Europe's newest Earth-watching satellite had to dodge an uncontrolled NASA spacecraft just after reaching orbit last week, putting engineers on edge until they could safely maneuver the nearly $400 million Sentinel 1A satellite out of the way.

An on-board camera sent back this image of one of Sentinel 1A's solar arrays and part of the satellite's C-band radar antenna after deployment during the craft's first stage in orbit. Photo credit: ESA

European Space Agency officials disclosed the close call and the nail-biting response by ground controllers in a blog post this week.

The news of a potential collision came a day after Sentinel 1A's successful launch April 3 aboard a Soyuz rocket from French Guiana.

The nearly 2.4-ton satellite had just completed the complicated deployment of its solar panels and radar antenna, a critical phase in Sentinel 1A's mission to supply European governments and scientists with timely data on oil spills, natural and man-made disasters, ice sheets and icebergs, and Earth's climate.

Sentinel 1A is the first satellite launched in the Copernicus program, a nearly $10 billion Earth observation initiative managed by ESA and the European Commission, the executive body of the European Union.

"As the first day shift nears its end, a serious alert is received: there is a danger of a collision with a NASA satellite called ACRIMSAT, which has run out of fuel and can no longer be maneuvered," officials wrote on an ESA blog.

The U.S. Air Force tracks all objects in orbit bigger than a baseball and issues warnings to commercial and international satellite operators when space debris is predicted to fly close to operating spacecraft.

There are now about 23,000 objects tracked by the Air Force, according to Gen. William Shelton, head of Air Force Space Command.

With so many debris fragments, decommissioned satellites and spent rocket components flying around Earth, operational spacecraft are routinely tasked with maneuvering out of the path of space junk.

In Sentinel 1A's case, however, the warning came before engineers could fully activate the satellite's systems, including its rocket thrusters, which were needed to adjust the craft's orbit away from ACRIMSAT.

Artist's concept of Sentinel 1A in space. Photo credit: ESA/ATG medialab

Late on the night of April 4, officials at Sentinel 1A's control center in Darmstadt, Germany, received news there are two threatening close calls with ACRIMSAT on the following morning, including a predicted miss distance of just 20 meters, or about 66 feet.

"This is serious," says the ESA blog. "No Hollywood fiction, this is 'Gravity' for real!"

When controllers were alerted of the projected brush with ACRIMSAT, Sentinel 1A was not yet in "normal pointing mode," a milestone in the satellite's commissioning phase that would allow the craft to maneuver in orbit.

After hastily readying Sentinel 1A for the avoidance maneuver, controllers uplink commands for a 39-second rocket burn to safely move the satellite out of ACRIMSAT's path.

The maneuver occurred as planned at 0514 GMT (1:14 a.m. EDT) on April 4, when Sentinel 1A was out of communications with engineers on Earth.

When the satellite re-established contact a few minutes later, the telemetry data stream showed Sentinel 1A had changed its orbit.

"For the first time that night, loud laughs and cheers bursts through the room," officials described in the blog post.

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