1
$\begingroup$

We hear a lot about space elevators that could place something in orbit, but what about a space road between planets? Suppose you had a long tube or flat road placed between Earth and Mars by chemical rockets, could you use a vehicle with solar panels and electric motors to reach Mars using just electricity? Also since there's no force of gravity the engineering would be simpler than a space elevator.

$\endgroup$
8
  • $\begingroup$ Well technically yes, but it wouldn’t be a straight line, it would be lots of orbital rings attached to each other. Check out this episode from Isaac Arthur m.youtube.com/… $\endgroup$ Commented Mar 28 at 18:53
  • 1
    $\begingroup$ Hasn't this been debated for a long time? $\endgroup$ Commented Mar 28 at 20:56
  • 10
    $\begingroup$ "Also since there's no force of gravity the engineering would be simpler than a space elevator"; except there IS the force of gravity, both at the planet you leave, at the planet you arrive at, everywhere in between and everywhere else too. It's why the planets orbit and stars collect into galaxies. $\endgroup$
    – Erin Anne
    Commented Mar 28 at 21:54
  • 3
    $\begingroup$ @phil1008 oh gee, The Atlantis Project and Newtonian Screw Shafts. Where have I seen this before without someone clearly disclosing their interest in promoting it? $\endgroup$
    – Erin Anne
    Commented Mar 28 at 23:02
  • 5
    $\begingroup$ @phil1008 topics you generally like to promote? you're listed as one of the editors. I think you've changed timestamps now so it just shows a concept from a paper you have commercial interest in promoting instead of immediately also mentioning The Atlantis Project, which you also have commercial interest in promoting, but it's your responsibility to mention that to not be a spammer $\endgroup$
    – Erin Anne
    Commented Mar 29 at 17:26

1 Answer 1

12
$\begingroup$

The trouble you're going to run into there is that the planets spin and move, which makes it hard to have a road or whatever in place. It's a little like trying to build a tower to reach an airplane that's in flight. You can't do it, the plane won't hold still.

First, we have the rotation problem. Suppose you want to build an elevator to the moon. We have a base somewhere on Earth's surface, and attach the other end to the Moon. Only... if you wait twelve hours, the earth has turned around to face the other way, and the moon is now on the wrong side of the planet. Your structure can't go through the planet to reach the moon, so that's gonna be a problem right there.

The concept of a space elevator is to string your elevator between the surface and a station placed in geosynchronous Earth orbit ("GEO") -- that is, an orbit at a very particular altitude where it takes exactly 24 hours to go around Earth, and thus hovers directly over a single point on the surface.

What if we don't try to fix it to Earth's surface, though? We could start from somewhere in orbit, maybe beyond the moon, and built a highway to Mars.

Well... This is the revolution problem. Before we build a highway, let's just put a string in place. If you imagine stretching a single thread between Earth and Mars, well, Mars takes around 687 days to go around the sun once, while Earth makes the same revolution in 365 days (give or take a bit). If we string our thread when the planets are at the absolute closest point, then wait a bit, the planets will move apart, so we have to feed out more thread or it'll break.

Worse, it takes nearly 2 Earth years for Mars to go around once, so if we wait six months or so, we inevitably end up in a situation where Mars and Earth are on opposite sides of the sun, which is to say, our thread is passing directly through the sun. This is generally considered unsafe for travel.

There are other options, though. If you build a space elevator on Earth (impossible with current technology) and then also build a space elevator on the Moon (which could be done with currently available material, if you could get it there or make it on the moon in sufficient quantities), you could go up to the GEO station by solar-electric power, make a relatively cheap rocket trip to the moon station, and then go down the elevator there by solar-electric power, and that would cut most of the cost off of your trip.

Actually, if you have a GEO station and then extend a counterweight out into an orbit beyond GEO, in theory the counterweight would be getting pulled along faster than orbital velocity, which is to say you'd feel as if the counterweight was "down" from the GEO station -- centrifugal force would be pulling you away from Earth, and grow stronger the further from GEO you got. You could slide down and let go at the end of the counterweight line to be flung into an elliptical orbit that could potentially reach the moon without any fuel at all. There's some tricky physics involved, but it's entirely possible that with the right timing, you could have regular moon service without spending any rocket fuel, which isn't a road, but might be the next best thing.

$\endgroup$
8
  • 5
    $\begingroup$ "which isn't a road, but might be the next best thing." Honestly, it's probably better in every way, both from the economic and engineering standpoint of not having to build the road, and the time standpoint of not having to travel interplanetary distances at the kind of speeds you can interface with a road or track at. $\endgroup$
    – Erin Anne
    Commented Mar 28 at 21:56
  • 1
    $\begingroup$ The limitation is timing -- a road would always be there, ready to use, while the counterweight scheme is more like a daily launch window. I agree that it's probably a better plan from the engineering standpoint than building hamster tubes to other planets. $\endgroup$ Commented Mar 29 at 15:09
  • 3
    $\begingroup$ "our thread is passing directly through the sun. This is generally considered unsafe for travel." lol $\endgroup$ Commented Mar 29 at 18:22
  • $\begingroup$ @DarthPseudonym The travel+wait time will still be shorter than trying to go along a road. $\endgroup$
    – SF.
    Commented Apr 2 at 12:37
  • $\begingroup$ @SF. I'm not sure how you can state that categorically without presupposing the characteristics of the 'road'. If we imagine a tube from Here to There that's lined with solar collectors that power magnetic accelerators, for example, then I would think we'd be traveling much faster, since we could potentially apply constant acceleration for the entire trip. $\endgroup$ Commented Apr 2 at 13:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.