When Asteroid 2024 YR4 was first discovered last year, it was estimated to have a 3 percent chance of striking Earth. This caused quite a stir. It no longer has a chance of striking Earth, but it still has a 4 percent chance of striking the Moon in December 2032, according to updated models.
We'll have a better idea of the likelihood as that time approaches, but scientists and engineers are also preparing for what we would have to do to make sure it doesn't strike our only natural satellite at all.
NASA and several other researchers have published a new paper, published by Universe Today, outlining possible missions and schedules that could prevent the Moon from being struck by a large asteroid in less than ten years.
It wouldn't directly impact any human activities because there are no firm plans to establish a permanent human presence on the Moon by then. However, for a few days, an asteroid of this size could produce a massive debris field that would increase the number of micrometeoroids striking Earth by up to 1,000 times the background average.
Even though it would be one of the most spectacular meteor displays in recent memory, it would also be dangerous for satellites orbiting the Earth and, if they are still there in 2032, astronauts on the ISS and other space stations.
Actually, if 2024 YR4 is going to reach the Moon—which is still incredibly unlikely, to be honest—there are only two ways to prevent that possibility.
Deflecting it is one way. Destroying it is the alternative.
It would be better to deflect it. It would miss the Earth and the Moon if it only made a small adjustment to its orbit. It is preferable to do it sooner rather than later because the earlier we could do it, the smaller the nudge would be required.
However, we need to know its weight in order to deflect 2024 YR4 accurately.
We can estimate its diameter to be about 60 meters (200 feet), give or take 10 percent. However, estimates of its mass rely on its density, which is difficult to determine from such a distance.
The weight of the asteroid may vary from 51 million kg to more than 711 million kg, and the energy required to move each of those weights precisely differs greatly.
It might inadvertently alter its trajectory to exacerbate the issue, possibly rerouting it towards Earth, if a mission to deflect it is founded on an incorrect mass calculation.
The best time to try to get a more accurate estimate of the mass of 2024 YR4 would be in 2028, which is just three years away. To that end, engineers could plan a reconnaissance mission. It has never been done before to design and launch a mission with a specific purpose in such a short amount of time, and while it might be for a mission to solve a problem with a high enough threat level, 2024 YR4 is most likely not it.
However, we could repurpose current missions that are in development or already in space. The OSIRIS-APEX mission is an extension of the OSIRIS-Rex mission, which is currently en route to another near-Earth asteroid, Apophis.
On its way to its main-belt target, Psyche might also be captured. However, in order to get close enough to 2024 YR4, those missions would have to sacrifice their intended targets. Janus, which is presently in storage, is an additional choice, but it's unclear how well they would define its weight.
The study points out that the alternative option is at least possible given the ambiguities surrounding the deflection option. There are two ways an asteroid can be destroyed.
The first is "kinetic"—basically, striking it with a large, heavy object that can shatter it into smaller pieces of 10 meters.
Recently, DART demonstrated the concept of rerouting an asteroid by performing the same action. It would be more difficult to hit it with the goal of destroying it, but we could definitely design and construct it in time for a launch window sometime between April 2030 and April 2032.
The alternative of destroying it by nuking it will make nineties kids feel something. Setting off a nuclear explosion at a certain altitude off the surface of 2024 YR4 might be more realistic than Bruce Willis giving his life.
We would still require some reconnaissance to try to customize the explosion, which is known as the "height of burst." However, the paper estimates that, regardless of its size, a 1 megaton nuclear weapon would be sufficient to "disrupt" 2024 YR4, which is well within the scope of our current nuclear arsenal.
To be fair, we have never tested a nuclear explosion in space with the goal of deflecting an asteroid, but physics clearly indicates that it is feasible. And we have just conducted a nuclear test in space, albeit in the 1960s, most notably with the July 1962 launch of Starfish Prime.
The question of whether that would be a practical solution for this specific threat to our planetary system is as much a political one as a technical one. It won't be known until 2028 whether 2024 YR4 will truly reach the moon.
However, if we discover that it is, it is preferable to at least be able to interfere if we so choose. The window for launching missions, whether or not they are nuclear, is closing daily, so that choice will need to be made in the coming years.
