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ottomadenedamotto
03-26-2011, 08:31 AM
So, I've got no real education in physics, etc. and I could definitely use a hand. I'm working on a science-fiction story involving commercialized inter-planetary travel. It seems to me that everything orbits the sun in the same direction, and even comets and the like move in that direction, once they get near enough.

For a vessel which is able to travel in under a month to any planet in its star-system (which, for the sake of argument, let's say is identical to our own,) would it likely be feasible (especially in terms of fuel-usage, the difference in required power, and the physical stress exerted on the vehicle) to shorten a trip by traveling in the opposite direction as its initial orbit? Obviously, this would be a more expensive option, but I'd like to get at least a rough estimate of how much more fuel, power, and vehicular fortitude it would require, or if it would probably be beyond the technology and means of a civilization whose typical starship can at best be expected to make the sort of trip specified above.

Thanks in advance.

Drachen Jager
03-26-2011, 08:54 AM
The fuel difference would depend entirely on the nature of the trip. Nobody could really guess unless they knew the specifics. But if you had the power to travel anywhere in the solar system within a month power would not be a problem. You're talking about fighting the gravity of the sun, which is less than the force the Moon imparts on Earth, which is very very little. Getting from the surface into orbit takes far more power and requires a vehicle that can handle far more stress.

So, fuel, probably minor difference (if the new anti-orbit path is faster it probably also consumes less fuel though) power, no big deal, stress, no big deal.

blacbird
03-27-2011, 10:31 PM
So, I've got no real education in physics, etc. and I could definitely use a hand. I'm working on a science-fiction story involving commercialized inter-planetary travel. It seems to me that everything orbits the sun in the same direction, and even comets and the like move in that direction, once they get near enough.

The bolded is not correct. Comets did not originate in the original planetary debris disk within which the major planets formed and the sun rotates. An individual comet may come at the sun from any direction, and swing around it in any orientation.

Kenn
03-28-2011, 12:07 AM
...For a vessel which is able to travel in under a month to any planet in its star-system (which, for the sake of argument, let's say is identical to our own,) would it likely be feasible (especially in terms of fuel-usage, the difference in required power, and the physical stress exerted on the vehicle) to shorten a trip by traveling in the opposite direction as its initial orbit?
I must admit, I can't see a logical reason why that should be the case. Maybe I've misunderstood what you are asking. What do you mean by 'its initial orbit' (the spacecraft?). If so, then it is irrelevant. Its initial velocity will depend on the vector components of the Earth's orbital velocity and the velocity of the spacecraft orbiting the Earth. The direction of orbit around the Earth is irrelevant unless you are viewing it from a point on the Earth's surface.

The orbital speed of the Earth will dominate (about 30 km/s as opposed to an orbital speed of a satellite of several km/s). So the logical solution is to time the planetary poitions and to slingshot the spaceship in the right direction.

ottomadenedamotto
03-28-2011, 11:11 PM
Thanks, everyone. I think I've got what I need.

Anaximander
03-29-2011, 01:15 AM
There's no particular force that makes anything orbit a particular way; it's just dependent on how they get dropped into the gravity well. If you're launching from an orbiting body like a planet, though, then you'll need quite a shove to reverse direction - it'll take a lot of fuel. Plus, as you slow, you'll drop in toward the star, and then depending on where you're going you'll have to climb back out of the gravity well - again, that'll take a fair bit of fuel.

movieman
03-29-2011, 03:49 AM
If I remember correctly, reversing your orbit takes around 60km/second at Earth's distance from the sun. While that would also result in falling towards the sun, so long as you change orbit fast enough it wouldn't be a problem; I haven't done the numbers, but 'fast enough' would probably be in the order of hours to days.

Note of course that you then need to reverse your orbit again at the other end of the trip, so without far-future engines it's an expensive operation.

Michael Davis
03-29-2011, 07:22 PM
Ok, I'm going back many years to my astrophysics classes but here's my take:

Some planets/moons do go in reverse orbit to the spin of the planet. It depends on how they were formed (e.g. aggregation of pre-formation gas cloud, sucked off orbit and captured (like one of Neptune moons, or by collision).

