I’ve seen it said (but will have to check and provide links) that the kinetic energy of a hypervelocity warhead arriving from orbit is so overwhelming that nuclear or chemical explosives are unnecessary. I suspect that’s the reason for some moratoria on space weapons.

I don’t support the development or escalation of military technology or influence, particularly the militarisation of space, but I do acknowledge that we have had some 70 years of “relative” global peace, in part because of nuclear weapons.

So this is an idea about something that might be technically possible, without an explicit view on whether it’s a good thing or not.

As of January 2019, more than 128 million bits of debris smaller than 1 cm (0.4 in), about 900,000 pieces of debris 1–10 cm, and around 34,000 of pieces larger than 10 cm were estimated to be in orbit around the Earth.

And it’s worth noting that Spacex Starlink has started it’s programme to add 12,000 satellites to LEO.

These will be in a highly diverse set of orbits, and with a great range of orbital vectors. A sufficiently advanced space administration could send a vehicle to intercept appropriate, massive debris objects (say dead satellites), and compact them into a dense mass (perhaps with some degree of structure/ablative shielding) to maximise post re-entry integrity and kinetic energy.

Such a mission could easily be posed as “a service to mankind” clearing space junk. But it could also have very significant strategic value.

Any such vehicle would need to have very good manoeuvring, tracking and capture capability. But nothing beyond current technology.
-- Frankx, Nov 19 2019

I see problems.

OK, to begin with, are hyperkinetic weapons really comparable to nuclear warheads? Consider 1000kg of material travelling at 10km/s. Its kinetic energy is otoo 10^13 Joules. This is (surprisingly; at least I was surprised) not manifold different from the energy of a 1000kg fission bomb. So, you get an A for that one.

However, does this mean that 1000kg of orbiting mass is as lethal as a nuclear warhead? Not really. To get it to hit the surface of the Earth at 10km/s, you need to put a lot of energy into it. In particular, you have to change its orbit quite drastically - you'd want to put it in a very elliptical orbit, and then tweak it so that the Earth got in the way. In effect, you've got to put in most of the kinetic energy before you can drop it on someone.

Another option would just be to decelerate it, so that its (originally circular) orbit decays and it spirals down to the ground. But that still means you've got to put the energy in to slow it down; and most of that energy will be lost by aerobraking during re-entry (even if your 1000kg of stuff doesn't just burn up).

In effect, having a mass in a roughly circular Earth orbit is energetically equivalent to having it sitting on the ground*. If you want to slam it into a city at high speed, you've got to put that speed into it first.

I've glossed over a few details, and it'll take a while to dry, so please don't touch it in the meantime.

*This whole discussion ignores gravitational potential energy, but the GPE of a 1000kg mass orbiting at say 300km above the earth is only 3x10^8J, which is trivial compared to kinetic energies. That's how much energy you could deliver if you could just stop the orbiting mass in its tracks so that it fell straight down; and even that energy would be mostly lost as reentry heat.
-- MaxwellBuchanan, Nov 19 2019


Having paid an excruciating price (in energy) to haul the stuff out of your planet's gravity well, why not collect it up and do something constructive with it right where it is ? You have huge amounts of free heat (solar input) and there are probably a proportion of the photovoltaics that can be re-used. It will more than likely be something of a steampunk/Heath Robinson (Will Robinson? ) job, but you'll learn a lot in the process.

Of course there will probably be quite a few deaths along the way, but all progress comes with a price tag.
-- 8th of 7, Nov 19 2019


//A for that one// Thanks! <proudly showing off badge/>

I was also surprised when I came across this.

There are other benefits to hyperkinetic missiles from orbit: there’s no launch signature, very little warning, and they’re essentially unstoppable.

Yes, disadvantages too. Orbital mechanics are complicated, and require energy/delta-v/fuel. Re- entry is essentially blind, and some energy is lost to atmospheric heating.

I suspect loss to heat is competitively small- most orbital reentries are designed to maximise this by approaching at a shallow angle to bleed off ke (aerobraking/aerocapture etc) - this would be the opposite, presumably 100km straight through.

[8th] yes, it would be far better to harvest the materials and components in orbit to make something useful and beneficial to mankind. Rather than killing people. Are you feeling ok?
-- Frankx, Nov 20 2019


At some point, someone will presumably build a space-pirate- machine that simply wanders from satellite to satellite, stealing a few solar panels from this one, an antenna from that one... and using them to build an ever-more-powerful space-pirate-machine.

Actually, given the amount of gold in spacecraft (thermal shrouding; all those very expensive aerospace connectors), an orbiting cannibalistic refinery might be the way to go.
-- MaxwellBuchanan, Nov 20 2019


//misfiled from orbit// Captain, it seems we were wrong when we filed that one under S for Satellites...
-- MaxwellBuchanan, Nov 20 2019


That was quick! Already corrected
-- Frankx, Nov 20 2019


Bearing in mind [Max]'s first annotation, one way to effectively use the kinetic energy stored in space junk would be to use a long tether from a second satellite to sling the junk at the target. You need two large satellites. The first is orbiting east like most satellites, picking up the space junk and compacting it. The second is launched into an orbit in the opposite direction, but at a slightly higher or lower orbit. To launch the collected trash, wait until the two orbits happen to line up somewhere above and not too far from the target. Launch a tether that snags the junk as it goes past. The junk starts spinning around the other satellite on the end of the tether. Release the tether at the right time to hit the target. I'm not sure if you want to try to make the second satellite massive enough that it's orbit isn't disturbed too much, or if you just make it about the same mass as the projectile and send it sailing up as the other is thrown down.
-- scad mientist, Nov 21 2019


Remarkably elegant, [scad].

// Are you feeling ok? //

Just lulling you into a false sense of security.

Presumably there must be such a thing as "Lulling someone into a false sense of insecurity" which sounds like it could be usefull in some circumstances.
-- 8th of 7, Nov 21 2019



random, halfbakery