stein-c@acsu.Buffalo.EDU (Craig Steinberger) (10/20/89)
Can anyone explain how Galileo will be gaining energy by flying near planets? I understand that the spacecraft will be gaining velocity due to gravity forces as it approaches Venus, etc., but wouldn't all of that energy be lost as Galileo goes away from the planet and has to go against gravity forces? The only way I can see it is if Galileo picks up some of the energy from the velocity of the planet. Thank you, Craig Steinberger =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Transcendentalism is all very fine, | Craig Steinberger but how does that relate to what | SUNY at Buffalo, Aerospace Engineering you see in the subway? | stein-c@cs.Buffalo.EDU
gwh@typhoon.Berkeley.EDU (George William Herbert) (10/20/89)
In article <12027@eerie.acsu.Buffalo.EDU> stein-c@autarch.acsu.buffalo.edu (Craig Steinberger) writes: >Can anyone explain how Galileo will be gaining energy by flying near >planets? I understand that the spacecraft will be gaining velocity due >to gravity forces as it approaches Venus, etc., but wouldn't all of that >energy be lost as Galileo goes away from the planet and has to go >against gravity forces? The only way I can see it is if Galileo picks up >some of the energy from the velocity of the planet. That's the idea. It's a momentum transfer... Planet slows down by some mniscule amount, spacecraft speeds up a LOT. **************************************** George William Herbert UCB Naval Architecture Dpt. (my god, even on schedule!) maniac@garnet.berkeley.edu gwh@ocf.berkeley.edu ----------------------------------------
aae391aa@uxh.cso.uiuc.edu (10/20/89)
4:00 pm Oct 19, 1989, stein-c@acsu.Buffalo.EDU writes: > Can anyone explain how Galileo will be gaining energy by flying near > planets? I understand that the spacecraft will be gaining velocity due > to gravity forces as it approaches Venus, etc., but wouldn't all of that > energy be lost as Galileo goes away from the planet and has to go > against gravity forces? The only way I can see it is if Galileo picks up > some of the energy from the velocity of the planet. The spacecraft does not get a boost in its speed (it speeds up approaching a planet, e.g. Venus, but slows back down leaving), but rather a change in its direction of motion as it sling-shots around Venus, and thus a change in the orbital trajectory about the sun. Erik A. Johnson johnsone@uxh.cso.uiuc.edu
henry@utzoo.uucp (Henry Spencer) (10/20/89)
In article <12027@eerie.acsu.Buffalo.EDU> stein-c@autarch.acsu.buffalo.edu (Craig Steinberger) writes: >Can anyone explain how Galileo will be gaining energy by flying near >planets? I understand that the spacecraft will be gaining velocity due >to gravity forces as it approaches Venus, etc., but wouldn't all of that >energy be lost as Galileo goes away from the planet and has to go >against gravity forces? The only way I can see it is if Galileo picks up >some of the energy from the velocity of the planet. That's exactly what happens. Galileo hasn't gained anything as seen from the planet -- it leaves at the same velocity, *with respect to the planet*, as it approached at. But the planet is moving, so to really understand the situation you have to add (using vector addition, in which directions matter) the planet's velocity to Galileo's inbound and outbound velocities. Then you see a difference. -- A bit of tolerance is worth a | Henry Spencer at U of Toronto Zoology megabyte of flaming. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
johnsonr@boulder.Colorado.EDU (JOHNSON RICHARD J) (10/21/89)
stein-c@acsu.Buffalo.EDU writes: > > Can anyone explain how Galileo will be gaining energy by flying near > > planets? [...] > > The only way I can see it is if Galileo picks up > > some of the energy from the velocity of the planet. johnsone@uxh.cso.uiuc.edu (Erik A. Johnson) writes in reply: >The spacecraft does not get a boost in its speed (it speeds up approaching a >planet, e.g. Venus, but slows back down leaving), but rather a change in its >direction of motion as it sling-shots around Venus, and thus a change in the >orbital trajectory about the sun. The spacecraft does indeed its direction of motion about the sun when it encounters a planet. However, as others have already pointed out, it also gains some (or loses some) speed from the encounter. The Pioneers and Voyagers currently leaving the solar system are a case in point. They didn't reach solar escape velocity on the strength of human-made boosters alone. The description of how a gravity assist works in terms of roller-skaters on intersecting sidewalks was a great explanation. You can also think of it in terms of a slingshot. Imagine Big John tied to the sole big yellow, glowing tree on an island in the middle of an icy lake. Big John is keeping the rope taught by moving in a circle around the island. Little Joey the prankster tosses a rock so it passes just behind Big John, who catches it momentarily in his sling. Big John's lazy, so he doesn't try and add any velocity to the rock - he just lets it swing around him until its heading more along his path. When he releases it, perhaps it's heading out so as to pass just behind Big Bertha... Swinging it around toward his front did slow him down a little bit, but because he's so big and heavy, no-one really notices. The rock, however, is small enough