Planetary Defense: A threat and opportunity
Asteroids pose a threat to Earth as shown from human and geologic history. Asteroid impacts led to the extinction of the dinosaurs 66 million years ago and more recently asteroid or comet strikes have been shown to be destructive from instances like the Tunguska event of 1908 and the Chelyabinsk event of 2013. Despite the real danger of asteroid threats, humanity has only recently begun to track potentially hazardous asteroids and only one or two space missions have been dedicated to planetary defense so far.
This makes asteroid impacts a significant threat to humanity that is also neglected. Addressing this threat would allow humanity to build a planetary defense system to prevent the destruction of human civilization or Earth’s biosphere. It would also eventually allow us to intercept asteroids for other reasons, such as resource utilization and creating potential future habitats for humanity and other Earth life. In this way, addressing planetary defense, both protects life on Earth, the only living planet we know of at the moment, and enables us to potentially spread Earth life and intelligence beyond our home planet into the cosmos.
In 2013, an incoming ~20-meter (65-foot) asteroid exploded in the atmosphere above the town of Chelyabinsk, Russia, resulting in shattered windows and a few people being knocked off their feet, but relatively little real damage. If the asteroid had instead reached the surface, however, Chelyabinsk wouldn’t exist anymore. Chelyabinsk is a recent example of the real danger of impacts by large asteroids and comets. Other examples include the Tunguska event of 1908, where a comet likely exploded over a forest in Siberia, flattening the trees for miles. There is also of course the ~10 km asteroid that created Chixculub crater in the Yucatan peninsula that led to the extinction of the dinosaurs 66 million years ago.
Asteroid or comet impacts represent low probability, high impact (no pun intended) events. They may not happen very often but when they do, large asteroids could destroy a city, a country, or even wipe out all life on Earth depending on their size. For this reason, asteroid strikes represent a real threat to human civilization that should be considered alongside other threats like climate change or non-aligned artificial intelligence. The good news is that asteroid impacts are also 100% preventable if we can detect the incoming asteroid in time.
Despite the demonstrated risk of asteroids strikes and the fact that they are preventable unlike other natural disasters, like volcanic eruptions or earthquakes, planetary defense has received little attention or funding. For example, there has so far only been one spacecraft mission dedicated specifically to planetary defense (the DART mission in 2021-2022). The first telescope launched specifically to find potentially hazardous asteroids, the NEO (Near-Earth Object) Surveyor, is set to launch no earlier than September 2027.
NASA was officially tasked with finding 90% of potentially hazardous asteroids larger than 140 m in diameter in 2005 by the end of 2020. Scientists have made progress but are not quite there yet.
An asteroid large enough to wipe out a city hits about once every 100-1000 years on average, but they usually hit sparsely inhabited parts of the planet or the ocean. Asteroids that are large enough to cause a mass extinction or wipe out civilization (~10 km) hit Earth even more rarely, about every 100,000 years or more on average. The last one that hit killed off the non-Avian dinosaurs 66 million years ago. For all we know, we might be due for another. Don’t worry though, astronomers have not detected one of that scale likely to hit us soon, not yet anyways. Asteroids are not disasters to lose sleep over, but they are disasters that we should prepare for. Otherwise, if we discover a giant asteroid on a course to collide with Earth in the next six months, not even Bruce Willis will be able to save us.
After detection, the other part of planetary defense is deflection. Once you have identified the asteroid that is on a collision course with Earth, how do you prevent it? A popular depiction that appears in films like Armageddon is of course to destroy the asteroid with bombs or projectile weapons. The problem with this approach is that it will leave a debris field that might hit Earth anyways. The debris would also devastate multiple parts of the surface rather than being concentrated in one area, making it more dangerous than it otherwise would be.
The more feasible approach is to use a projectile weapon to redirect the asteroid. This was tested in 2022 by DART (Double Asteroid Redirect Test), the first asteroid re-direct mission dedicated specifically to planetary defense. The DART mission used a projectile to collide with asteroid Dimorphos, a 160 m (530-foot) “moonlet” of the larger asteroid Didymos (D = 780 m or 2560 ft.), on September 26, 2022, successfully altering Dimorphos’s orbit.
Alternatives to using projectile weapons is to use a gravity tractor. In this case you send a spacecraft to fly alongside the asteroid and the spacecraft’s gravity slightly tugs on the asteroid until its course is shifted. This takes longer but is less likely to cause debris that would hit Earth anyways. A related option is to use a spacecraft that attaches itself to the asteroid and pushes or pulls it using thrusters.
If humans can alter the course of asteroids, this does more than ensure that we can protect our home planet from asteroid strikes. It also means that we eventually could control and reach asteroids for other purposes, such as to mine resources or to create dwellings.
For example, some asteroids are rich in critical minerals containing metals important for electronics. A major problem with the transition to renewable energy, and the more recent growth in demand for data centers for AI use, is reliance on mining practices that are destructive to the environment and in some cases sourced from organizations with unethical labor practices depending on the country of origin.
While I am not suggesting that we strip mine every asteroid or that the green and AI transitions need asteroid mining to become sustainable, asteroid resources could become a long-term solution to the problem of electronics manufacturing relying on environmentally destructive and ethically questionable mining practices on Earth since most asteroids are unlikely to have indigenous life and the extraction methods would involve robots not human laborers (the plot of The Expanse notwithstanding).
Asteroids are also good candidates for natural space stations and future dwelling places for humanity and other Earth life. I am not saying we leave Earth, but since humans and other earthlings are vulnerable to extinction by a major impact event as long as we are restricted to one planet, settling other worlds would put our eggs into more than one basket.
Why not just settle Mars instead of tiny asteroids? I am not opposed to settling Mars, but there are drawbacks to living on a planet if we move off-Earth. For one thing, the larger gravity well would make it expensive to travel there and back. Settling an asteroid would mean that future settlers could travel more freely across the solar system with less fuel if their asteroid ran out of resources. Also, unlike the relatively gigantic Mars, future settlers could alter the asteroid’s orbit relatively easily or choose to leave the solar system altogether and become interstellar nomads. DART proved that we can alter the trajectory of small asteroids. It is not obvious that we will be able to alter the trajectory of a planet any time soon. That is probably a good thing for now. Furthermore, Mars in particular may have life, which in that case it could be argued that Mars belongs to the Martians.
In this way, planetary defense, like AI safety, for example, represents both a response to a threat and investment in an opportunity. If we can mitigate the threat of asteroids colliding with Earth and destroying civilization, we may also be able to utilize asteroid resources for the benefit of life on Earth and to move beyond Earth into the wider solar system and beyond.
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