Depending on the size and composition of the asteroid and the power of the nuclear device, an explosion could potentially shatter an asteroid into pieces. Whether the fragments missed the planet altogether or continued on the same impact trajectory might depend on the same factors — size, composition, and power of the explosion. Although some fragments might be small enough to burn up in the atmosphere, other chunks of asteroid could still pose a serious threat and might rain down over a wide area of the planet.
Another approach to nuking an incoming asteroid would be to detonate the nuclear device near, rather than on, the surface of the asteroid. This way, the explosion could cause the asteroid’s surface material to vaporize and ablate, creating a thrust that could alter the asteroid’s trajectory without causing significant fragmentation.
Any attempt to destroy or divert an asteroid using nuclear detonations would likely require a mission to study that asteroid up close beforehand to find out what it’s made of, how it’s structured, and what kind of force would be needed to effectively deflect or destroy it without turning one impact threat into many.
International treaty concerns
The use of nuclear weapons in space is strictly prohibited under the international Outer Space Treaty, and for very good reason. During the Cold War, the Soviet Union and the United States competed to develop powerful rockets and nuclear weapons, going so far as to test nuclear weapons in space. In response to this arms race, the United Nations drafted the Outer Space Treaty, to which all major spacefaring nations have agreed.
While sending a nuclear weapon into space for asteroid deflection is clearly not the same as using a nuclear weapon for war, it’s possible that the use of nuclear arms in space could create geopolitical tensions or set a bad precedent for future military activities in space. This is one reason why the nuclear option is not necessarily the most straightforward way of dealing with an asteroid threat.
Other options are out there
Luckily, there are other ways of deflecting an asteroid that do not pose the same challenges as nuclear detonations.
One technique is the slow gravity tractor, where a massive spacecraft is launched from Earth to meet the asteroid and follow next to it, using the spacecraft’s own gravity to pull the object’s path away from a future collision. Another technique is laser ablation, where one or many spacecraft apporach the asteroid and use lasers to vaporize rock to create jets that slow or speed the object’s path around the Sun. The Planetary Society worked with a team at the University of Strathclyde in Scotland to develop early lab research into this technique, through their Laser Bees project.
There is also the kinetic impactor technique, where one or more spacecraft slam into the asteroid to impart a little force to shift its orbit. NASA’s DART (Double Asteroid Redirection Test) mission proved the effectiveness of this technique in September 2022 when the spacecraft intentionally crashed into the asteroid Didymos’ small moon, Dimorphos. The crash changed the time it takes Dimorphos to orbit Didymos by 33 minutes, proving the kinetic impactor technique works to change an asteroid’s trajectory through space.