Detection & Warning
How much warning
would we have?
For a 1-kilometre asteroid: decades. For a 20-metre rock approaching from the Sun: possibly none. Warning time is the single most important variable in planetary defence - and it scales with asteroid size.
How planetary defence works →Warning time depends on size
The most important factor is size. Large asteroids reflect more sunlight and can be detected decades before they approach Earth. Small objects are faint until they are close - and by then there may not be enough time to do anything beyond issue warnings.
The practical consequence is stark. A 1-kilometre asteroid heading for Earth would almost certainly be known decades in advance, leaving time for a deflection mission. A 20-metre rock like the Chelyabinsk impactor might be detected hours before arrival - or not at all. Between those two extremes lies a range of scenarios, each with different response options.
Warning time by size class
| Size | Likely warning |
|---|---|
| < 10 m | Hours to zero (often found after the flyby) |
| 20–50 m | Zero (Sun-direction approach) to days |
| 50–140 m | Days to months (many objects in this range undiscovered) |
| 140 m – 1 km | Years to decades |
| > 1 km | Decades (most already catalogued) |
The Chelyabinsk lesson
On 15 February 2013, a 20-metre asteroid entered the atmosphere over Chelyabinsk, Russia without any advance warning. It approached from the direction of the Sun - the one region of sky that ground-based telescopes cannot observe during daylight. No survey network in operation at the time could have detected it in advance.
Roughly 1,500 people were hospitalised, primarily from glass injuries caused by the shockwave. The building damage was widespread across several cities. No one died directly, but the event made clear that an object in the 20-50 metre range can cause significant casualties with zero warning.
The event directly accelerated investment in short-warning detection systems, including ATLAS (Asteroid Terrestrial-impact Last Alert System) - a network of telescopes designed to provide at least several days of warning for objects in the 50-140 metre range.
The 2008 TC3 exception
On 6 October 2008, asteroid 2008 TC3 was discovered 19 hours before it struck Earth's atmosphere above Sudan. It was only 4 metres across - too small to cause surface damage. The object broke up at altitude, scattering meteorites across the Nubian desert.
The discovery marked the first time a predicted impact was confirmed before it happened. The 19-hour window was short, but observers managed to record spectra before atmospheric entry. Meteorite hunters later recovered fragments. The event proved that short-warning detection is achievable in principle, even for very small objects.
Why lead time is everything
Warning time is the key variable for planetary defence. With decades of lead time, a kinetic impactor - like the DART spacecraft - can change an asteroid's velocity by a fraction of a millimetre per second. That tiny change, accumulated over years, moves the asteroid's eventual Earth-encounter point by thousands of kilometres.
The earlier the intervention, the smaller the force needed. With a few months of warning, options narrow sharply: only a nuclear standoff detonation could disrupt a significant object in that timeframe, and even that is uncertain for larger targets. With days or hours of warning, only evacuation remains as a realistic response. This is why finding objects early matters - decades of lead time turn a potential catastrophe into an engineering problem.
Current system limits
Ground-based surveys cover the sky repeatedly but cannot see objects approaching from the Sun's direction, or certain southern sky regions during northern hemisphere observing seasons. The Catalina Sky Survey, Pan-STARRS, and ATLAS all contribute to the global detection network, but each has coverage gaps.
The upcoming NEO Surveyor space telescope, operating in an orbit near the Sun, is designed specifically to detect objects in these blind spots. Until that telescope is operational, sub-140-metre objects approaching from sun-adjacent directions remain the least predictable class - the gap where Chelyabinsk-style events remain possible without warning.
Where the tracking is good
For the objects that matter most - those 140 metres and above - tracking is comprehensive. More than 95% of near-Earth asteroids at 1 kilometre scale have been found. The gap is in the 50-140 metre range, where many objects remain undiscovered. These are large enough to destroy a city, but small enough to have escaped cataloguing.
Related pages
Will an asteroid hit Earth?
Current impact risk and the state of NASA tracking.
Planetary defence
Detection surveys, deflection options, and coordination.
How asteroids are discovered
Survey telescopes, detection methods, and what happens next.
The DART mission
How NASA proved kinetic deflection works in practice.
