a reply to:
Flyingclaydisk
During my 30+ year career as a NASA Aerospace Engineer, one of the areas of responsibility in my portfolio was Planetary Defense--meaning detection
and neutralization of potentially hazardous Near Earth Objects (NEOS). So I think there are a few things I can add to the discussion here.
The field more or less got going in the 1980s by a bunch of non-governmental astronomers who specialize in studying the small bodies of the Solar
System (Comets, Asteroids, Meteors, and small planetary moons). After studying the record of NEO impacts on the Earth (for example the Chixulub crater
on the Yucatan Peninsula) they became convinced that while most of the significant impacts occurred long ago and far away, there was still a non-zero
threat of impact today and into the future. So lesson number 1 was that you can't just assume that there is no longer any problem--you have to go out
and look with telescopes and actually measure the risks.
By 1998 Congress had gotten over the giggle factor and provided funding for NASA to go out and figure out how big the problem is. NEOs are not
self-luminous. The only way you can see them is by reflected sunlight or by the infrared radiation they give off by being warmer than the background
space. Bigger NEOs carry more kinetic energy and also reflect more light, so the most dangerous ones are also the easiest to detect and track. After
about a decade or so, NASA had identified about 1,000 NEOs 1 kilometer or larger in diameter and estimated that that was more than 90% of the risk in
that size range. None of them were on a collision course with Earth, and many of them were discovered by backyard astronomers using relatively small
telescopes.
So the NEO search program was extended down to NEOs of 140 meters in diameter or greater. There are more of those (approx. 15,000) and they need even
bigger telescopes to detect and track.
Eventually, it was figured out that most of the risk of being impacted by an NEO is presented by a small number of them that are on orbits that cross
the orbit of the Earth frequently and can be easily perturbed by Earth's gravity field onto a collision course. Those NEOs get many chances to impact
Earth in any given 100 year period and are the ones that should receive constant attention. Apophis is one of those Earth-crossing NEOs.
Once we figured out how big the problem is it became possible to start engineering solutions for deflecting a collision, if we should forecast one
ahead of time. The two most important parameters that determine the solution are 1) how big the NEO is, and 2) how much time we have to respond.
The Earth's atmosphere filters out NEOs below a certain size (about 50 meters in diameter, as I recall). So we only have to worry about deflecting
the ones in space that are bigger than that. NEOs smaller than 50 meters can still cause local damage, but they aren't going to wipe out cities or
cause huge tidal waves. The meteor that exploded over Chelyabinsk, Russia in February 2013 is a good example. It was about 18 meters in diameter,
and was entirely undetected before it entered the atmosphere, so we had zero warning time. But because it also disintegrated in the atmosphere, we
didn't have to do anything about it.
The bottom line is that Apophis is a pretty good example of the kind of NEO risk that we should plan for. If it is ever found to be on a collision
course with the Earth, we will know it 10 years or more beforehand, and it is big enough to cause enough damage that we SHOULD do something to avoid
the collision.
The scenario presented by FCD where you land a rocket on Apophis and use the thrusters to change the trajectory is not the one that would be used,
however, for at least two reasons. First, it is extremely difficult, if not impossible to "land" on an Asteroid. The gravity field of an Asteroid a
few hundred meters in diameter is practically negligible and most Asteroids are tumbling, so you can't really plant a rocket on the surface and have
it stay there. Second, even if you could land a rocket on the surface, you would have to expend a lot of fuel to null out the velocity of the rocket
relative to the Asteroid, and that wouldn't leave much fuel to actually shove the Asteroid out of the way. (It's the Rocket Equation).
Many studies have been done by Universities and government agencies on engineering the preferred approach, which is to simply collide a small
spacecraft with the Asteroid. The closing velocities of a spacecraft with an object like Apophis are easily calculated and typically are around 10
km/s. The momentum of the spacecraft would, of course, be transferred directly to the NEO and contribute toward deflecting the trajectory of the NEO.
However, a larger effect occurs because at those closing speeds, the spacecraft and an equivalent mass of the NEO are instantly vaporized and ejected
backward in a plume. That plume transfers up to 10 times as much momentum toward deflecting the NEO trajectory as the momentum of the spacecraft.
It's an amplification effect.
This effect was measured for the first time during NASA's DART mission of 2022. In that mission, a 500 kg spacecraft was impacted on the Didymos
binary Asteroid system at about 6.6 km/s and the amount of ejected matter and the amount of trajectory change were measured. This provides the data
required to design a kinetic impactor Asteroid deflection mission for an NEO like Apophis, if we ever had to.
People always ask the question about "nuking an Asteroid". In 2005, Congress asked NASA to look at the range of possibilities for performing the
Planetary Defense mission, and there was a special, classified annex to the study in which we looked at some of the issues involved with the nuclear
option. The first and most important conclusion was that it is very unlikely that the nuclear option would be needed. As previously mentioned, the
most likely threatening NEO would be something like Apophis, and a simple kinetic impactor like the DART mission would get the job done cheaply and
efficiently. There would likely be more than 10 years of warning, so you could make one attempt at deflection, measure the results, and then make
another one or two attempts, if you had to.
The unlikely scenario where you might need to use a nuclear device is where there is only a few months of warning time instead of 10 years. If you
only have months of warning time, you have to supply a much bigger push in order to make the NEO miss the Earth, and that's where the energy of a
nuclear detonation comes in. The most efficient way to couple the energy of a nuclear detonation to an Asteroid is with an "enhanced radiation" device
(also known as a Neutron Bomb) detonated at a standoff distance. The radiation vaporizes the surface layer of the NEO and creates the same kind of
ejecta plume as with the kinetic impactor, except on a much bigger scale. You probably can't just pull a nuclear warhead out of a missile silo in
Montana and go Asteroid hunting, but it's not that big of a deal to build a device for that special purpose, and the National Nuclear Security Agency
has set aside a couple of warheads for that use, if the need ever arises.