NASA says India has threatened the International Space Station and the astronauts on board by shooting down a satellite with a missile in March.

NASA Administrator Jim Bridenstine said the satellite shattered into small pieces of space junk that posed an "unacceptable" threat to astronauts and called it a "terrible, terrible thing."

India said that it deliberately chose to destroy a satellite in low orbit with the goal of keeping the debris from harming the station or other satellites and that the debris would fall back to Earth and disintegrate, but previous cases suggest that could take a long time.

Bridenstine said pieces were moving above the station and "that kind of activity is not compatible with the future of human spaceflight."

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NASA called India's destruction of a satellite in March a "terrible, terrible thing" and said the space debris created by the explosion should be considered a threat to the International Space Station and the astronauts on board.

India intentionally destroyed one of its satellites with a missile on March 27, a move Prime Minister Narendra Modi welcomed as one that established India "as a space power."

But NASA Administrator Jim Bridenstine told employees on April 1 that it posed an "unacceptable" threat to astronauts on board the ISS.

Read more: India's anti-satellite missile test just moved humanity closer to a space-junk nightmare scenario

He said the satellite shattered into pieces, many of them large enough to pose a danger to the space station but not large enough to track. It is unclear how many pieces of debris were created.

The International Space Station in orbit. NASA

"What we are tracking right now, objects big enough to track — we're talking about 10 cm (4 inches) or bigger —about 60 pieces have been tracked," he said.

He said 24 of those pieces were traveling above the ISS just after the explosion, even though the satellite had been orbiting 185 miles above the Earth, lower than the station, which orbits roughly 250 miles above the Earth.

While the station has remained safe since the satellite was destroyed, Bridenstine was critical of the India for what he said was a threat that would last some time, and for potentially inspiring other countries to take similar actions.

"When one country does it, then other countries feel like they have to do it, as well," he said.

"That is a terrible, terrible thing to create an event that sends debris at an apogee that goes above the International Space Station," Bridenstine added.

"That kind of activity is not compatible with the future of human spaceflight."

Read more: India's anti-satellite missile test may have created 6,500 pieces of space junk larger than a pencil eraser, according to a new simulation

He said the risk of the ISS colliding with debris had increased by 44% in the 10 days after the Indian missile launch.

Indian Prime Minister Narendra Modi. Reuters

"It's unacceptable and NASA needs to be very clear about what its impact to us is," he said.

Six crew members are living aboard the ISS.

A software-engineering company called Analytical Graphics made a simulation of the debris created by the anti-satellite test, Business Insider's Dave Mosher reported.

"We modeled 6,500 fragments, basically those that were larger than half a centimeter," Tom Johnson, the vice president of engineering for Analytical Graphics, said.

India downplayed the risk of debris after its missile launch, with its top scientists saying last week that the country expected the debris to burn out in Earth's atmosphere in less than 45 days.

Bridenstine acknowledged that "over time this will all dissipate." But this could take longer than what India suggested, The New York Times noted — space debris created when China destroyed a satellite in 2007 is still in orbit today. Read more: India says space debris from its anti-satellite test will 'vanish in no time'

G. Satheesh Reddy, the chief of India's Defence Research and Development Organisation, said a low-altitude military satellite was targeted with the goal of reducing the risk of debris.

"That's why we did it at lower altitude — it will vanish in no time," he told Reuters. "The debris is moving right now. How much debris, we are trying to work out, but our calculations are it should be dying down within 45 days."

Acting US Defense Secretary Patrick Shanahan warned a day after India's test that the event could create a "mess" in space.

Dave Mosher contributed reporting to this story.