If an asteroid heads for Earth, can we stop it?

Still, rare is not the same as impossible. As NASA’s Planetary Defense Coordination Office has said, “The good news is that we know of no asteroid or comet currently on a collision course with Earth”. The key word is currently. Planetary defense is about ensuring it stays that way.

Step One: Find the asteroid threat early

Saving Earth from an asteroid begins not with rockets, but with telescopes. The earlier an asteroid is discovered, the easier it is to deflect. If scientists detect a potentially hazardous object decades in advance, even a tiny change in its velocity — perhaps just a few millimeters per second — can cause it to miss Earth entirely. 

Ground-based surveys like the Catalina Sky Survey in Arizona and the Pan-STARRS telescope in Hawaii scan the skies nightly, searching for moving points of light. NASA estimates that more than 95% of large, civilization-threatening asteroids have already been identified.

But smaller objects — city-killers in the 50-to-150 meter range — remain harder to track. This is why new missions are being developed, including NASA’s upcoming NEO Surveyor space telescope, designed specifically to hunt for dark asteroids invisible in visible light. The rule is simple: detection buys time. Time buys options.

Nudging an asteroid, not blowing it up

Hollywood has long suggested that the solution to an incoming asteroid is a nuclear explosion, à la the 1998 film Armageddon.

In reality, scientists prefer subtler approaches. The DART mission proved the simplest strategy: kinetic impact. Slam a spacecraft into the asteroid and change its speed slightly. The earlier you do this, the smaller the push required. After DART’s success, NASA Administrator Bill Nelson stated, “NASA has proven we are serious as a defender of the planet. This mission shows that NASA is trying to be ready for whatever the universe throws at us”. Another possible method is the “gravity tractor.” A spacecraft would hover near an asteroid for months or years, using its own gravitational pull to slowly tug the asteroid onto a safer path. It sounds delicate — and it is — but gravity, given time, is powerful.

Nuclear devices remain a last resort. Not to blow the asteroid apart — which could create multiple dangerous fragments — but to nudge it off course through a carefully calculated detonation nearby.

The principle behind all these methods is the same: change the orbit slightly, long before impact.

A lesson from Chelyabinsk

When the Chelyabinsk asteroid exploded over Russia in 2013, it came from the direction of the Sun, making it difficult for telescopes on Earth to detect beforehand. There was no warning. Dashcam footage from that morning shows a brilliant flash streaking across the sky, followed minutes later by a thunderous shockwave. Windows shattered. Car alarms screamed. Confused residents stepped outside — some injured by flying glass. The event injured more than 1,500 people, mostly from broken windows. No one had predicted it.

Chelyabinsk was small on cosmic scales. But it underscored the need for better detection systems, including space-based telescopes that can observe objects approaching from sunward directions. It also demonstrated that even when an asteroid does not hit the ground, atmospheric explosions can be dangerous.

Global cooperation above politics

Asteroid defence is one of the few challenges that unites nations automatically. A large asteroid does not respect borders. After DART, the European Space Agency launched the Hera mission to follow up on Dimorphos and study the impact crater in detail. Together, these missions form a coordinated planetary defense effort.

International bodies such as the United Nations have established advisory groups to coordinate responses in case of a credible impact threat. This global cooperation is essential. If an asteroid were discovered on a collision course with Earth decades in advance, decisions would need to be made collectively: which nation launches the deflection mission? Who pays? Who leads?

Fortunately, for now, these remain planning exercises rather than emergencies.

The quiet work of planetary defense

Unlike climate change or pandemics, asteroid impacts are not slow-moving crises. They are low-probability, high-consequence events. The challenge is psychological. It is difficult to invest in preventing something that may not happen for centuries. Yet the technology required is within reach, and the stakes are enormous. 

As Carl Sagan once warned, “Extinction is the rule. Survival is the exception” (Pale Blue Dot, 1994). Planetary defence is about bending those odds. The DART impact in 2022 marked a turning point: humanity moved from passive observation to active capability. We can detect threats. We can calculate their orbits. And now, we have demonstrated that we can change them.

The sky is no longer something that merely happens to us.

If an asteroid is ever found on a collision course with Earth decades in advance, the solution will not involve heroic drilling crews or last-minute gambles. It will involve early detection, careful mathematics, and a precisely timed push. Saving Earth from an approaching asteroid, it turns out, is not about brute force.

Shravan Hanasoge is an astrophysicist at the Tata Institute of Fundamental Research.