An Asteroid Doesn’t Have to Hit Earth to Cause Catastrophe

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• Ioana Patringenaru- [email protected]

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The threat of a civilization-ending asteroid hitting the Earth is well-known - as a plethora of Hollywood movies on the subject has demonstrated. But there is a less-known threat that could have equally dramatic consequences: a collision between a large asteroid and the moon. The debris from such a collision would damage satellites that underpin the infrastructure for everything from telecommunications, to defense, to navigation here on Earth. Now a team of engineers at the University of California San Diego is working to improve tools to calculate the likelihood of these collisions and predict their occurrence far enough in the future to prevent impact.

The worst-case scenario for these types of collisions is known as the Kessler syndrome. In this scenario, low-Earth orbit becomes so crowded with satellites and space debris that a few collisions with these objects lead to a chain reaction. This in turn would make a significant portion of our planet's low-Earth orbit regions unusable for centuries. GPS, satellite-provided internet and cable, as well as weather forecasts would be severely impaired, if not completely destroyed. Military applications, such as missile defense, would also be impacted.

This scenario is realistic if an asteroid that is about 200 feet across were to hit the moon. "Because the moon has no atmosphere to slow or burn up debris, a big impact can loft vast amounts of rock into space. Some of that ejecta can be steered by Earth's gravity onto complicated paths that pose a direct hazard to satellites in orbit between the Earth and the moon, including future communications and navigation infrastructure around the moon," said Aaron Rosengren, a faculty member in the Department of Mechanical and Aerospace Engineering at UC San Diego.

With a team from the University of Arizona, Rosengren investigates what would happen when such a sizable asteroid hits the moon - how much debris would be launched off the lunar surface, where that debris would go, and what risks it would pose to satellites and future infrastructure in the Earth-facing side of the moon and low-Earth orbit. The team found that a surprisingly large fraction of pieces of the moon ejected after a meteoroid impact can make their way back to Earth. Some of these fragments can be parked for long periods in special "co-orbital" trajectories that accompany Earth around the sun.

"So the problem is no longer just 'Will something hit the ground?' but 'What are the long-term consequences for the Earth-Moon system we now rely upon?" Rosengren said.

What can be done

To avoid a major impact, early detection is crucial. "For the class of objects we worry most about from a planetary-defense standpoint - hundreds of meters across - a realistic goal is to have at least 5-10 years of warning. That sounds like a long time, but in engineering terms it is barely enough: we would need to detect the threat, converge on a reliable impact probability, design and fund a deflection mission, build and launch a spacecraft, and then give it time to reach the asteroid and gently 'nudge' it so that, many orbits later, it misses Earth," Rosengren said.

The dangerous scenarios live out in the "tails" of the uncertainty distribution where most traditional methods perform poorly. Rosengren, mechanical engineering professor Thomas Bewley and Ph.D. student Ben Hanson, all in the UC San Diego Department of Mechanical and Aerospace Engineering, are trying to follow not just the "average" path of an asteroid, but the rare, low-probability paths that could still lead to impact. The approach they are using has not yet been widely applied in this field, and they are finding that it can give more reliable estimates of very small probabilities - the ones that really matter for making a decision on whether to launch a deflection mission. The team has published five papers on the topic in the last three years.

In collaboration with the University of Arizona, Rosengren found that a fragment launched off the moon's surface can spend tens of thousands of years looping through the Earth-Moon system, sometimes settling into strange "companion" orbits where it appears to follow Earth around the sun as a faint, secondary moon. Recent discoveries like the near-Earth asteroid Kamoʻoalewa and the object 2024 PT5 are strong candidates for exactly these kinds of debris.

Deflection of these types of objects has become more realistic in recent years. In September 2022, a NASA spacecraft, during a mission known as DART (Double Asteroid Redirection Test), intentionally impacted the asteroid moonlet Dimorphos. A recent study published in Science Advances shows that the impact didn't just change the motion of Dimorphos around its larger companion, Didymos; the crash also shifted the orbit of both asteroids around the sun.

"This is difficult mathematical and computation-focused work, with a potentially huge impact - pun intended," said Bewley.

How the risk is calculated

The risk assessment begins with data gathered from all around the world, including the Pan-STARRS telescopes in Hawaii, the Catalina Sky Survey in Arizona, and ATLAS, with sites in Hawaii, South Africa, and Chile, which repeatedly scan the sky each night looking for "moving dots" against the background of stars. These data are then sent to the Minor Planet Center, which is the global clearinghouse that links observations together and estimates preliminary orbits.

For the most interesting objects - those that pass relatively close to Earth - the Solar System Dynamics group at NASA's Jet Propulsion Laboratory refines the object's orbit using all available data and maintains the solutions in the Small-Body Database. Space telescopes such as NEOWISE add a crucial piece: by observing in the infrared, they can estimate the size and reflectivity of an asteroid, including very dark ones that are hard to see in visible light.

"From a mathematical standpoint, every orbit solution comes with uncertainty - it isn't a single curve in space, but a smeared-out 'cloud' of possible paths," Rosengren said.

His group works on methods to project that cloud forward in time, taking into account gravitational tugs from the sun, planets, and moon. They then ask: "What fraction of that cloud intersects the Earth or the moon on some future date?" That fraction is the impact probability.

The good news is that surveys have now found on the order of 95% of the largest, kilometer-class "planet killers" in near-Earth space. The less-good news is that the much smaller "city-killer" objects that are tens of meters across are far more numerous and much harder to detect. Researchers estimate that only a tiny fraction of those have been identified, as events like the 2013 Chelyabinsk airburst in Russia made very clear.

"There are many near-Earth objects which could create such collisions, and astronomers identify several more each and every year," Bewley said.