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Brian Sand
2004-Dec-09, 04:06 AM
I remember reading recently that Voyager is due to reach the edge of our solar system in a few years and that the last few times they have tracked it down - it wasn't where they thought it should be.

Could this be due to an escape velocity that has yet to be reached? Spy planes can fly high, but not go into orbit. Is there such a thing as this for the solar system? And if so, would the same be true for an intergalactic escape attempt as well?

Once you broke free from the gravity and movement of each system, would it be like tossing an apple from the window of a speeding car?

Maybe I missed the point of the article... Any thoughts on this would be appreciated.

-Brian Sand

antoniseb
2004-Dec-09, 04:49 AM
Originally posted by Brian Sand@Dec 9 2004, 04:06 AM
Maybe I missed the point of the article...
Hi Brian,
Yes, you missed the point of the article. The Voyager 1 & 2 and Pioneer 10 & 11 have more than enough speed to escape from the solar system, and float around in interstellar space. They do not have enough to get to intergalactic space, but that's another story.

The article was saying that the Pioneer 10 & 11 spacecraft had slowed about one part in a hundred million from their expected velocity, and we weren't sure why. Many alternative theorists point to this and say it proves, is explained by, or demonstrates their theories.

There are several threads about this topic in the UT forum. If you do a search with the words 'Pioneer anaomaly', you should find them in an instant.

Brian Sand
2004-Dec-11, 01:28 AM
I guess I don't really care about the point of the article anymore. :rolleyes:

So what does the escape velocity need to be? Or is there one?

-Brian Sand

antoniseb
2004-Dec-11, 01:44 AM
Originally posted by Brian Sand@Dec 11 2004, 01:28 AM
So what does the escape velocity need to be? Or is there one?
For any massive object, like a planet, a star, a moon, or an asteroid, there is an escape velocity. The velocity depends on how massive the object it, and how far you are from the center of mass, when you decide to escape. Escape velocity from the surface of the Earth is 11.186 km/sec. Escape velocity from the Earth, when you are already at the moon's orbit is about 1.5 km/sec.

Similarly escape velocity from the surface of the sun is pretty fast, about 620 km/sec, but if you are already at the Earth's orbit, you are quite far from the Sun's mass already, and the velocity needed to escape is about 42 km/sec. Since the Earth is already going about 30 km/sec, only another 12 km/sec is required to get just barely out of the solar system. The Voyager spacecraft had that plus another 16 km/sec to speed them on their way [after getting boosts from the planets they buzzed].

ChromeStar
2004-Dec-11, 11:37 AM
esacape velocity is the speed needed to escape a objects gravity (antoniseb can correct me if i'm wrong) so the answer to your question: is there such a thing for escaping the bonds of the solar system is a no. once the moving object leaves the gravational field of an object it no longer has any thing to escape from.

the problem with the slowing down of probes will probably be because of something like interstellar dust or the gravity of near by planets or asteriods pulling the other way.

For Antoniseb:

why do you say theses probes can't make it out into inter galactic space - whats to stop them? I thought that in a vacum with no resistance, an object can travel for ever unless acted apon.

antoniseb
2004-Dec-11, 01:56 PM
Originally posted by ChromeStar@Dec 11 2004, 11:37 AM
is there such a thing for escaping the bonds of the solar system is a no. once the moving object leaves the gravational field of an object it no longer has any thing to escape from.
One of the important issues is, how fast does an object need to be moving to for an object to leave the gravitational field? [note well, the gravitational field of an object extends to the edge of the universe, but at some point the gravity of other objects become more important]. The speed is the escape velocity. So the answer is Yes, not No. There is an escape velocity.

why do you say theses probes can't make it out into inter galactic space - whats to stop them? I thought that in a vacum with no resistance, an object can travel for ever unless acted apon.
The near vacuum of space offers no serious resistance, but the fact is that the sun is in an orbit around the center of the galaxy, going about seven hundredths of a percent the speed of light, and these probes have not got anywhere near enough speed to escape far from that orbit, let alone escape the galaxy entirely.

GOURDHEAD
2004-Dec-11, 02:12 PM
and these probes have not got anywhere near enough speed to escape far from that orbit, let alone escape the galaxy entirely.

Has anyone computed the escape velocity for the MW or does the uncertainity of the amount of dark matter cause the answer to lie within a fuzzy range of values?

ChromeStar
2004-Dec-11, 02:37 PM
Thanx Anton

didn't think of the first answer like that, but how is it that a gravitational field can stretch the entire universe??

You've got a point. the sun deso rotate around the galaxy centre - i did read that somewhere, wasn't thinking!

bossman20081
2004-Dec-11, 03:54 PM
Originally posted by GOURDHEAD@Dec 11 2004, 09:12 AM

and these probes have not got anywhere near enough speed to escape far from that orbit, let alone escape the galaxy entirely.

