Planetary Defense: What Happens in the 48 Hours After Scientists Confirm an Asteroid Is Heading for Earth

In September 2022, NASA flew a spacecraft 11 million kilometres and deliberately crashed it into an asteroid at 22,500 kilometres per hour.

The asteroid moved. That was the rehearsal. The playbook for the real scenario already exists — and it is more procedural, more urgent, and more uncertain than most people know.

What did the DART mission actually prove?

The DART spacecraft — Double Asteroid Redirection Test — targeted Dimorphos, a 160-metre moonlet orbiting the larger asteroid Didymos. The mission's minimum success threshold was a 73-second change in Dimorphos's orbital period around Didymos. DART changed it by 32 minutes.

The reason it worked so well caught scientists off guard. The impact did not just push — it blasted a debris plume hundreds of thousands of kilometres long. That ejecta acted like rocket exhaust, multiplying the kinetic push far beyond the spacecraft's own momentum. In a sense, the asteroid helped deflect itself. The physics of that discovery are still being studied by teams around the world.

How does anyone find out an asteroid is heading for Earth?

Right now, thousands of robotic telescope-hours every single night are scanning the sky for Near-Earth Objects. The main hunters: the Catalina Sky Survey in Arizona, Pan-STARRS in Hawaii, and the upcoming NEO Surveyor infrared space telescope. Every new object gets an orbit calculated from multiple independent observations. That orbit then receives a Torino Scale score — a threat index from 0 to 10.

Zero means routine. Ten means a civilisation-ending impact is certain. Right now, no known asteroid scores above zero. However, roughly 60% of city-killer-sized objects — 140 metres and larger — have not been catalogued yet. They exist. We simply have not pointed a telescope at them.

What actually happens in the first 48 hours after a threat is confirmed?

There is an actual protocol. It was drafted before DART launched and updated after the mission succeeded. Here is how it unfolds:

Is Apophis going to hit Earth in 2029?

No. But it will pass closer than your satellite television signal travels.

Apophis (99942 Apophis) is a 340-metre asteroid — taller than the Eiffel Tower — first detected in 2004. On Friday, April 13, 2029, it will pass Earth at approximately 32,000 kilometres above the surface. Geosynchronous satellites orbit at 35,786 kilometres altitude. Apophis will pass inside that ring. For a brief window, it will be visible to the naked eye — a point of light drifting across the sky in roughly an hour. No telescope required.

Extensive radar and optical observations have ruled out an impact in 2029, 2036, and 2068. Apophis is not coming for us. But multiple space agencies — including ESA and JAXA — are planning to intercept it on approach, making the 2029 encounter the most closely observed near-Earth asteroid pass in history and a live, full-scale test of every planetary defence system humanity has built. You can follow near-Earth asteroid approaches live alongside 15,000+ tracked satellites on the SkyLens live tracker, where approach data updates daily from NASA's Jet Propulsion Laboratory.

How complete is our catalogue of actual threats?

For objects larger than 1 kilometre — the scale that would trigger mass extinction — scientists estimate we have catalogued more than 95% of them. None are on a collision course in any foreseeable timeframe.

For city-killers in the 140-metre class, the picture is less complete. Roughly 40% have been found. The remaining 60% are likely hiding in orbits that keep them near the Sun from Earth's perspective — lost in the solar glare where ground-based telescopes cannot see. That is precisely the gap NASA's NEO Surveyor telescope is designed to close, scanning in infrared from space where sunlight is no longer a barrier.

Here is the reassuring framing: every year of continued sky survey that passes without finding a threat is itself data. The catalogue is a growing bill of health for Earth's neighbourhood. DART proved we can act if we need to. The telescopes are doing the watching. For one of the few civilisation-scale risks humanity faces, this is one we are actually making measurable, verifiable progress on — year by year, rock by rock.

Read more deep dives into space science and planetary defence on the SkyLens blog, or explore the learn section for explainers on orbits, satellites, and how we watch the solar system.