Cheap Astronomy - Dear CA # 139: Clearing the Neighborhood

Crashing asteroids on the moon

0:00It's the 365 Days of Astronomy podcast, coming in 3, 2, 1.

0:20Hi, this is Steve Nerlick. Why, why, why, why, why, why, why, why Cheap Astronomy? Yeah, why? And this is Dear Cheap Astronomy, episode 139, Clearing the Neighbourhood. Yes, it's time once again to dive into Cheap Astronomy's wacky scheme to crash asteroids on the moon as a mining strategy.

0:51This time, we not only suggest that it could save money, but it could also save the world. Dear Cheap Astronomy, can we really clear all the potentially hazardous objects from around Earth? A key feature of Cheap Astronomy's CSOTM proposal, Crash S*** on the Moon, is that the most readily available source of s*** to crash are near-Earth asteroids and a small number of comets.

1:23This has the double benefit of putting all the raw materials in one place, plus you are clearing out potentially hazardous objects that could collide with Earth at some time in the future. It's currently estimated there are around 19 potentially hazardous objects that do carry a small but measurable risk of colliding with Earth in the next 100 years, and all are well under one kilometre in diameter, so do seem like good candidates to crash on the moon,

1:54insofar as their small size should make them relatively easy to move and more likely to properly crash, rather than fragmenting into rebounding shrapnel or create an orbiting lunar dust cloud. And of course, if you do somehow bollocks things up so badly that you send these objects on a trajectory towards Earth, they at least aren't big enough to be mass extinction objects. One good example is the infamous Apophis, which was a worry insofar as it was calculated

2:26to have an almost 3% risk of colliding with Earth in 2029, but now after further observation and calculation, it's no longer considered at all likely to impact the Earth within the next 100 years, and it's only 40 metres in diameter anyway. So, if we're looking for a poster asteroid for CSOTM, it's Apophis. The second poster asteroid in line is probably Bennu, which also has pretty much no chance of hitting Earth

2:57in the next 100 years, although it's more worrisome if it did, being about half a kilometre in diameter. So, we'd want to be well advanced in CSOTM procedures before taking on something this substantial. If it's not ringing any bells, Bennu was visited by the OSIRIS-REx spacecraft in 2018, which stayed there for about two years, and collected a sample in 2020, which was returned to Earth in 2023.

3:29The spacecraft has now been renamed OSIRIS-APEX, and will visit Apophis in 2029. The spacecraft no longer has a sample collection and return device, but the plan is to have a good look around Apophis, and probably blast its surface with retrofire, retrofire, so that we get a peek at its subsurface composition. We know from seeing Bennu up close, and collecting a sample, that it's a carbonaceous rubble pile asteroid,

3:59meaning it's more like dry clay than rock or metal, and it's a collection of small, loosely bound particles, rather than one solid object. This is likely to be a common finding amongst near-Earth objects, although we won't really know until we get a closer look at each one, and we should expect we will find a bit of variety out there. Nonetheless, there's limited value in collecting samples from each one, if we're just going to crash them on the moon anyway,

4:29so perhaps we'll just have robot scouts with cameras, to take a few snaps, before attaching a rocket engine, and sending them plunging to their doom, on the moon's surface. We assume most asteroids are clay, rock or metal, or some combination of those three, based on general observations of all the asteroids we can see, either in space, or crashed on Earth, where some, or bits of some, do survive the impact with Earth, as they've been initially slowed down by our atmosphere.

5:02However, let's remember that some near-Earth, and potentially hazardous objects, are comets, which are mostly water, or carbon dioxide ice, though they may have a rocky core. If we crash those comets on the moon, some of the water ice might be heated sufficiently by the impact, to dissociate into hydrogen and oxygen, and some of it would just vaporise, and then fall back to the surface's widely dispersed, fine particles of ice,

5:33or otherwise be blown out into space, by the solar wind. So, if you crash water ice, it will be mostly irretrievable. This might be less the case with carbon dioxide ice, but there's not much economic value in carbon dioxide. So, perhaps with comets, we'll just steer them into lunar orbit, and harvest the water from them, if that seems economically viable, which would still mean we mostly dismantle that object, and eliminate its risk to Earth anyway.

6:05Again, it's all about economics. Mining potentially hazardous near-Earth objects, in their solar orbits, sounds very slow and fiddly, and doesn't really deal with the fundamental problem, some of them being ticking time bombs, waiting to crash on Earth. Crashing them on the moon, at least takes them out of the picture, and leaves most of their raw material on the surface, some of which, could be potentially mineable, and no longer hazardous. It is hard to imagine a high-technology vision of humanity's future,

6:45where we'll just leave a bunch of civilization-destroying rocks, to continue floating out there. So, if we are going to get rid of them anyway, why not make a few bucks in the process? And that all sounds great, but how we actually make it happen, is a whole different question.

