NASA finds gravitational 'space freeway' that runs through solar system
July 19, 2002 6:25 PM   Subscribe

NASA finds gravitational 'space freeway' that runs through solar system... Vorgon jokes aside, this could seriously reduce the amount of energy it takes to move around the solar system. [this is good]
posted by SpecialK (23 comments total)

errr... that would be Vogon, I think. Sorry. I'm going to crawl back under my towel now.
posted by SpecialK at 6:26 PM on July 19, 2002

Hi Karl! Good link.
posted by corpse at 6:53 PM on July 19, 2002

*grin* Thanks, Jeff.
posted by SpecialK at 7:00 PM on July 19, 2002

Good link, but why oh why do they ALWAYS have to mention that killer asteroid...
posted by slater at 7:00 PM on July 19, 2002

Good link, now let's send an unmaned probe to all the planets and moons except Europa.
posted by riffola at 7:10 PM on July 19, 2002

i wanna go to Sirius and get a tan.. *checks space road map* .. bugger, they're doing roadwork , of course.
posted by dabitch at 7:13 PM on July 19, 2002

You can't be sirius about that, dabitch. (sorry, had to be said.)
posted by SpecialK at 7:28 PM on July 19, 2002

*argh* , ok, I'll set 'em up you knock 'em down.
posted by dabitch at 7:42 PM on July 19, 2002

The story mentions that traditional space missions use the slingshot effect to gain momentum from the gravity of near by planets, moons and sun.

I've never understood, and I'm hoping that somebody here can explain, why the energy the craft gains accelerating towards a mass isn't balanced and lost when the craft is pulling away from the mass.
posted by willnot at 8:12 PM on July 19, 2002

Vogons? Double-post.

willnot: it's quite simple. The energy is transferred from one body to another. When you slingshot past Jupiter, your spacecraft "steals" some inertia from that giant planet, by dint of its own minuscule gravitational attraction. The energy is a vast increase for the small craft, but an immeasurable -- in human terms -- decrease for the massive planet. Slowing down works just the opposite, of course.

While this may be an oversimplification, the report seems to suggest that a slingshot effect may be able to attain a kind of "sweet spot" where it takes acceleration from multiple bodies rather than just one. Certainly, those other forces are there; but usually they are dwarfed by the attraction of the body your craft most closely approaches (gravity being an exponential force). This simply increases the number of options in mission planning, which will remain important for a long time to come: the biggest limitation on any interplanetary journey is the amount of fuel you have to carry. Even a small percentage increase in one's range can be invaluable.
posted by dhartung at 8:24 PM on July 19, 2002

Hmm. I should add, to more specifically answer your question, that the reason less energy is lost on the outbound leg depends on the angle. Since you can use the same effect to transfer energy "back", i.e. slow down, what you want to do is select an approach that will maximize -- within your craft's capabilities -- the acceleration, and then shoot out at an angle that will minimize the amount stolen back by the planet -- which, after all, is in motion. Your sentence presupposes a static body, but all bodies in the real universe are actually in motion. It's not the gravity itself that creates the energy transfer; think of that as the means of communicating the transfer of the acceleration of the planet in one direction back to the craft.
posted by dhartung at 8:33 PM on July 19, 2002

There's a bit more detail in the JPL press release:

"Each planet and moon has five locations in space called Lagrange points, where one body's gravity balances another's. Spacecraft can orbit there while burning very little fuel. To find the Interplanetary Superhighway, Lo mapped all the possible flight paths among the Lagrange points, varying the distance the spacecraft would go and how fast or slow it would travel. Like threads twisted together to form a rope, the possible flight paths formed tubes in space. Lo plans to map out these tubes for the whole solar system."

Lagrange points. Learn something new everyday.
posted by homunculus at 8:52 PM on July 19, 2002

willnot, more info on the gravitational slingshot effect can be found here. An explanation with math can be found here.

Key idea: "A gravitational interaction with Jupiter is just like bouncing off it." Gravitational fields are conservative, so ignore the gravitational potential energy gained when you fall in and lost when you climb out. That's not where you're getting your boost: it's Jupiter's kinetic energy that you're tapping into. When you do this trick, Jupiter slows down in its orbit.
posted by Hieronymous Coward at 9:05 PM on July 19, 2002

This calls for a celebratory round of Pan-Galactic Gargle Blasters :-)
posted by clevershark at 9:19 PM on July 19, 2002

No jinnintonix?
posted by NortonDC at 10:38 PM on July 19, 2002

There goes the neighborhood. :(
posted by pekar wood at 12:04 AM on July 20, 2002

Kudos for an interesting thread.
posted by ParisParamus at 12:48 AM on July 20, 2002

Ok, for anyone who's interested and awake I can take a shot at explaining whats going on here.

I think what most people are unaware of is that while the equations of Gravity seem simple, they are in fact intractable in most practical situations. We know how two bodies interact, how the earth orbits the Sun and can calculate these things precisely, we can frame this in equations and provide whats called an analytical solution.

Ok fine. But, now throw a third body into the mix. A interacts with B and C. B interacts with A and C and so on. The thing is that we dont have any exact solutions as to how these bodies will behave. All we can do is well, guess, and we can use the equations we do know to predict how these bodies will interact on short time-scales. We cant do these computations for large time-scales because the interactions of these three bodies has a strong dependence on initial conditions - as your calculations get buried in more and more estimates you get farther from the truth. This is now in the realm of chaotic dynamics.