Has anyone computed the escape velocity for the MW or does the uncertainity of the amount of dark matter cause the answer to lie within a fuzzy range of values?
I, too am interested in this question; I dont see why dark matter would interfere with the measurement though. Just because we cant see doesnt mean we cant measure its gravity that it exerts on other objects.

ChrisRT
2004-Dec-12, 05:12 AM
I remember reading recently that Voyager is due to reach the edge of our solar system in a few years and that the last few times they have tracked it down - it wasn't where they thought it should be.


Can't tell you about the velocities and physics involved in order to escape the Sun's or the Milky Ways gravitational influence but I can tell you that most scientists believe that the Oort cloud or and other acting particles have pushed or even slowed down these space craft from their expected points of arrival...

antoniseb
2004-Dec-12, 06:53 PM
Originally posted by GOURDHEAD@Dec 11 2004, 02:12 PM
Has anyone computed the escape velocity for the MW or does the uncertainity of the amount of dark matter cause the answer to lie within a fuzzy range of values?
It's a fuzzy range because it depends somewhat on which direction you are escaping in, and where you are starting.
For something to start near the sun, and escape from the galaxy, it would need to add about 0.0004 c to its velocity in the direction the sun is travelling. Measured from a fixed location in space [fixed relative to the line from the center of the galaxy, and some distant galaxy], this would take a velocity of about 0.0011 c to barely escape from the galaxy.

imported_WINSTON
2004-Dec-13, 06:56 AM
LOW SPEED SPACE TRAVEL

One wouldn't have to reach an escape velocity to leave a body or galaxy.
To turn off the engines and coast away, yes. But if you continue to propel a craft, enough to overcome the acceleration of graviational pull, you can travel at any speed away from a body.

HIGH SPEED SPACE TRAVEL

Seeing nothing theoretically wrong with moving faster than c, I see other [non-propulsion related]problems. Your craft would be encountering debris striking with incredible force, moving faster than c in relation to the craft.

Erimus
2004-Dec-15, 07:45 PM
The local galactic escape velocity is about 525 km/sec. Someone posted this link in the Space.com forums some time ago:

http://adsabs.harvard.edu/cgi-bin/nph-bib_...IAUS..117...39C (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1987IAUS..117...39C)

Greg
2004-Dec-17, 05:20 PM
I originally thought that this was a silly post, but now thinking about these concepts turns out to be a bit mind-bending. So it is a good thought exercise after all. The goal of propulsion is to accelerate an object. Acceleration imparts increased velocity to an object which remains constant unless acted upon by another force (friction from the ground or atmosphere or acceleration in another direction.) Gravity essentially imparts an acceleration to an object in the direction of the center of the object generating the gravity (9.8m/s2 on Earth) So you need to accelerate an object to a velocity that will overcome the negative acceleration of gravity and resistance in the atmosphere.
A thought I had that might explain the Pioneer anomaly has to do with the center of gravity of the galaxy. At this point we assume that the exact center is the SMBH, but there is also the dark matter halo which is diffusely spread throughout the galaxy. Perhaps the dark matter halo is not evenly distributed and therefore the center of gravity is moved a little from where we would expect it to be.
At lower velocities you can achieve orbit which means you are able to speed around the object as fast as you are falling towards it. At lower orbits you will lose velocity due to (ever so slight) friction with the atmosphere and fall towards the Earth. At the escape velocity you will be able to leave the influence of the object entirely.
Another thing to recall is that the effect of gravity weakens with the distance from it. The reach of gravity may be infinite, but not equally strong over infinity. Thus the further away you get the less of a grip the object holds on you and the less acceleration you will need to achieve a velocity from rest to escape that object. But if you already have achieved that velocity then it doesn't matter, you can escape in any direction you choose.
Now the assist that the voyagers got from their flybys I thought was an effect of their stealing rotational energy from the planets they passed. Depending on which side you approach a rotating object you can either steal rotational energy and accelerate yourself to gain velocity or lose velocity and accelerate the rotation of the object. I thought that this effect could also explain the pioneer anomaly. Depending on the vector of the craft's departure, perhaps it is losing some of its velocity to the sun's rotation. (ie its moving against or opposite the sun's rotational direction.) Correct me if I am wrong about this concept, since I could not figure out how the voyagers could accelerate using an effect of gravity since any acceleration gained approaching an object's gravity well would be equally lost heading away from it.

TheThorn
2004-Dec-17, 11:58 PM
Hi Greg.

The gravitational assist maneuver steals energy from the planet's rotation around the sun (i.e. it's orbital motion) not its rotation about its own axis.

If the spacecraft flies by in front of the planet it is slowed down, if it passes by behind the planet it is speeded up - like "cracking the whip" when you're on skates.

