Title: Don’t Forget To Look Up
Authors: Philip Lubin, and Alexander N. Cohen
Authors’ Institution: Department of Physics, University of California – Santa Barbara, CA, USA
Status: Published on arxiv, 27 Jan 2022
This Astrobite contains spoilers for the movie “Don’t Look Up”.
What if we were to discover a gigantic asteroid—a planet killer—that is on course to directly collide with Earth in six months? This is both the premise of the recent Netflix movie “Don’t Look Up” and of today’s paper, fittingly titled “Don’t Forget To Look Up”. Like the movie, the paper investigates the daunting scenario where an object of diameter 10 km and density 2.6 g/cm3 is on collision course with Earth. Due to its rapid velocity of 40 km/s, the asteroid is set to impact on Earth in only six months, leading to an event similar to the one that wiped out the dinosaurs 65 million years ago.
Now, even small Asteroids can have devastating effects when crashing into Earth. The Chelyabinsk asteroid (Fig. 1), which impacted Russia in 2013, briefly outshone the sun and caused a blast wave that injured almost 1500 people. This was despite a relatively small size of only 20m, and entering the atmosphere in a relatively remote area. The impact of a 10km-sized asteroid would have truly catastrophic consequences. In addition to the enormous blast wave, global earthquakes and towering tsunamis, temperatures could rise up to 300 Celsius as the energy injected by the asteroid eventually dissipates as heat. The excess heat could even ionize the atmosphere, which in turn would ignite a global fire. Lastly, the dust produced upon fragment burn up would eventually block sunlight and lead to a nuclear winter. Such an impact has more than ten thousand times the energy of the entire global nuclear arsenal and would almost certainly mean the end of life on the Earth’s surface.
As these events do happen about once per 100 million years, and smaller but more frequent, asteroids also can pose significant threat, the authors deem it worth investigating how such a threat could be mitigated—in particular if the warning is only six months ahead of impact (as in “Don’t Look Up”).
Pulverize it – Peeling an asteroid like an onion
The authors’ idea for preventing a catastrophic asteroid impact seems simple at first: blow it up! For smaller asteroids, the Earth’s atmosphere acts like armor: They burn up in the upper layers and only cause small(ish) blast waves. However, just blowing up a planet-killer sized asteroid into a bunch of smaller fragments is not good enough. The combined kinetic energy of the fragments would be too large to be absorbed by the atmosphere. It is thus crucial that most of the fragments miss Earth. This is achieved by intercepting the asteroid early enough, and injecting the fragments with radial kinetic energy, such that the fragment cloud spreads out with time to a size (much) larger than the Earth. Then, most fragments are expected to miss Earth, and those that don’t will be small enough to not pose a catastrophic threat.
The authors propose an array of high velocity penetrators equipped with nuclear explosive devices. The explosive energy must be sufficient to fragment the asteroid and give its fragment enough kinetic energy to scatter in different directions. This is achieved by peeling the asteroid like an onion, i.e. with successive waves of penetrators detonating on the asteroid’s surface.
Is six months enough to stop the asteroid?
There is a tight tradeoff between warning time, target speed, nuclear energy requirements and launcher requirements. For example, if we were to intercept the Asteroid 75 days prior to impact, an average fragment escape speed of 1 m/s is required, whereas an interception 30 days prior to impact already requires a fragment speed of more than 5 m/s (see Fig. 2), and thus much more energetic nuclear devices.
The authors argue that required nuclear devices could be carried by multiple heavy rockets currently under development such as the SpaceX Starship or NASA’s Space Launch System (SLS). A key uncertainty is coupling of explosive energy to fragment velocity, which would determine the nuclear energy requirements and thus the exact launch requirements of the planetary defense system. Past underground nuclear tests, like the Storax Sedan test in 1962, give some sense of the relation between explosive energy and fragment speed, but are not easily transferable to space.
In addition, the planetary defense system must also have enough time to actually get to the asteroid. If the asteroid travels at 40 km/s, even an interception 30 days prior to impact would already require a four month long journey for the interceptors. If the threat was discovered six months prior to impact, we would be left with about one month to plan and prepare the launch. One month is not a long time to save the world, but it could be done.
We are not dinosaurs (an Astrobiter’s opinion)
Humanity is just on the verge of being able to defend itself from a planet-killer type asteroid impact. This, the authors point out, makes us distinct from dinosaurs “who never took a physics class and failed to fund planetary defense”. I do find it worth pondering, however, whether the next existential threat to humanity really is a 10km sized asteroid as it was for the poor dinosaurs as well as Leonardo DiCaprio and Jennifer Lawrence in “Don’t Look Up”, or something else entirely. For example, I’m not convinced the relative usefulness of maintaining a global nuclear arsenal for destroying extremely rare killer-asteroids outweighs the dangers of nuclear weapons.
The authors’ plan to prevent a planet-killer type asteroid from posing an existential threat to humanity requires determined, coordinated, and immediate action. I believe this also applies to a successful climate change strategy. After all, the movie “Don’t Look Up” is an allegory for climate change, and the relative governmental and medial indifference to it. Maybe, today’s paper can be interpreted that way too.
Astrobite edited by Jessie Thwaites and Luna Zagorac
Featured image credit: NASA