In regard to a craft, a space vehicle would sense more degrating force vectors if in reverve ordit, but unless you're going to be a station orbiting for long periods, it should be a 3rd order affect (e.g. very minor).

That's my take, hope it helps.

Forlorn Radiance
03-30-2011, 08:46 AM
If you want to get really fancy with your fuel issues, google ghostly neutrinos. If we could find a way to harness those buggers, we could have a renewable, indefinite, efficient, free (minus the equipment need and upgrades) energy source. Hope you check it out and that its useful.

SOURCE: I was an astrophysics major. Articles on the above subject are plentiful, though the technology is still beyond us.

RJK
03-30-2011, 08:58 PM
What I know about astrophysics would fit in a shoe box. If you're referring to reaching orbit from earth, you would need to take the earth's spin into affect. Launching from near the equator gives you nearly a 915 mile an hour boost. If you want to launch to a retrograde orbit, you need to overcome that 915 MPH speed in addition to the 17,000 MPH, needed to reach orbital speed. It's not much, but it adds to the fuel requirements.

Terry L. Sanders
04-02-2011, 09:44 PM
Sorry for coming in late on this one...

Unless you're talking "magic warp drive," it's gonna make things much worse. What you're talking about is commonly called a "retrograde orbit." Putting a space vehicle in one is usually considered a Bad Idea, for several reasons.

1.
The thing that uses fuel in a spacecraft isn't distance and/or speed, the way it is on Earth. It's how much you have to CHANGE your velocity (You speed up to go outward, to Mars, the asteroid belt, or beyond. You slow down to go inward, to Venus or Mercury.) The space travel guys call it "delta-V."

You push yourself to the right speed and then coast for most of the distance. No friction, nothing to slow you down. Then when you get there, you burn more fuel to match velocities with your destination planet (or whatever).

To go to Mars retrograde, you'd have to use fuel to push yourself backward to "speed zero" relative to the sun. As some here have said, that's about 30 km/s. Then you'd keep pushing until you were back to something vaguely like orbital velocity in the other direction. another 30 km/s. Then, once you got to Mars, you'd have to reverse the process. That's something like 120 km/s of speed change, most of which you didn't need to get there if you were a bit more patient!

Most of the "magic space drives" assume you don't have to worry about fuel--you just keep accelerating the whole trip. But that means you're moving at a really incredible velocity at the midpoint, and you have to turn around to slow down for the whole second half of the trip. A ship that can do that can take just about any course it wants, but it has other problems.

2.
If you're moving backwards to everything else in the solar system, there are no minor collisions. You're moving 30km/s one way, and that grain of sand is moving 30 km/s the other. Gives a whole new meaning to "head-on."

If I remember right, once upon a time, the United States had (may still) an anti-satellite weapon system, for use in wartime against spysats. It didn't use an explosive device at all--an F-15 would launch a missile up to orbital altitude, where the missile would simply maneuver into the satellite's path.
The missile is just hanging there, almost ready to fall down. Relative to the earth, it's not really moving at all. The satellite moves at 10+ km/s. Bam. Instant debris.

And that's a lot slower than the speeds we're talking about here. Better add a good deflector shield.

--

Not to say it doesn't work--with unlimited power you can do just about anything. And if the planets are in the right position a retrograde path of sorts would be just what the supership would do. But there would be a fuel penalty for doing it that way.

Then again, if you've got that kind of power, it might not be that big a difference. You're already using so much...

You might try this site (http://www.projectrho.com/rocket/). It's mostly aimed at people doing old-fashioned rockets, but it has sections covering how to use a "magic space drive" in a solar system...

Rammstein
04-08-2011, 07:19 PM
Check up on Langrangian points (http://en.wikipedia.org/wiki/Lagrangian_Point). They're basically points in and around the orbit of the planets and moons of our solar system where a spacecraft (or pretty much anything) could remain stationary by orbiting a gravitationally stable point. These points can and have been used already in interplanetary flight. Here's how (http://ccar.colorado.edu/asen5050/projects/projects_2003/cain/) they could be used in your story.