that being swung around passively like that causes a very noticeable increase in speed. In case you haven't figured it out :-), Big John is a planet, the rock is a spacecraft, Big Bertha is another planet, the Tree is the sun, Little Joey is JPL, and Big John's sling is a poor analog for the force of gravity. Now, if Little Joey tosses the rock *in front* of Big John, the rock will be slowed down, and Big John will pick up a miniscule amount of speed. On a lighter note, if such is possible after that analogy, think of some possible future names of environmental groups... How about "The Momentum Conservation Society", dedicated to keeping angular momentum, a very valuable and non-renewable resource, where it belongs. =:-O | Richard Johnson johnsonr@spot.colorado.edu | | CSC doesn't necessarily share my opinions, but is welcome to. | | Power Tower...Dual Keel...Phase One...Allison/bertha/Colleen...?... | | Space Station Freedom is Dead. Long Live Space Station Freedom! |
navas@cory.Berkeley.EDU (David C. Navas) (10/21/89)
In article <34700003@uxh.cso.uiuc.edu> aae391aa@uxh.cso.uiuc.edu writes: >4:00 pm Oct 19, 1989, stein-c@acsu.Buffalo.EDU writes: > > Can anyone explain how Galileo will be gaining energy by flying near > > planets? I understand that the spacecraft will be gaining velocity due [deleted] >The spacecraft does not get a boost in its speed (it speeds up approaching a >planet, e.g. Venus, but slows back down leaving), but rather a change in its >direction of motion as it sling-shots around Venus, and thus a change in the >orbital trajectory about the sun. No, that's true, but it's not. I mean -- it *does* get a change in its speed (think, relative to something else -- relativity's a tricky thing where there's little to be relative to...). It also gets a change in direction. Otherwise the darn thing would have to [bounce] off the surface of a planet. Not a <good> thing... :) Think of it this way. get one of those big 'ole super-bouncing rubber ball things. Throw it at a stationary car. ooo, look how it bounces back. Now throw it while the car's moving toward you at 60 mph. Get the picture?? [Oh, and duck real fast :)] It's a simple, if not exactly accurate, description. I always went for simple, myself :) [That was a disclaimer, folks] David Navas navas@cory.berkeley.edu
aae391aa@uxh.cso.uiuc.edu (10/21/89)
I (aae391aa@uxh.cso.uiuc.edu) wrote > 4:00 pm Oct 19, 1989, stein-c@acsu.Buffalo.EDU writes: > > > Can anyone explain how Galileo will be gaining energy by flying near > > planets? I understand that the spacecraft will be gaining velocity due > > to gravity forces as it approaches Venus, etc., but wouldn't all of that > > energy be lost as Galileo goes away from the planet and has to go > > against gravity forces? The only way I can see it is if Galileo picks up > > some of the energy from the velocity of the planet. > > The spacecraft does not get a boost in its speed (it speeds up approaching a > > [etc] As has been pointed out, I was incorrect in that last statement. Sorry. Here is some of the e-mail I got pointing out the correct answer: sw@groucho.att.com (Stuart Warmink) states: > I'm afraid you are mistaken; the probe *does* gain speed. Just changing > the trajectory is pointless, they might as well have launched it in the > right direction in the first place. > > See Mariner 10, Pioneers 10 & 11, Voyagers 1 & 2, ... > > The probes *gains* (or loses, depending on the approach) momentum at > the expense of the planet's. It does *not* loose all the speed gaines at > approach. See the many articles on the Net... jwp@cupcake.sal.wisc.edu (Jeffrey W Percival) writes: > I'm sure you'll hear a lot about this, but the spacecraft > actually does pick up speed. Remember, the planet is moving too, > and the idea that Galileo "gives back" all the speed it gained > falling in only works for a stationary planet. Erik A. Johnson johnsone@uxh.cso.uiuc.edu
thomas@mvac23.UUCP (Thomas Lapp) (10/23/89)
johnsonr@boulder.Colorado.EDU (JOHNSON RICHARD J) writes: > On a lighter note, if such is possible after that analogy, think of some > possible future names of environmental groups... How about "The Momentum > Conservation Society", dedicated to keeping angular momentum, a very > valuable and non-renewable resource, where it belongs. =:-O I've wondered about this idea myself. By "using" this unrenewable resource, when the planets do finally fall into the sun, it will be man's fault again for speeding up the process. Of course, it may be a little difficult to bring this to people's attention since it will be Billions And Billions of years before there is any noticable effects... :-) - tom -- internet : mvac23!thomas@udel.edu or thomas%mvac23@udel.edu uucp : {ucbvax,mcvax,psuvax1,uunet}!udel!mvac23!thomas Europe Bitnet: THOMAS1@GRATHUN1 Location: Newark, DE, USA Quote : Virtual Address eXtension. Is that like a 9-digit zip code? -- The UUCP Mailer
state@unc.cs.unc.edu (Andrei State) (10/23/89)
