The seven astronauts and cosmonauts on board the International Space Station sheltered inside their respective spacecraft, a Crew Dragon and Soyuz, on Monday morning as the orbiting laboratory passed through an unexpected debris field.

This was not a pre-planned collision avoidance maneuver in low Earth orbit, in which the station would use on board propulsion to move away. Rather, the situation required the astronauts to quickly take shelter.

Had there been a collision during the conjunction, the two spacecraft would have been able to detach from the space station and make an emergency return to Earth. Ultimately that was not necessary, and the astronauts reemerged into the space station later Monday. However, as the crew on board the station prepared for their sleep schedule, Mission Control in Houston asked them to keep as many of the hatches on board the space station as possible closed for the time being, in case of an unexpected collision during subsequent orbits.

"We’re looking forward to a calmer day tomorrow," NASA astronaut Mark Vande Hei said from on board the space station, in response to the final call from Mission Control on Monday.

It appears likely that the debris field that had alarmed flight controllers on Monday was caused by an anti-satellite test performed by Russia's military early on Monday.

The US Department of Defense on Monday issued the following statement on the test: "US Space Command is aware of a debris-generating event in outer space. We are actively working to characterize the debris field and will continue to ensure all space-faring nations have the information necessary to maneuver satellites if impacted."

Aside from this statement, as of early Monday afternoon in the United States, there have been no official confirmations of details about the test. However, satellite and orbital dynamics experts have pieced together the most likely scenario of what happened based on publicly available data and observations. Brian Weeden, of the Secure World Foundation, summarized these conclusions here.

It appears that Russia launched a surface-to-space Nudol missile on Monday, between 02:00 and 05:00 UTC, from the Plesetsk Cosmodrome in the northern part of the country. The missile then struck an older satellite, Cosmos 1408. Launched in 1982, the satellite had been slowly losing altitude and was a little more than 450 km above the Earth.

This is a large satellite, with a mass of about 2,000 kg. As of Monday afternoon, US Space Command said it was already tracking more than 1,000 pieces of new debris. Although the satellite's altitude is higher than the International Space Station, which is about 400 km above the surface, a kinetic impact would spread a large cloud of debris. Satellite expert Jonathan McDowell believes the Cosmos 1408 satellite is the likely candidate for the space station's ongoing debris event.

What seems clear is that this story is only beginning. NASA has not had any kind of a formal response, but, doubtless, senior officials are livid about another debris cloud in low Earth orbit that will threaten the valuable International Space Station and other assets. Russia's involvement—they are a major space station partner, after all—must be doubly perplexing.

It's also difficult to fathom why Russia would knowingly destroy a satellite that might threaten the station, where two of its cosmonauts are presently living and significant money has been invested. So far, Russia's space corporation, Roscosmos, has issued only a bland statement that reads like something the former Iraqi information minister might have said: "The orbit of the object, which forced the crew today to move into spacecraft according to standard procedures, has moved away from the ISS orbit. The station is in the green zone."

2:40 pm ET Update: During a daily briefing today, US State Department Spokesman Ned Price said the test had created more than 1,500 pieces of trackable debris and hundreds of thousands of pieces of un-trackable debris.

"The Russian Federation recklessly conducted a destructive satellite test of a direct-ascent anti-satellite missile against one of its own satellites," Price said. "This test will significantly increase the risk to astronauts and cosmonauts on the International Space Station as well as to other human spaceflight activities. Russia's dangerous and irresponsible behavior jeopardizes the long-term sustainability of outer space."