City Killing Asteroids: NASA Scientist Warns of Unstoppable Threats to Earth

Asteroids are large rocks orbiting the Sun. Most remain in the asteroid belt between Mars and Jupiter. Some, however, pass near Earth’s orbit. When they are large enough to cause regional devastation but remain undetected until too late, scientists call them City Killing Asteroids. These space rocks are not massive enough to end global civilisation like the one that likely wiped out the dinosaurs, but they would be powerful enough to destroy entire cities or large regions on impact.

Many of these asteroids are around 140 meters or larger in diameter. That size is big enough to cause severe damage if one were to hit a populated area, but small enough that tracking and defending against them is very difficult with current tools.

NASA’s Planetary Defense Challenge

NASA’s Planetary Defense Coordination Office (PDCO) is responsible for finding, tracking and studying near-Earth objects (NEOs) that could threaten Earth. The goal is to catalogue as many of these objects as possible, especially those big enough to be City Killing Asteroids. The agency also works on missions that could test ways to deflect or disrupt dangerous asteroids before they reach Earth’s vicinity. (NASA Science)

Despite sophisticated ground and space telescopes, NASA faces a big challenge. According to the latest reports, only about 40% of the estimated 25,000 mid-sized asteroids larger than 140 meters have been located and tracked. That means roughly 15,000 remain invisible to current surveys. These unknown asteroids are of greatest concern because scientists cannot predict where they are or if they might head toward Earth.

Planetary defense experts like Dr Kelly Fast, NASA’s acting planetary defense officer, have openly admitted that the threat from undetected city killers “keeps me up at night”. This warning highlights how limited our detection capabilities are when it comes to these mid-sized space rocks.

Why Some Asteroids Are Hard to Detect

There are several reasons City Killing Asteroids can slip past detection:

• Low Reflectivity: Many asteroids don’t reflect much sunlight, making them difficult to spot with optical telescopes.
• Similar orbits to Earth: Some follow paths that keep them in rough sync with Earth’s orbit for long periods, making them blend in with the night sky.
• Size: These rocks are big enough to be dangerous but not large enough to stand out easily in telescope surveys.

Because of these challenges, scientists rely on a mix of ground-based observatories and proposed space telescopes equipped with infrared sensors that can pick up the heat of dark asteroids. One such mission, the Near-Earth Object Surveyor, aims to improve our detection of hidden threats.

Past and Potential Near-Earth Risks

Occasionally, specific asteroids draw public attention when surveys show they have a small chance of impacting Earth. For example, an object designated 2024 YR4 once had a calculated chance of hitting Earth around the year 2032. Initial assessments put the probability above 3%, making it one of the highest risk objects ever recorded in modern history. Further observation downgraded the risk, but the situation showed how uncertain asteroid path predictions can be without sustained tracking.

Similarly, previous large asteroids such as Apophis were once thought to pose a threat in the future, but refined observations eventually ruled out any hazard for the next several decades. These events reflect how evolving data can change our understanding of impact risks.

Current Detection and Deflection Efforts

1. Tracking Mission and Surveyor Telescopes

NASA’s planetary defense strategy includes increasing the number of known near-Earth objects. The planned NEO Surveyor space telescope will use heat detection in infrared to find dark asteroids that current optical telescopes can miss. The goal is to identify up to 90% of potentially hazardous asteroids within the next decade.

2. Planetary Defense Missions

NASA’s Double Asteroid Redirection Test (DART) successfully altered the orbit of a small asteroid’s moon in 2022 as a proof of concept for planetary defense. This mission showed that a spacecraft impact could change an asteroid’s trajectory. Future similar missions could be used to deflect threatening asteroids if they are detected early enough.

3. International Collaboration

Global cooperation is also key. Agencies like the European Space Agency (ESA), Japan’s space agency (JAXA) and others work together to share data and improve early warning systems. Collective observation helps build a more complete picture of near-Earth space.

What Happens If an Asteroid Hits?

Not all asteroids that approach Earth are City Killing Asteroids. Many are smaller and burn up in the atmosphere, causing meteor showers but no major ground effects. Larger ones, however, can:

• Create large shockwaves over populated areas
• Trigger fires and structural collapse
• Cause regional devastation that disrupts infrastructure and lives

The impact of a 140-meter asteroid could release explosive energy equivalent to millions of tons of TNT. While not global, the local and regional consequences would be significant.

Public and Scientific Response

The possibility of City Killing Asteroids has both intrigued and alarmed the public. Space agencies emphasise that although the number of unknown asteroids is concerning, no known object currently holds a significant risk of striking Earth in the next century according to existing official data. Detection and improved monitoring remain the best defence.

Scientists and astronomers often stress the importance of continuous funding for planetary defence and enhanced telescopes. These investments help reduce future uncertainty and give humanity more time to respond if any threatening asteroid is discovered.

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