The "gravity boost", more correctly referred to as "swing-by" or "gravity-assist" maneuver, has been used by NASA at least since the days of Pioneer and Voyager, possibly earlier. It involves approaching a planet with a spacecraft under a certain angle on a hyperbolic trajectory. Roughly the spacecraft intersects the planet's orbit around the sun passing from "inside" to "outside", with the spacecraft's velocity having a component parallel to the planet's tangential (orbital) velocity (i. e., we approach the planet from "behind" and from "inside" its orbit -- I wish I could draw a sketch, I'm no good at this). During the approach, spacecraft and planet attract each other, but since the spacecraft's mass is much smaller than the planet's, it gets a higher delta v; nevertheless, the planets gets a delta v as well (you guessed it: it's NEGATIVE), i. e. it slows down a little bit (tanstaafl). Assuming the planet's orbit was (ideally) circular in the beginning, then the swing-by point on the planet's orbit becomes the apohelion of an elliptical orbit, so the (poor) planet will be nearer to the sun exactly half a year later, at the perihelion of the (new) elliptical orbit. Theoretically, if we do this often enough, we can bring a planet down into the sun; but then again, we don't have enough mass on Earth to do it. ________________________________________________________________________ Andrei State alias state@cs.unc.edu more movie buff than grad student at the University of North Carolina
rogers@SRC.Honeywell.COM (Brynn Rogers) (10/24/89)
Okay, related to gravity boost there is something I would like to ask. I think that the best (most efficient) time to make a course correction or add delta-V is when a spacecraft is closest to a planet (moon,star,asteroid, or gravity well). Any small changes made in course or speed of the spacecraft are much more effective than changes made in deep space. Why doesn't Galileo (or future craft) use this fact to shorten its trip time? (Other than the fact that it dumps the IUS close to Earth.) If there were a small kick motor on Galileo that fired on the closest approach to Venus (apogee or perogee, I forget which), wouldn't it give a great improvement to the trip time, at small cost? Isn't the reason Galileo goes to Venus, then back to Earth, then to Jupiter, still taking 6 years due to the fact that the IUS has a significantly smaller amount of specific impulse than the Centaur upper stage? (that sentence is too long.) How long would the trip take with a Centaur upper stage as originally planned? Did that trajectory bring it around Venus? Brynn Rogers Honeywell S&RC rogers@src.honeywell.com home 612 874-7737
henry@utzoo.uucp (Henry Spencer) (10/24/89)
In article <36082@srcsip.UUCP> rogers@src.honeywell.com (Brynn Rogers) writes: > I think that the best (most efficient) time to make a course >correction or add delta-V is when a spacecraft is closest to a planet >(moon,star,asteroid, or gravity well). Any small changes made >in course or speed of the spacecraft are much more effective than >changes made in deep space. Correct. How much gain there is depends on how deep the gravity well is; you don't gain a *lot* unless you're talking about something like Jupiter. (For those who aren't up on this, you gain because on leaving the planet's gravity well at higher speed, the gravitational field has less time to slow you down, so you don't lose all the speed you gained going in. The "extra" energy comes from the fuel you carried in but aren't carrying out again.) > Why doesn't Galileo (or future craft) use this fact to shorten its >trip time? (Other than the fact that it dumps the IUS close to >Earth.) If there were a small kick motor on Galileo that fired on the >closest approach to Venus (apogee or perogee, I forget which), wouldn't it >give a great improvement to the trip time, at small cost? I suspect the answer is (a) it wouldn't help that much in such a relatively shallow gravity well, and (b) the IUS is pushed to its limit just getting Galileo out, and the extra mass of a kick motor would make it impossible. There are *lots* of ways the trip could be speeded up if more fuel or an extra kick motor could have been added. > Isn't the reason Galileo goes to Venus, then back to Earth, then to >Jupiter, still taking 6 years due to the fact that the IUS has a >significantly smaller amount of specific impulse than the Centaur upper >stage? ... Yes. Actually the deficiency is in delta-V (total velocity change), although lower specific impulse is the reason for that. > How long would the trip take with >a Centaur upper stage as originally planned? I think it was a couple of years shorter. No numbers on hand. >Did that trajectory bring it around Venus? No; there wasn't any need for either the Venus flyby or the Earth flybys. -- A bit of tolerance is worth a | Henry Spencer at U of Toronto Zoology megabyte of flaming. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
rfc@briar.philips.com (Robert Casey;6282;3.57;$0201) (10/25/89)
In article <93.UUL1.3#5131@mvac23.UUCP> mvac23!thomas@udel.edu writes: >I've wondered about this idea myself. By "using" this unrenewable >resource, when the planets do finally fall into the sun, it will >be man's fault again for speeding up the process. Of course, it may >be a little difficult to bring this to people's attention since it >will be Billions And Billions of years before there is any noticable >effects... :-) Just wait 'till they find out that a space probe is gonna steal some from the Earth! "You're going to make global heating worse!" "Keep that up, and you're going to crash the Earth into the Sun!" Is it necessary to :-)? :-) :-) ---------------------------------------------------------------------------- "Better dead than red!"