Deflecting asteroids with rocket engines

7:05Dear Cheap Astronomy, Can we really deflect asteroids, with rocket engines? This is a follow-up episode, to our recent one about crashing near-Earth objects, on the moon. Where a listener wrote in, to critique a throwaway line we made, about attaching a rocket engine to an asteroid. Particularly, if it was a rubble pile asteroid. They were too polite to say it's a dumbass idea,

7:35but yeah, it probably is a dumbass idea. Well, sort of maybe. The age of asteroid deflection, is really with us now, after the success of NASA missions, like DART, and Deep Impact. China is planning to launch an as-yet-unnamed asteroid deflection mission, and otherwise there are a lot of good ideas out there, about how to either deflect, or fragment an asteroid, on a seeming collision course with Earth,

8:07and none of them involve attaching rocket engines. And here's a fun fact for you. Since the Earth's orbital speed around the Sun, is 30 metres a second, and the Earth is nearly 13,000 kilometres in diameter, it takes about seven minutes, for the Earth to move one planetary diameter. So if an object is on a collision course with Earth, all you have to do is either delay, or advance its arrival, by up to seven minutes,

8:37and it will miss. Indeed, if it was heading for the midpoint of Earth's planetary disk, you'd just have to shift its arrival by three and a half minutes. It's also the case that the earlier you shift its trajectory, the less you have to shift it by. One suggested strategy, which does need a large amount of lead time, is to paint part of an asteroid, which will change its surface radiation absorption properties. Normally a sun-heated asteroid radiates that heat away,

9:10hence giving it a small push, which is known as the Yarkovsky effect. Painting some or all of the asteroid with a reflective paint, stops that long-term effect, and also gains a more immediate effect, by acting like a solar sail. All this really guarantees, is that there will be some change in the asteroid's trajectory, which direction it moves in, is largely unpredictable, particularly if the asteroid is spinning.

9:41And so, painting is largely useless, as a method of directing the asteroid to a particular target, like crashing on the moon, for example. A more accurate method, is a gravity tug. Essentially a moderate-sized spacecraft, which flies close to the asteroid, so that its mass generates enough gravity, to pull the asteroid off its previous trajectory. This method can be used to steer the asteroid onto a definite alternate course,

10:12and that course can continue to be modified further, either by the same tug, or by successive rendezvous with different tugs. However, like painting, this method will only very slightly modify an asteroid's trajectory, so you have to start with a lot of lead time, particularly for larger asteroids. So, if you don't have a lot of lead time, there's the kinetic impact method, where you crash something into the asteroid, imparting a kinetic energy,

10:43which is derived both from the mass, and the velocity of the thing, that hits the asteroid. This was measurably successful, with the DART mission, in 2022. So, with this method, you can be confident, of achieving a change in the asteroid's trajectory, but it's still not ideal, for steering the asteroid, onto a particular trajectory, since the effect of each impact, will be a bit unpredictable. So, if you do want a fast, and accurate,

11:14trajectory altering method, this is where a rocket engine comes in. It's by far, the least energy efficient, and hence would be the most costly, of all the options mentioned so far. Part of the problem, is that you'll need fuel and propellant, so the rocket engine, not only has to shift the asteroid's mass, but also its own mass, plus a sizable fuel tank. And so, that leads us to perhaps, the best of all options, a mass driver, which would be some kind of,

11:46electromagnetic slingshot device, that you'd load up, with bits of the asteroid, firing them off into space, and hence gaining a push, in the opposite direction. And so, no fuel or propellant, is needed. This approach, would be most useful, for rubble pile asteroids, where large chunks of rock, will be readily available, to fire into space. So you just land the catapult, which might be sufficiently, powered by solar arrays,

12:16and a bank of rechargeable batteries, and it would use, an attached robotic arm, to grab and load up, nearby chunks of rock. Piece of cake.

Using high tech mass drivers

12:28This, is the end bit. So, there you go. Cheap astronomy saves the world, with high tech catapults. Of course, that approach just replaces, big potentially hazardous objects, with lots more small, potentially hazardous objects, but small potentially hazardous objects, are only a bit hazardous, since they will mostly burn up, in Earth's atmosphere. But that's it, for another episode,

12:59of Dear Cheap Astronomy. If you've got a space science question, or you just want to fling some dross, into outer space, why not write to, cheapastro, at gmail dot com, and we'll load it up for you. Thanks, for listening. Steve Nerlick, Cheap Astronomy. You are listening, to the 365 Days of Astronomy podcast.

13:32Cool. The 365 Days of Astronomy podcast, is produced by, the Planetary Science Institute. Audio post-production, is by me, Richard Drumm. Project management, is by Aviva Yamani. And hosting, is donated by, LibSyn.com. This content, is released, under a Creative Commons, attribution, non-commercial, 4.0 international license.

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