Chaos may sound like a new concept but this problem has been known for a long time - The Three-Body Problem of dyamics.

Now, we have found some stable solutions of the three-body problem and we can sort of solve it if we assume that one of the masses is too small (e.g. a satellite) to affect the other two masses but its still a surprisingly tough nut to crack.

My favorite solution is the figure 8

Now, Lagrange came along and showed that for the restricted problem (small third body) and if the second body orbits the first (e.g. earth orbits Sun) we have a set of solutions that are stable if the third body is placed in specific locations with respect to the first two. These points are now known as the five Lagrange points.

Here's another way of looking at lagrange points: Say we wanted to create a satellite that orbits the Sun but we wanted it to orbit along with us. Well, theres a problem. Since the satellite weighs much less than the Earth, it cant possibly be in the same orbit with the same period. However, if we place the satellite between the Earth and the Sun so that it still orbits the Sun but the Earth's gravitational attraction is able to balance the Sun's so as to provide a stable orbit well then - that will work.

We do in fact have a satellite at the Lagrange point L1 - its called SOHO.

The Lagrange points are really neat because essentially if we place a satelllite at any of these five points, it will "stay there". It will still orbit around the larger body but it will stay a fixed distance from the smaller body. An asteroid placed at L1 for example, would always be between us and the Sun.

L4 and L5 are really interesting. These two points are also called the Trojan points. A satellite placed here will share the Earth's orbit but always stay a fixed distance away.

There may already in fact be asteroids there and we have been searching for them.

Now, L4 and L5 are stable points. What this means is that if you place a satellite at L4 and then give it a small push away, it will tend to come back to L4. So one way to think of L4 (and L5) is as virtual planets around which you can orbit satellites. Theres actually nothing there - its empty space but the peculiariaties of gravitational laws and orbital dyamics conspire to create this strange zone of stability in space.

So now you probably see where this is going. As dhartung describe above, satellite trajectories are planned so as to maximize the outcomes of gravitational interactions so that satellites will gain momentum as needed. Lo seems to have found a computational model which also takes into account the lagrangian points as an extra source for dumping/acquiring angular momentum and velocity. Throw this in with what we already know about satellite/planetary interactions and we get a kind of roadmap that shows us the paths of minimum energy through the solar system depending on the goals of the mission.
posted by vacapinta at 1:03 AM on July 20, 2002 [1 favorite]

Thanks, vacapinta -- LaGrange points make a lot more sense than faraway planets.

L5 of the Earth-Moon system -- the LaGrange point trailing the moon's orbit around us -- is considered a good place not only for satellites, but for freestanding space colonies; hence the L5 Society of the 1970s.

Larry Niven, especially, wrote about Trojan asteroids in many of his stories, and they show up in a lot of hard sf these days. Jupiter's Trojans {not to scale!} have been known for centuries, long before the LaGrange equations. Another important concept in this mix are the original Hohmann Transfer Orbits, essentially the math for the minimum-energy route from one orbit to a "higher" one and vice-versa. This can be planetary -- say, low-earth orbit to geosynchronous -- or interplanetary, say Earth to Mars. If you're not concerned with how long it takes to get there (bulky supplies, perhaps), you can use a minimum-energy orbit that looks like an oval that touches the inner orbit at one side and the outer orbit at the other side. If it's timed correctly, you could actually have a spacecraft sweep continuously from planet to planet; Kim Stanley Robinson proposes such a commuter method in his Mars trilogy. The big trouble with that is that you have so much angular momentum at your destination -- you're zipping past rather than easing into an orbit.
posted by dhartung at 1:26 AM on July 20, 2002

Technically, we cannot "reduce the amount of energy it takes to move around the solar system." It takes whatever it takes; all we can change is which cheap solutions we know about.

Hieronymous Coward gets the prize for first correct explanation of the slingshot effect: you are not borrowing the planet's gravity energy, you are borrowing orbital energy.

Note that the slingshot is also used to slow down after a fast trip: instead of coming in behind the planet and vaulting past with more speed, you come in ahead and loop backwards with less speed. And you can use it to escape the orbital plane by coming in near the poles. Some of this will be applicable to the Lagrange "freeway" as well.

One of the reasons we have gotten so adept at gravity assist solutions in the last 15 years is that after Challenger, the shuttle astronauts refused to fly with the "Death Star," the Centaur liquid-fueled upper stage, in the cargo bay. Thus anything the Shuttle carries into space has to use the wimpier solid-fueled Inertial Upper Stage (IUS), necessitating lots of slingshotting to get the delta Vee for planetary missions.
posted by anser at 7:51 AM on July 20, 2002

And, since all the planets and moons and their gravitational influences are in motion, the layout of the "freeway" network will be constantly moving itself; mission planners will need to calculate the network as a function of time, rather than a static map. These paths will be more like trade winds than roads, with launches needing to coincide with the right conditions (or future conditions) in the network.
posted by cardboard at 8:47 AM on July 20, 2002

Hmn, trade winds... I wonder if the 'freeway' network will make solar sails a practical way to provide most of the power for a probe?
posted by SpecialK at 9:36 AM on July 20, 2002

anser: While my response may not have been concise, I do not believe it was incorrect, Mr. Superior-Acting Ass. And I did beat HC. So there.

SpecialK: solar sails will be a practical means of providing power, which is quite a different thing from using gravity assists. They may be combined, or not. It's all about how quickly you want/need to get there.
posted by dhartung at 1:47 PM on July 20, 2002

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