Gravity is weird

In My Humble Opinion
121

To clarify that, I assume you mean “sending something into the sun from Earth’s surface”? Because otherwise, the energy required to send an object from the asteroids to Earth would be pretty similar to sending it from the asteroids into the Sun - just steer it differently.

This is a concern, if you want asteroid-based resources for use on Earth’s surface. You have to account for damage / loss due to atmospheric friction, plus the potential for damage if the object is big enough or hits in the wrong place - just ask those dinosaurs.

Most SF stories seem to concentrate on using such resources in space - either for building space stations, or on other planets, or whatever.

Sending such resources to a space destination shouldn’t be all that costly, ignoring the cost / effort of getting out to the asteroid belt, mining, etc. If time isn’t an issue, you don’t have to accelerate them all that much, just point them where you want them to go, and give them a nudge. They’ll keep going until something stops them. You’d want a way to do minor course corrections, and of course a way to decelerate them when they get to their destination - but compared with the cost to get out there to do the mining, that all would be relatively small.

122

We don’t need an average asteroid, nor do we need to slow it down to 0 MPH (relative to what, the sun? That would just make it fall straight in…)

We can scan the asteroid belt to find asteroids, figure out their weight, and their orbits. Then we can figure out exactly how much fuel and time it would take a given tug to bring said asteroid into a desireable orbit. Most likely the ideal asteroid would be one that can be nudged towards Mars or Jupiter for a gravity assist that flings it towards Earth, where we could perhaps use the moon’s gravity to slingshot towards the desired orbit.

In space, “how far” matters much less than “how much fuel does it take to change your orbit to the desired orbit”; and there’s all kinds of tricks you cna use to cut down the needed fuel, such as gravity assists.

Rockets, obviously - same way you move anything else in space.

Ion thrusters could work if you had a way to generate lots of power, like big solar panels or RTGs; there’s no rush, so having the engines burn for many hours or days is totally fine. We can set the asteroid coasting where it needs to go and let it get there a decade or two later, if we wanted.

As noted, you don’t have to. You just nudge it towards a gravity well, and use that to do most of the dV changes. And you don’t want to remove all its velocity relative to the sun anyways, because the Earth is going around the sun at 67,000 miles per hour, and you want to match the Earth’s orbit, not fall into the sun.

The benefits are, again, very simple to compare.

Once we decide whether it’s cheaper in rockets to launch a tug and bring in the asteroid, or cheaper to launch a big lump of rock from Earth, we can compare the cost of doing so (and of manufacturing a space elevator), to the capacity that the elevator will have for putting things in space; and then we can easily calculate, the space elevator will pay for itself after it lifts Y tons to orbit, and this will take Z years at the space elevator’s capacity.

123

LOTS of chocolate pudding.

124

Here’s the fun part of orbital dynamics. The asteroid orbits at 40,000 MPH. Earth orbits at 67,000 MPH. How can the asteroid get to Earth?

Well, it has to slow down (burn retrograde). Whaaa? Why does it need to slow down if Earth is moving way faster?

Well, because its orbit is way too big! By slowing down, the asteroid’s periapsis will drop, until it is at the same altitude (above the sun) as the Earth. As the asteroid drops in altitude, it will gain speed, and when it is at the Earth’s orbit, it will be moving much faster than Earth. At that point, to capture into Earth’s orbit, we need to speed up (relative to the sun) or slow down (relative to the Earth) and match orbits (which we can do partially through a rocket burn and partially by using Earth’s gravity well).

And to get even MORE confusing, the best way for the asteroid to drop its periapsis enough to meet Earth might be for it to actually speed up initially, RAISING its apoapsis until it meets Jupiter; if it passes in front of Jupiter’s path and then swings behind it, it will bleed off a ton of speed into Jupiter’s gravity well, dropping its periapsis down past the Earth’s orbit.

125

Yes, the questions themselves are easily answered, but putting those equations into actual projects are, as you state, “are beyond our current capacity”. I can draw up plans for any number of projects that are easily enough engineered on paper but would require technological leaps and financing that is just not available right now or even practical.

I disagree. Just because someone can run the delta-V equations of doing a thing doesn’t mean the cost of doing that thing wouldn’t be prohibitive to the cost/benefit ratio to the overall goal. And I did say in my post I didn’t think it was impossible.

For instance, there have proposals to launch nuclear by-products and waste into the sun. Could we do it? Of course. It’s easy enough to engineer but very difficult to do, though not impossible. The Parker Solar Probe is perhaps the poster child for how we would, from Earth, successfully launch a rocket payload into the Sun. So why aren’t we doing it. Cost to benefit ratio.

In fact, if dragging asteroids is relatively easy, why haven’t we done it yet? Evidence suggests that there are trillions of dollars’ worth of minerals and metals buried in asteroids. That seems like a more immediate payoff than fabricating space cables.

126

What benefit is there to launching nuclear waste into the sun that isn’t gained by putting it in a geologically stable hole in the ground? We will never launch waste into orbit, much less the sun, because there is no reason to. Not unless it’s a couple orders of magnitude worse than nuclear waste, at least.