Matthew
2004-Dec-18, 02:11 AM
A good point is that you cannot create energy, as such when you "steal" energy from a planets orbital energy you actually take some enrgy from a planet. Though because of the relatively small mass of any craft you there is no noticeble loss, but the loss is still there.

Greg
2004-Dec-18, 05:57 AM
The loss of orbital potential energy to the extremely large planets involved that gets converted to kinetic energy for the space probes is infinitesimally small. I guess you can call the process the "drag" effect. Maybe a better analogy would be the "drafting " effect for runners, cyclists, and automobile racing enthusiasts? I am not clear yet on how the conversion takes place, but I think it has to do with exploiting the slight difference in the gravity well in the wake of the speeding planet (as it rotates in orbit) vs its leading edge.
Although this thought is slightly irrelevant to this post topic, I got to thinking about the Pioneer effect some more and looking into those threads while pondering this topic. I agree that Ort cloud dust cannot explain the lower than expected velocity of those probes. The more I think about it the more convinced I am that the sun is responsible for it (due to the exactness of the deviation on two different vectors from the sun) and therefore it is an unexpected property of gravity at large distances. A troubling thought indeed. Is anyone aware of a good link to a more detailed discussion on that?

GOURDHEAD
2004-Dec-18, 02:51 PM
I, too am interested in this question; I dont see why dark matter would interfere with the measurement though. Just because we cant see doesnt mean we cant measure its gravity that it exerts on other objects. The evidence for dark matter is that the orbital velocities of stars near the periphery of the MW are greater than they should be if the estimated mass of the total number of visible objects is the only contributor to the strength of the MW's gravitational field. How can we be sure about DM distribution within or near the MW? Is the velocity discrepancy of the stars with ever increasing apoapses and decreasing eccentricities (orbits with ever increasing areas) such that the gravitational contribution from dark matter is computable with confidence? How much dark matter and degree of homogeniety of distribution are assumed in the 0.0004 c and 0.0011 c escape velocity computations? How can we know the orbital eccentricities of stars, even the sun, hence know the orbital areas from data collected over such short intervals since our instruments have come of age (if they have)? Also, how was the "pre-discovery-of-dark-matter" value for the mass of the MW computed exclusive of DM contributions? Isn't the orbital speed of the peripheral objects the only source we have?

bossman20081
2004-Dec-18, 08:51 PM
Whoah, Gourdhead, no need to highlight stuff like that, I can read it just fine :D

I know why we think there is dark matter, I just didnt see why we needed to know exactly how much DM there is because we can just measure its gravity, but I see what you mean now. I wasnt thinking about how the distribution could affect our calculations and such; I was thinking that it would be distributed evenly (I dont know why...) But if we could measure DM(density, mass, etc.) we would be able to map it all based on the gravity, correct?

Greg
2004-Dec-18, 09:50 PM
Gourdhead,

Those are all good questions about the nature of DM distribution in the MW. Now if only we had a way to detect dark matter, then we may begin to be able to answer these questions. That is of course if DM even exists at all. If the pioneer anomaly is evidence of unexpcted strength of gravity over large distances, then it may not be necessary to invoke dark matter to explain orbital velocities of objects around the center of the milky way. I will feel a whole alot better about all of this once somebody finally detects dark matter or is able to prove there isn't any (WIMPS ar least), or maybe if the pioneer anomaly is adequately explained by more mundane means. It may take an ESA deep space mission now being proposed to do that, however.

antoniseb
2004-Dec-18, 10:44 PM
Originally posted by bossman20081@Dec 18 2004, 08:51 PM
If we could measure DM(density, mass, etc.) we would be able to map it all based on the gravity, correct?
There are efforts being made, and some already concluded that attempt to map the dark matter density and normal matter density in this way for several nearby galaxies. I'd say that between one and five papers a month in arXiv are about measuring dark matter in exactly this way. Another two to ten seem to be about trying to measure its distribution by the lensing effect that galaxies have on the distant galaxies behind them, and yet another, smaller set of paper appear regularly on other methods, that make assumptions about the nature of the particles, and look for specific effects from the particles. A number of studies assume that the WIMPs that make up dark matter are their own anti-particle, and so they annihilate when they hit each other, giving off very high energy gammas, concentrated in places where dark matter density is highest, such as galactic centers.

There are also several Earth-bound efforts to build detectors that will observe dark matter particles making certain subtle interactions with normal matter. These also depend on the specific nature of the WIMPs to work. Some of these projects have been going on for a while, and are about to scale up from kilogram scale detectors [little chance of detecting DM] to ton scale detectors [fair chance of detecting if the model of the kind of particle interactions is right]. If some detections are made with these, huge detectors will probably get funded, so we can learn more with better statistics.

GOURDHEAD
2004-Dec-19, 02:51 AM
Whoah, Gourdhead, no need to highlight stuff like that, I can read it just fine That was just my attempt to add emphasis which I might have done with voice inflection had we been conversing face to face.