NASA simulated the terrifying scenario of attempting to stop an asteroid from hitting Earth but in the process breaking off a fragment that hurtles toward New York and forces an evacuation of the entire city.

In an exercise at April's Planetary Defense Conference, participants from NASA, the Federal Emergency Management Agency, and other federal agencies were given eight years to try and stop an asteroid from hitting Earth. But when NASA and space agencies around the world came together to deflect the asteroid, they caused a chunk to split off.

Image: NASA

During the simulation, the 500- to 850-foot-wide (140- to 260-meter-wide) asteroid was on course to hit Earth near Denver when the space agencies deflected it using "kinetic impactors" — a type of fast-moving spacecraft designed specifically to knock asteroids off course.

However, that deflection resulted in a 165- to 260-foot (50- to 80-meter) fragment breaking off the asteroid. This then ended up "on a certain collision course with Earth," and scientists were forced to watch from the ground to try and figure out where it would hit the planet.

They learned 10 days before it hit that it was headed for New York City and would come into the atmosphere at 43,000 miles per hour (19 kilometers per second).

This would produce "a large fireball or 'megabolide'" and release between 5 and 20 megatons of energy on impact. For comparison, the nuclear bombs dropped on Hiroshima and Nagasaki combined created 0.04 megatons of energy.

Those involved in the simulation determined that the only way to keep people safe was to evacuate the entire city of New York, USA Today reported.

NASA says the risk of such a disaster is low, but they need to be prepared

NASA said that the simulation "is designed to help key decision makers practice for a real asteroid impact" but reassured people that: "Currently, there is no known asteroid with a significant probability of impacting Earth in the next century."

Paul Chodas, the director of NASA's Center for Near-Earth Object Studies and the author of the simulation, said: "We need to challenge ourselves and ask the tough questions."

He said that a scenario in which an asteroid hit a major city was unlikely and that if an asteroid hit Earth, it would likely hit the ocean.

Chodas said that at NASA: "Each day we ask, what if?"

"You don't learn anything if you don't study the worst possible case each day."

FEMA also said that such simulations are important because they help the agency be prepared for major disasters.

Leviticus Lewis from FEMA's response-operations division said: "Bringing together the disaster management community and the scientific community is critical to preparing for a potential asteroid impact in the future."

NASA Administrator Jim Bridenstine at the Planetary Defense Conference in Washington, DC, on April 29, 2019. Image: Solar System Exploration Research Virtual Institute/YouTube

NASA Administrator Jim Bridenstine at the Planetary Defense Conference in Washington, DC, on April 29, 2019.Solar System Exploration Research Virtual Institute/YouTube

NASA is taking the threat of an asteroid impact seriously. NASA Administrator Jim Bridenstine told the conference that people need to take the idea that an asteroid could crash into Earth more seriously.

He said that people often dismiss it because of the "giggle factor," or scientific theories that seem too ridiculous to be likely.

But he said that the risk was real, even if people tend to think of Hollywood films when they consider the possibility.

"We have to make sure that people understand that this is not about Hollywood. It's not about movies. This is about ultimately protecting the only planet we know right now to host life, and that is the planet Earth," he said.

NASA and other space agencies around the world scan the skies for near-Earth objects (NEOs), such as asteroids and comets.