Because moving mass in space is relatively easy, but moving mass out of Earth’s gravity well and through Earth’s atmosphere is an enormous challenge. One that we are barely in the infancy of meeting economically. Once we are putting big things into orbit reliably - an immense hurdle - nudging things around in space is unquestionably an easier problem to solve.

127

No, not that’s not what I mean. I was referring to the point I was trying to make in Post #92 and then again in Post #96.

Basically the issue is that the difference in the sun’s gravitational potential at the radius of the asteroid belt compared to at the radius of the earth’s orbit is much, much less than between the earth’s orbit and one very close to the sun, or spiraling into it.

To put it a different way, for any mass in a solar orbit, you can’t just arbitrarily “steer” it because no matter what you do, you just end up changing the radius and/or shape of the orbit. To get from the asteroid belt to earth, you have to bleed off enough energy from the higher (albeit slower) orbit that the mass falls into a lower orbit, in the process picking up speed, and if you do it just right you end up more or less in a co-orbit with the earth. You might want to park the asteroid at a stable L4 or L5 sun-earth Lagrange point.

Whereas when trying to descend into an orbit close to, or into, the sun, you’re dealing with a gravity well whose strength increases very rapidly due to the inverse-square law. That strength is represented by the potential energy of the higher orbit, and rapidly translates into kinetic energy (higher speed) as you descend, and so that’s a lot more energy that you have to bleed off or else you end up just whizzing round and round at just a slightly lower orbit.

128

For clarity’s sake, I was talking about bringing an asteroid into geostationary orbit around earth where it could be mined for it’s ‘trillions of dollars worth of minerals and metals’.

129

So am I. That would require an immense investment in terms of lifting an orbital tug and enough fuel to bring the asteroid over off the surface, which I presume is where they would be made.

130

Then that seems to validate my post that, ‘just bringing over an asteroid’ for a counter-weight on a space elevator is hand-waving away the complexity and prohibitive costs.

131

I think putting anything significant into orbit is prohibitively difficult. If we could wave a wand and put whatever we built into orbit instantly, then it would be easy (I won’t say trivial because we haven’t done it, but certainly easier than putting mass into orbit from the surface for sure) to go and tug an object into a different orbit.

Which is exactly why a space elevator is so enticing - almost regardless of the cost, the benefit is basically “nearly free access to orbit”, which is huge.

132

To be sure, capturing an asteroid would be an impressive feat of engineering. But moving a mass from the asteroid belt to Earth orbit is much easier than moving the same mass from the surface of the Earth to orbit. You don’t have to deal with the Earth’s gravity well, and it’s all in vacuum and in orbit, so you can get away with using low-thrust, high-efficiency engines for the whole time. It’d be slow, but what’s a few years to a lump of nickel?

133

You’d want to be sure you had reliable reverse low-thrust engines, too … wouldn’t want us to end up like the non-avian dinosaurs.

;~)

134

It should be pointed out that not all asteroids are in the asteroid belt:

Some of the Near Earth asteroids are in orbits that are actually more difficult to reach than going out to the asteroid belt, but at least some of them are pretty accessible in terms of ∆v. But to move large masses around in space is certainly going to require advances in propulsion technology as well as new methods to capture and control asteroids or bulk masses. As for a counterweight, would makes sense to build something useful, such as an array of solar panels shading radiators, rather than a raw asteroid.

The general problems with constructing, maintaining, and protecting a space elevator still remain.

Stranger

135

Just out of curiosity, with our current engine technology, what kind of time scales are we looking at - jetting a tug out to the asteroid belt, connecting it up, and pushing it back to earths orbit? (Assuming we had advances in propulsion technology and a way carry enough fuel)

136

There is no extant (or proposed near term) propulsion technology which would enable to a spacecraft to travel out to the asteroid belt, attach itself to a large asteroid, and propel it back to Earth orbit in a human timescale (i.e. a few decades). Even if there were a propulsion technology that could send a sizable spacecraft to the asteroid belt with sufficient impulse to return an asteroid, it would be essentially required to extract and process propellants in situ. This is notwithstanding that most asteroids of moderate size are loose aggregations of material held together by their tenuous self-gravitation.

Stranger

137

Emphasis mine.

We had the proposed ARM mission from NASA a few years ago, that was eventually cut.

Aside from the “large” qualifier, ARM would have been exactly that.

138

By the time the carpet on the upper parts was replaced from wear and tear, it would be necessary to start replacing the carpet at the bottom again, therefore we cannot have a staircase to the moon.

139

Property Maintenance Coordinator here, can confirm.

140

The Asteroid Redirect (or Retrieval) Mission (the name and goals changed over the course of its evolution) was focused on bringing a small Near Earth asteroid (likely from the Atens or Apollo groups) back to lunar orbit. Although the linked white paper says “a near-Earth asteroid with a mass up to 1000 t”, detailed planning was actually focused on a return payload on the order of 1 ton, captured in a net or bag to prevent it from disaggregating. Solar insolation at the distance of the asteroid belt is 25% to 7% of what it is at Earth orbit, which would require scaling up solar panels and the associated inert mass by a factor of 4 to 16 to get the same power as to intercept a Near Earth asteroid.

Stranger