If an asteroid is determined to be on an Earth-impacting trajectory, scientists typically want to stage a deflection, where the asteroid is gently nudged by a relatively small change in velocity, while keeping the bulk of the asteroid together.
A kinetic impactor or a standoff nuclear explosion can achieve a deflection. However, if the warning time is too short to stage a successful deflection, another option is to couple a lot of energy to the asteroid and break it up into many well-dispersed fragments. This approach is called "disruption" and it is often what people think of when they picture planetary defense.
While scientists would prefer to have more warning time, they need to be prepared for any possible scenario, as many near-Earth asteroids remain undiscovered.
Now, new research takes a closer look into at how different asteroid orbits and different fragment velocity distributions affect the fate of the fragments, using initial conditions from a hydrodynamics calculation, where a 1-Megaton-yield device was deployed a few meters off the surface of Bennu, a near-Earth asteroid discovered in 1999. The research was published in the latest issue of the journal Acta Astronautica.
Lead author Patrick King, a former Lawrence Livermore National Laboratory Graduate Scholar Program fellow who currently works at the Johns Hopkins University, said: "The results highlighted in the paper are reassuring: for all five asteroid orbits considered, carrying out the disruption just two months before the Earth impact date was able to reduce the fraction of impacting mass."
In a report posted on the website of the Lawrence Livermore National Laboratory (LLNL), King said the principal finding of the work was that nuclear disruption is a very effective defense of last resort.
"We focused on studying late disruptions, meaning that the impacting body is broken apart shortly before it impacts. When you have plenty of time — typically decade-long timescales — it is generally preferred that kinetic impactors are used to deflect the impacting body," he added.
Kinetic impactors have many advantages: for one, the technique is well-known and is being tested on real missions, and is capable of handling a wide range of possible threats if you have enough time.
However, they do have some limitations, so it is important that if an actual emergency does arise that multiple options are available to deal with a threat, including some ways that can handle pretty short warning times.
Owen said this paper is critically important for understanding the consequences and requirements for disrupting a hazardous asteroid approaching Earth. "If we spotted ahazardous object destined to strike the Earth too late to safely divert it, our best remaining option would be to break it up," he said.
"This is a complicated orbital question though — if you break up an asteroid into pieces, the resulting cloud of fragments will each pursue their own path around the sun, interacting with each other and the planets gravitationally. That cloud will tend to stretch out into a curved stream of fragments around the original path the asteroid was on. How quickly those pieces spread out, combined with how long until the cloud crosses Earth's path tells us how many will strike the Earth," he concluded.
