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Comment by ithinkso | Hacker News Reader

How do you get enough fuel to the asteroid to bring it back if even sending a 'measly' rover to Mars is a multi-year endeavor. Asteroids weight a lot more than a rocket and you need to change their velocity by the same amount

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ARM was going to bring back a moderate sized asteroid the size of a small car or so. The reason this works is that there are no deep gravity wells involved. The delta-v needed to move things over long periods of time is feasible if difficult.

To be explicit, the ARM mission propulsion bus, canceled in 2017, is now to be the main component of the lunar gateway program. But it's like putting the cart before the horse with no intention of getting a horse ever. All this talk about lunar gravity well exploration is just sour grapes.

https://en.wikipedia.org/wiki/Power_and_Propulsion_Element#A...

That mission is targeting something the size of a washing machine not a car. Something around 5,000kg is reasonable, but asteroids are denser than your thinking.

How much metals are in this 5k kg rock?

That’s dependent on the rock. An metallic asteroid would be something like 80% Iron which is 7,873 kilograms to the cubic meter.

However, the limitation is mass not volume so while an asteroid 1/3 that density might be chosen it’s still going to be fairly compact.

https://www.space.com/20610-nasa-asteroid-capture-mission-in...

This is the low cost version of the ARM program that funding was chosen for. It is kind of marginal because there are literally only 3 near earth asteroids with little enough spin that this design would work.

Yes, but you don't need to use chemical propulsion for 100% of the needed energy -- just a well-timed and well-aimed change in angle to slingshot it off of a gravity well.

The real answer: It's entirely beyond our capabilities and not achievable at all.

The energy cost of changing something's orbit is not just mass times distance, otherwise launching a 1 metric tonne probe to Mars would have been impossible. Thanks to orbital dynamics, there are a lot of opportunities to get huge bang for your delta-V buck.

Not true. Well timed gravitational slingshots have been used on many existing missions. e.g. https://en.wikipedia.org/wiki/Parker_Solar_Probe

Of stable masses capable of analysis in advance to their mechanical properties under thrust. Asteroids are known solely by their orbital elements and we have literally no idea how the specific mass works as an aggregate or bonded entity, under load or stress.

You can line up a slingshot with a very low rate of acceleration, though.

Yes. I probably overstated my case. How quickly would that deliver a benefit?

It could take years for each mission, but of course at industrial scale you could have a continuous delivery queue and processing each asteroid could provide months or years worth of material. Think of each asteroid as a new mine being opened up. These initial missions are just proof of concept to test out techniques. You have to start somewhere.

So strap a rocket to one and find out. If it doesn't work grab another.

Having created literal mayhem in an unstable orbit of uncounted masses, ie for a giant externality. Next flight up, don't be within a large amount of space nearby in orbital terms, without care. (Maybe. Im not a kerbal person or an astronomer or flight engineer. This is a precautionary question to risk and to whom it applies)

Asteroids are very far apart from each other on the whole. Kicking one off in a different direction with a rocket isn't so different from a change in direction from a (rare) collision, except that it will almost certainly only affect one asteroid. ~25% of all the mass in the asteroid belt is the dwarf planet Ceres, which is only 0.0128 the mass of our moon. https://en.wikipedia.org/wiki/Ceres_(dwarf_planet)

There’s a vast amount of rubble flying about any which way up there already. About 100 tons of rock and dust enter the earths atmosphere every day, it’s just that space is so vast the chances of any of it hitting a vehicle are quite small.

The asteroid is the reaction mass. Send a solar collector and a rail gun sled that fires bits of asteroid really fast backwards.

That's a massively complicated endeavor and would require a lot of knowledge we don't have at the moment principally zero G mining and remote robotic assembly (time lags will mean it'll have to be very automated).

And reliable railguns.

That sounds like experience we’ll never get if we do not even make the attempt.

Eventually, it's not the first step you take though.

Forget railguns; an automated burger flipper could launch chunks of rock at escape velocity.

The faster you can accelerate them the more dV you get out of each chunk so you want to get them moving as fast as possible. If you're barely tossing them off the asteroid you might need to throw away a majority of the asteroid's mass to move it into Earth's orbit.

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