3I/ATLAS: Probably NOT an alien interstellar probe

[Loren Pechtel]

Many things impact earth and even after the impact, are still cold.

Two points:

1) what’s the delta v in these cases? Neither object is moving at a velocity that can escape the solar system.

2) the earth has an atmosphere, so the impact happens over a large distance and the impactor goes through a lot of ablation.

3) what do you mean by "cold"? Anything arriving from deep space is at a couple of Kelvins; It will arrive at the Earth's surface at more like hundreds of Kelvins. That sounds pretty "hot".

Most things that hit Earth will be in the ballpark of Earth temperature before they burn in.

 

[Loren Pechtel]

I have a hypothetical question unrelated to the present interstellar comet. Assume a space-craft launched from Earth with only a limited amount of stored energy aboard. After achieving a desired initial trajectory it has enough fuel left for some course corrections. (Instead of wasting matter on impulse, suppose it steers magnetically.)

My question is: ASSUMING that some of the major planets line up propitiously for the space-craft to approach them dozens of times in a long series of ever-changing ellipse-like trajectories, gaining more and more energy from the "sling shots" what is the maximum speed it could have when it finally escapes the solar system? Assume that the missionaries are very patient and are willing to spend hundreds of centuries on the "sling-shotting." (Assume as well, perfect computation of future orbits.)

Once you have solar escape you can't do more than a few encounters because you won't come back. Soon you run out of planets in the right place and that's it. Note that for our system Jupiter is enough to give escape, only three more encounters are possible--and that happens less than once a century. Pioneer and Voyager were about the best that can be done--that's why they flew when they did and nothing like them since.

 

[Loren Pechtel]

The maximum velocity increase achievable relative to the Sun, by gravitational slingshot from a planet, is twice the planet's orbital velocity. So the best you can do in the Solar System with a single manoevre is about 96km/s (twice the orbital velocity of Mercury).

Each slingshot manoevre is effectively a perfectly elastic 'bounce', transferring momentum to your spacecraft from tne planet. As far as I can see, if you have infinite time for the task, you could keep doing such manoevres until you have stolen all of the momentum from everything in the Solar System, and all the planets have fallen into the Sun.

Your final velocity would be huge in such a scenario; although your accelerations would get very small as your probe's mass increased due to its speed. You should achieve a sizable fraction of c for any probe with a reasonable rest-mass.

Two problems:

1) They're not in the right place to do this.

2) While in theory you could get 96km/sec out of Mercury look at the periapsis. It's way, way below the surface.

 

[Loren Pechtel]

AFIAK you can't build something to survive a supersonic impact--measured using the speed of sound in the object, not the environment. Force can't be transferred faster than local sound velocity, thus any possible crush mechanism on the front can't slow down the payload. In the real world we see this with armor-penetrating weapons. Once the weapon is hitting the target all that matters is mass and energy. Strength only matters to the extent that you need to hold things together--note that plasma jets are plasma, they have no strength whatsoever. Lithobraking is usually treated as a joke, in practice it's never been used beyond tens of miles an hour. (Those Mars rovers that were basically wrapped in airbags.)

Interesting!
So an ablating suspension system is required? In the case of something moving a at tens or hundreds of miles per second, it might have to be a pretty elaborate so the actual payload experiences no acceleration greater than the rate at which it can be transmitted through the body of that payload… get to work Loren, I know you can figure it out!

No, the point is there is no possible suspension as you can only transmit the force at the speed of sound.

 

[Elixir]

No, the point is there is no possible suspension as you can only transmit the force at the speed of sound.

Yet you can continuously apply 16000 gs to a tardigrade for a full minute without killing it?

 

[Shadowy Man]

One problem seems to be an assumption that rocket sizes scale linearly with payload sizes. This may be true down to some size, below which I presume the rockets themselves and the fuel needed to gain a certain velocity become the primary driving parameter. A payload the size of a grain of sand may require the same size rocket as one the size of a refrigerator to reach the same velocity.

That being said, slamming a refrigerator into an interstellar comet at 10 km/s will likely result in only grain of sand size parts surviving.

 

[Elixir]

That being said, slamming a refrigerator into an interstellar comet at 10 km/s will likely result in only grain of sand size parts surviving.

Does that imply that slamming an interstellar comet with a grain of sand at 10kps will likely result in only refrigerator-sized parts surviving?

 

[steve_bank]

I am working on a movie script, 'Escape From The Planet Of The Humans'.

 

[Jimmy Higgins]

One problem seems to be an assumption that rocket sizes scale linearly with payload sizes. This may be true down to some size, below which I presume the rockets themselves and the fuel needed to gain a certain velocity become the primary driving parameter. A payload the size of a grain of sand may require the same size rocket as one the size of a refrigerator to reach the same velocity. That being said, slamming a refrigerator into an interstellar comet at 10 km/s will likely result in only grain of sand size parts surviving.

We could leave it in its original polystyrene protective packaging.

 

[Swammerdami]

I'm afraid I should disassociate myself from my foolish question. I imagined a long series of ever-enlarging ellipses, with ever-increasing velocities at perigee, while ignoring that the velocity (relative to escape direction, but prior to actual escape) is always just zero at apogee.

Can we please just pretend that I was held prisoner in the nearby dungeon while an ignorant Grok fiend took control of my laptop to embarrass me?

 

[bilby]

The maximum velocity increase achievable relative to the Sun, by gravitational slingshot from a planet, is twice the planet's orbital velocity. So the best you can do in the Solar System with a single manoevre is about 96km/s (twice the orbital velocity of Mercury).

Each slingshot manoevre is effectively a perfectly elastic 'bounce', transferring momentum to your spacecraft from tne planet. As far as I can see, if you have infinite time for the task, you could keep doing such manoevres until you have stolen all of the momentum from everything in the Solar System, and all the planets have fallen into the Sun.

Your final velocity would be huge in such a scenario; although your accelerations would get very small as your probe's mass increased due to its speed. You should achieve a sizable fraction of c for any probe with a reasonable rest-mass.

Two problems:

1) They're not in the right place to do this.

The setup in the question assumes that we can wait until they are, however long that takes.

2) While in theory you could get 96km/sec out of Mercury look at the periapsis. It's way, way below the surface.

Sure, but we are looking for an upper bound to the achievable velocity relative to the Sun. Some assumptions are justified, such as treating planets as point masses. It's not like I am asking you to consider a spherical planet in a vacuum...