NASA Scientist Warns of Undetected City-Killing Asteroids

Planetary defense officer says there could be 15,000 undetected asteroids that could devastate cities.

Published on Feb. 18, 2026

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Kelly Fast, a planetary defense officer at NASA, warned that there could be up to 15,000 undetected asteroids measuring around 500 feet that have the potential to cause 'regional damage' and take out a city. Fast said these 'city killers' are small enough to avoid detection but large enough to be destructive, and that there is currently no way to actively deflect them.

Why it matters

Undetected asteroids pose a serious threat to cities and populations, as there are currently no effective measures in place to stop them from striking. This raises concerns about the vulnerability of urban areas and the need for improved planetary defense systems.

The details

Fast explained that these mid-size asteroids, known as 'city killers,' are hard to detect 'even with the best telescopes' as they accompany Earth in orbit around the Sun, preventing them from reflecting sunlight. While NASA has tested methods to deflect asteroids, like the DART mission that crashed a spacecraft into the mini moon Dimorphos, experts say replicating this with a 'city killer' would be difficult due to a lack of dedicated deflector crafts and funding.

  • In 2022, NASA successfully crashed a spacecraft into the mini moon Dimorphos to test asteroid deflection methods.
  • The notorious 'city-killer' asteroid YR4 has a 4% chance of striking the moon in 2032.

The players

Kelly Fast

A planetary defense officer at NASA who is warning about the threat of undetected city-killing asteroids.

Nancy Chabot

The leader of the DART mission, who suggested that replicating the asteroid deflection method used in that experiment would be difficult with a 'city killer' asteroid due to a lack of dedicated deflector crafts and funding.

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What they’re saying

“What keeps me up at night is the asteroids we don't know about.”

— Kelly Fast, Planetary defense officer, NASA (The Daily Star)

“We would not have any way to go and actively deflect one right now.”

— Nancy Chabot, Planetary scientist, Johns Hopkins University (The Times Of London)

What’s next

Scientists are planning to deploy a 'Near-Earth Object Surveyor space telescope' next year to help detect dark asteroids and comets that were previously hidden from view.

The takeaway

This report highlights the urgent need for improved planetary defense systems and increased funding to address the threat of undetected city-killing asteroids, which could strike without warning and cause devastating regional damage.