NASA’s asteroid collision: how many asteroids are really out there?

Episode Summary

Executive Summary: The episode uses NASA’s DART asteroid-deflection mission as a springboard to revisit BBC More or Less’s earlier fact-check of Armageddon and to explain real asteroid risk. Experts say catastrophic impacts are extremely unlikely in the short term, most large near-Earth objects are already catalogued, and smaller hazardous asteroids are harder to detect but can be estimated statistically and better tracked with new telescopes like Vera Rubin.

Main Topics: DART mission and asteroid deflection (Priority: 5/5): The programme opens with NASA’s successful DART impact, which intentionally crashed a spacecraft into an asteroid to test whether Earth-threatening objects could be nudged off course. Armageddon as a science-fiction benchmark (Priority: 4/5): The show revisits the film’s premise of drilling into and nuking an asteroid, asking whether such a scenario is plausible in real life and using it as a humorous framing device. How much asteroid danger exists? (Priority: 5/5): NASA scientist Steve Chesley explains that the largest asteroids are the easiest to find and the most consequential, but they are also very rare on human timescales. Why discovery percentages can be estimated (Priority: 5/5): Professor Catherine Haymans explains that even if not all asteroids are directly observed, astronomers can estimate total populations statistically, similar to counting shoppers or ants on a street. Detection limits and near-Earth objects (Priority: 4/5): The episode clarifies that the relevant population is near-Earth objects, not all asteroids, and that many smaller bodies remain undiscovered because they are too faint, distant, or small. Future mitigation and readiness (Priority: 4/5): The Vera Rubin Observatory is presented as a major upcoming tool for finding hazardous objects earlier, giving humanity more time to respond if a threat is found. Training astronauts vs drillers (Priority: 2/5): The episode briefly checks whether it would be easier to train a driller to be an astronaut than vice versa, concluding that there is no definitive real-world answer, despite Armageddon’s comedic logic.

Key Arguments: Large asteroids are easiest to detect because they are bright and cause the most damage, so NASA’s main goal has been cataloguing kilometre-scale objects. NASA says about 95% of asteroids larger than 1 km are already known and monitored, making a surprise global-killer impact highly unlikely. The missing large asteroids are rare enough that they would hit on timescales of many tens of millions of years, not imminently. A sudden humanity-threatening asteroid discovered only days before impact is possible in theory but is not the expected or most likely scenario. Smaller asteroids pose a greater practical risk because they are harder to detect and more numerous. NASA can estimate the proportion of undiscovered asteroids by observing the ones it can see and extrapolating the total population statistically, just as one estimates ants on a high street. The 40% figure cited in news coverage refers to a subset of near-Earth asteroids about 150 metres wide, not all asteroids in the solar system. A new telescope, the Vera Rubin Observatory, should improve detection of smaller hazardous objects and provide more warning time for mitigation. A direct-impact deflection mission like DART is a realistic planetary-defence method, unlike Armageddon’s drilling-and-nuking scenario. The show treats Hollywood logic humorously but emphasizes that real planetary defence depends on detection, tracking, and having enough lead time to act.

Data Points: DART mission result: Direct hit - NASA’s uncrewed spacecraft successfully crashed into the asteroid target Size of large asteroid monitored by NASA: Greater than 1 kilometre - NASA’s main catalogue focus for the most damaging asteroids Catalogue completeness for >1 km asteroids: About 95% - NASA says it has found and is monitoring nearly all kilometre-scale objects Undiscovered >1 km asteroid impact timescale: Many tens of millions of years - Chesley explains the missing large asteroids are so rare that impacts would be extremely infrequent Asteroid impact timescale for large asteroids generally: About 1 million to 10 million years - Chesley gives the approximate interval at which large asteroids hit Earth Near-Earth object distance threshold: Within 45 million kilometres - Haymans defines the category relevant to Earth-risk assessment Distance comparison: About 100 times Earth-Moon distance - Used to contextualize 45 million kilometres Reported identification rate for ~150 metre asteroids: 40% - The news report cited in listener questions about city-destroying space rocks Approximate asteroid size cited in report: 150 metres / 500 feet - The class of asteroid described as capable of obliterating a whole city Vera Rubin Observatory location: A mountaintop in Chile - The new telescope being commissioned to find smaller hazardous objects Mission training time for astronauts: Two years general training plus usually two years mission-specific training - Compared with driller career progression in the Armageddon question

Pivotal Quotes: "The most simple and obvious approach is just to send up a spacecraft as big as we can, as fast as we can, and run into it, knock it off course, and thereby eliminate the possibility of impact." — Steve Chesley: Explanation of real-world asteroid deflection strategy "We nuke this thing from the inside." — Armageddon script excerpt: Summarizing the film’s fictional plan to destroy the asteroid "So just because they're small doesn't mean that you can't make a good estimate of how many there are in total." — Professor Catherine Haymans: Why astronomers can estimate asteroid populations even when some are not directly observed

Implications: Listeners should take asteroid threats seriously but not fear an imminent Armageddon-style event. Better surveys, new telescopes, and deflection testing improve planetary defence by turning a rare, high-consequence risk into a manageable one.