Scientists have tested whether a nuclear weapon could one day help protect Earth from a large asteroid heading towards our planet. An asteroid about the size of a football field could be big enough to destroy a city. Some asteroids are also difficult to detect because they are dark and reflect very little light. If one were discovered too late for other methods to work, a nuclear weapon could potentially be used as a last line of defence.
A team led by astrophysicist Isaiah Santistevan from Lawrence Livermore National Laboratory in the United States used computer simulations to study what would happen if a 1-megaton nuclear weapon were used against a 160-metre-wide asteroid.
The researchers found that the weapon would not necessarily need to hit the asteroid. Instead, it could explode several metres away from its surface.
The key would be the huge amount of X-ray energy produced by the explosion. Around 70 to 80 per cent of the energy from a nuclear explosion can emerge as X-rays. These X-rays could heat the asteroid's surface so strongly that some of its material would vaporise and be blasted away.
As this material escapes, it could change the asteroid's speed and direction. At the same time, the energy could create powerful shock waves inside the asteroid, causing it to crack and break apart.
To study this, the researchers created three detailed 3D simulations. They based the shape and porous structure of the simulated asteroid on Bennu, the asteroid studied by NASA's OSIRIS-REx mission.
They also used information from two real meteorites, the Chelyabinsk meteorite, which exploded over Russia in 2013, and the Aba Panu meteorite, which fell in Nigeria in 2018.
In one simulation, the nuclear explosion took place 10 metres above the asteroid. About 98.2 per cent of the asteroid's material was badly damaged, while around 97 per cent was moving faster than the speed needed to escape the asteroid's gravity.
Large amounts of material were also moving in opposite directions. This suggested that the asteroid could be pulled apart by the explosion.
The researchers then tested an explosion 25 metres away. Surprisingly, this produced more widespread damage at one point in the simulation. Although less energy reached the asteroid, the X-rays spread over a larger area of its surface.
After 68 milliseconds, about 92.6 per cent of the asteroid was fully damaged in the 25-metre simulation, compared with 78.1 per cent in the 10-metre simulation at the same stage.
However, there is an important limitation. The simulations were extremely demanding. The longest simulation covered only 145 milliseconds of activity but took 59 days to complete using 1,680 computer processors.
Because of this, the researchers could not establish what would happen to the asteroid over a longer period. The fragments might spread out and become harmless, but some pieces could remain large enough to threaten Earth. There is also a possibility that gravity could pull some of the fragments back together.
The researchers say the results do not prove that nuclear weapons would definitely stop a dangerous asteroid. Instead, the simulations provide more information about whether nuclear disruption could be used as a last-resort option in planetary defence.
"Based on the extent of damage, the directionality of the material motion, and the [velocity change] that we impart on these asteroids, we suggest that disruption is highly likely for two of the three scenarios," the researchers write in a paper in The Planetary Science Journal.
"Simulations such as these help to further inform the viability of nuclear mitigation options for planetary defense and support emergency response planning."
The findings provide scientists with another piece of information as they work on ways to protect Earth from potentially dangerous asteroids.
