Great work, Stranger: http://www.straightdope.com/mailbag/mgravslingshot.htm
A great read. Thanks to Stranger.
Cool article.
Tiny, tiny nitpick: The reference to “the recent New Horizons mission” should probably be something more like “recently used in the ongoing New Horizons mission” because the former phraseology implies completion of the project. That won’t happen until the spacecraft arrives at Pluto/Charon in eight years. The New Horizons “slingshot” at Jupiter did happen less than a month ago, but the overall mission is far from over.
(And if you’re not impressed by New Horizons getting from Earth to Jupiter in 400 days, you should be.)
</tiny nitpick>
Regardless, a good and informative article. I especially liked having the quotes from the rocket-science types, who rarely get the opportunity to explain to the public what they do. Nice touch.
Nice article, but you didn’t really address one of the questions asked: Does the spacecraft pick up speed from the swing-by? Your answer is that it mostly changes direction, but according to the link you included in your answer. it also picks up some significant speed because the planet it is swinging by is not stationary. If the planet were stationary the speed after leaving the planet’s influence would be the same as when it entered the influence. But since the planet is also moving in it’s own orbit around the sun, the spacecraft leaves the planet’s influence with some additional speed due to the planets orbital speed.
I would be curious about how much of an effect this is numerically. Could your contacts at JPL give an example assuming a recent flyby example where they force Jupiter to be stationary vs moving and see what difference it makes in the speed of the spacecraft? Or is that too hard to do?
Great report, but I also had trouble working out the gravity assist mechanism. Those who are similarly confused should click the NASA link that Stranger provided: it’s a solid treatment (with diagrams), albeit a little dryer. Here it is again: http://www2.jpl.nasa.gov/basics/grav/primer.html
I assume you mean final speed relative to the Sun? I don’t see any way the final speed relative to the Sun could be any different post-swing versus pre-swing assuming a stationary planet with no atmosphere, provided there wasn’t also some active propulsion going on at the same time.
Yes, I agree, the link has a good description with diagrams, but it is also missing any numerical results. I repeat my request: I would be curious about how much of an effect this is numerically. Could your contacts at JPL give an example assuming a recent flyby example where they force Jupiter to be stationary vs moving and see what difference it makes in the speed of the spacecraft? Or is that too hard to do?
Why would you think the final speed would be significantly different relative to the sun post-swing around a stationary-relative-to-the-sun Jupiter? (If it is significantly different post-swing, it is I who am missing something here.)
Now, the direction might be different, and I’d imagine even with stationary (!) planets, space missions would still swing around them for changes in velocity (before they fell into the Sun that is :))
This was a great article. Thanks Stranger.
Great article. I’m reminded that, in his novelization of 2001: A Space Odyssey, Arthur C. Clarke has the American spacecraft Discovery using a gravity-assist “slingshot” to reach the moon Io. He noted that Mother Nature always balances her books: the force of acceleration the spacecraft picked up was offset by Jupiter slowing down just as much (a miniscule amount, compared to how massive the planet is).
Very good article, SoaT. I see that it’s future dated - can normal punters see the upcoming articles anywhere?
The reason I think the speed will be significantly different is because of what I read in the link to the JPL site from the original article ( http://www2.jpl.nasa.gov/basics/grav/primer.html ). Here is some text from that webpage (about half way down):
There is a drawing next to this paragraph where some velocity vectors are shown. By my eye, it looks like the spacecraft SPEED is almost doubled (and the direction is also changed, of course). But it is not clear if this diagram is drawn to scale or if the effect was exagerated to make a nice graphic. That is why I want to know the numerical significance of the speed change - is it 100%, 30% or 3%?
By the way, I continually talk about SPEED in these postings because that is the magnitude of the velocity vector. It is very clear that the DIRECTION of the velocity vector can change by an arbitrarily large amount during an encounter with a planet, but I am interested in the typical percentage change in speed of the spacecraft between a time long before it encounters the planet to a time long after it encounters the planet.
It won’t appear on the SD home page until Tuesday. Newsletter subscribers get the link on Fridays. Of course, by the following Tuesday there’s usually a comments thread about the report, so you can pick up the link there.
Take a look at the link: http://www2.jpl.nasa.gov/basics/grav/primer.html
I agree that with a stationary planet the final speed should equal the initial speed, but see the link where it explains that a moving planet can change the speed of the spacecraft. I just want to know the relative magnitude of the speed change effect.
A very good article, Stranger.
One question of my own, here, though - I thought that there were also benefits to using the (very limited) on-board reaction mass in the ‘bottom’ of a gravity assist maneuver by allowing the probe to ‘cheat’ a little, and keep some of the kinetic energy that had been converted from potential energy on the way ‘down’ the gravity well. A matter of getting more ‘bang for the buck’ on the fuel in the probe, as I understand it. Is this understanding correct?
Great article, but the thing needs a spell check: I noticed “Sllide rules” and that was on a cursory pass. Plus, the typeface changes for the last six words of the second paragraph and in various other places. (It becomes Times New Roman, according to the page source.) Finally, I’m a picky, pedantic man.
Because subscribers are alerted on Friday, but the Official Post isn’t until the next Tuesday, it is sometimes the case that the proofing has not been as thorough as one might like. THanks for those mentioning typos, we’re glad to hear of them and will (I hope) have them fixed before Tuesday’s Official Date.
Meantime, thanks, Stranger, for a very engaging (not to say “stellar”) report!
Yes! I had forgotten that I also had heard that burning the engine at the bottom of the gravity well will give “more bang for the buck”. I found a very good explanation of this fact on Wikipedia: Gravity assist - Wikipedia . Basically a given rocket motor burn will change the spacecraft velocity by a certain delta-V but if that velocity is added when the velocity is very high - like deep in a gravity well - the change in kinetic energy, which is proportional to the velocity squared, will increase much more than when the same delta-V is added when the spacecraft is at a lower velocity (as in deep space).
This same Wikipedia article basically answers my question about the numerical magnitude of the change in speed effect with an example of the Cassini probe: without the multiple gravity assists, the delta-V required to go directly from the Earth to Saturn would have been 15.7 km/sec but with the four gravity assists (Venus, Earth, Venus and Jupiter) the delta-V required was only 2 km/sec. Several of these delta-V engine burns were when the spacecraft was deep in gravitational wells. So the gravity assist effect is very large and does include doing engine burns deep in the gravity wells.
So before this article is published on the website next Tuesday, I think that some of these kinds of facts and examples should be added to it. I did like the article overall and enjoyed reading about the software history and the effort required to compute these complex orbits, but I think some of the fact outlined above should be included in the article. Just my $0.02.
FrankH, I’m not sure that the article will be made more effective by including the hijack we’re starting, here. You and I seem to have little problem with the concept of taking advantage of the changes between kinetic and potential energies, when dealing with when to make a burn on a space probe. (I’m not claiming I could describe it mathematically, just that I’m not claiming it’s PFM, either.) For Stranger to make that same set of explainations available and comprensible to a physics neophyte would likely double or treble the size of his already large article.
I don’t doubt he’d do an excellent job of it, just thinking that it’s a case of having chosen to limit the scope of the answer he gave to a question that could have gone on for textbooks worth of verbiage.
OtakuLoki, I guess I agree the article shouldn’t be excessively lengthened to try to include these effects, but I think that at least mentioning the 2 km/sec vs 15.7 km/sec difference for the Cassini probe would be very useful and I think there might at least be a brief mention of utility of the rocket engine burn while deep in the gravity well since Cassini definitely took advantage of that effect. I guess I forgot that these articles are aimed at the masses and not scientists. So I guess it is up to Stranger to make the final decision…