the previous year trying to figure out how good these telescopes were and what, if anything, they could be used for. Working with a small band of astronomers for the past couple of months, Dr. Grunsfeld, famous as the Hubble telescope’s in-orbit repairman, has now come up with a plan, which was presented to the public on Monday at a meeting at the National Academy of Sciences in Washington.