Enlarge / SpaceX launches a satellite for the National Reconnaissance Office on May 1.

About six weeks ago, SpaceX launched a spy satellite into low Earth orbit from Launch Complex 39A at NASAs Kennedy Space Center. As is normal forNational Reconnaissance Office launches, not much information was divulged about the satellite's final orbit or its specific purpose in space. However, a dedicated group of ground-based observers continued to track the satellite after it reached outer space.

Then something curious happened. In early June,the satellite made an extremely close pass to the International Space Station. One of the amateur satellite watchers, Ted Molczan,estimated the pass on June 3 to be 4.4km directly above the station. Another, Marco Langbroek, pegged the distance at 6.4km. "I am inclined to believe that the close conjunctions between USA 276 and ISS are intentional, but this remains unproven and far from certain," Molczan later wrote.

In recent days, Ars has run these observations by several officials and informed sources. They are credible, these officials say, and curious indeed. "This is strange," said one astronaut who hascommanded the International Space Station. "I don't really believe in coincidences. But I can't really think of anything that would be worth highlighting a close approach."

One expert in satellite launches and tracking, Jonathan McDowell, said of the satellite's close approach to the station, "It is not normal." While it remains possiblethat the near-miss was a coincidence due to the satellite being launched into similar orbit, that would represent "gross incompetence" on the part of the National Reconnaissance Office, he said. Like the astronaut, McDowell downplayed the likelihood of a coincidence.

Another option is that of a deliberate close flyby, perhaps to test or calibrate an onboard sensor to observe something or some kind of activity on the International Space Station. "The deliberate explanation seems more likely, except that I would have expected the satellite to maneuver after the encounter," McDowell said. "But it seems to have stayed in the same orbit."

Another question, if the maneuver was deliberate, is whether the US government informed Russia or other international partners on the space station. The Russian segment of the station controls the thrusters that generally are used to maneuver the station away from orbital debris, so such coordination might seem prudent.

In regard to these questions, so far the US government has declined to provide answers. A NASA spokesman offered to look into the matter on Monday but as of Wednesday afternoon had nothing to say. A query sent to public affairs at theNational Reconnaissance Office went unanswered. We will update this story if we receive any official responses.

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A spy satellite buzzed the space station this month, and no one knows why - Ars Technica