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View Full Version : questions about Mars, Uranus, and Neptune



clemmentine
2004-Feb-21, 06:13 AM
I'm new here so please forgive me if these questions had been answered before.

Mars gravity assist. we've sent many missions out to the outer planets, how come none of them used Mars for gravity assist? is it possible to employ one in the future? the last time I checked, Mars still sits between us and the outer planets.

Uranus and Neptune orbiters. I've looked around on the net and there seems to be many more mentions of a Neptune orbiter than one for Uranus. why is that?

Neptune orbiter. many of the pages I've read about the Neptune orbiter mission talk about using NEP and aerocapture. are those technologies really needed for this mission? Cassini uses a chemical rocket and will be able to break into orbit around Saturn. why can't we do the same at Neptune? is it because it would take too long to get there?

thanks for the help.

- clemmentine

Rc2000
2004-Feb-21, 01:00 PM
Remember, search engines are your friend. :D

ToSeek
Everything I need to know I learned through Googling.


Here's a neat explanation and animation on gravity assists.

Let gravity assist you... (http://www.esa.int/esaSC/SEMXLE0P4HD_index_0.html)

2 and 3 I don't know about, but for gravity assisted maneuvers, the planets have to be in certain positions to be able to use them.

Rc

ToSeek
2004-Feb-21, 02:16 PM
I'm new here so please forgive me if these questions had been answered before.

Mars gravity assist. we've sent many missions out to the outer planets, how come none of them used Mars for gravity assist? is it possible to employ one in the future? the last time I checked, Mars still sits between us and the outer planets.

Rosetta (http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31397) will use a Mars gravity assist to aid in its rendezvous with a comet. I assume that Mars was just inconvenient for other missions.


Uranus and Neptune orbiters. I've looked around on the net and there seems to be many more mentions of a Neptune orbiter than one for Uranus. why is that?

Neptune is a livelier and more interesting planet than Uranus.


Neptune orbiter. many of the pages I've read about the Neptune orbiter mission talk about using NEP and aerocapture. are those technologies really needed for this mission? Cassini uses a chemical rocket and will be able to break into orbit around Saturn. why can't we do the same at Neptune? is it because it would take too long to get there?

Cassini also cost about 3 billion dollars. NASA is not likely to be able to come up with that amount of money for any future robotic probes.

Rc2000
2004-Feb-21, 03:46 PM
Clemmentine, I should not read and post before breakfast. 8-[ I just saw that you already had been looking and reading across the internet about what you're looking for, but couldn't find exactly the right thing. :oops:

Rc

Jason Thompson
2004-Feb-21, 07:16 PM
Neptune is a livelier and more interesting planet than Uranus.

Indeed. From an Earth based telescope, Uranus appears to be a tiny blue-green orb with no visible features.

From the HST, Uranus appears to be a reasonably sized blue-green orb with no visible features.

When Voyager 2 flew by, it discovered that Uranus was in fact, in close up... a large blue-green orb with no visible features.

In terms of 'surface' detail, Uranus is the plain, boring member of the solar system. Still, maybe that's why I find it so intriguing. And besides, it's still got the unusual axial tilt, the rings, the odd moons (especially Miranda).

But it is true that Neptune is far more interesting in many ways.

Poor old Uranus. It's sat there watching while we send orbiters to Venus, Mars, Jupiter and Saturn and then, just when it thinks 'oh it must be my turn next,' we talk about skipping it completely and popping further afield to Neptune...

ToSeek
2004-Feb-21, 07:54 PM
Actually, NASA just doesn't want to deal with all the jokes that will result from announcing that they're sending a probe to Uranus. ;)

kenneth rodman
2004-Feb-21, 07:54 PM
WE ACTUALLY used gravity assit to launch one of our earlier probs to jupiter.
the probe went around venus, then around earh thenaround venus one more time before hurling towards its destnation if I remember correctly.

Rc2000
2004-Feb-21, 09:56 PM
ToSeek
Actually, NASA just doesn't want to deal with all the jokes that will result from announcing that they're sending a probe to Uranus.


TOSEEK!!! GEZZZZZZZZZZ!!! :o

Well, at least it wouldn't be as bad as saying they're inserting a probe into Uranus and hope it radios back analysis of the samples taken. #-o
*bangs head against keyboard*

Oh, checked out your site and spent a long time in the Dr. Who Companions section. Most excellent! Brought back memories of the series. I used to watch the good Doctor all the time when it used to come on PBS.

Rc

JohnOwens
2004-Feb-21, 11:10 PM
WE ACTUALLY used gravity assit to launch one of our earlier probs to jupiter.
the probe went around venus, then around earh thenaround venus one more time before hurling towards its destnation if I remember correctly.

That was the Cassini/Huygens probe illustrated in rc2000's link above. It was actually Venus-Venus-Earth. But it doesn't really answer the actual question, since OP was asking why not Mars, since it's between us and the outer reaches?

ToSeek
2004-Feb-21, 11:46 PM
Oh, checked out your site and spent a long time in the Dr. Who Companions section. Most excellent! Brought back memories of the series. I used to watch the good Doctor all the time when it used to come on PBS.


Glad you enjoyed it. It really needs to be updated and revamped, but I haven't gotten around to it yet. Btw, I met Janet Fielding ("Tegan") at a convention last weekend. ;)

parejkoj
2004-Feb-22, 12:09 AM
That was the Cassini/Huygens probe illustrated in rc2000's link above. It was actually Venus-Venus-Earth. But it doesn't really answer the actual question, since OP was asking why not Mars, since it's between us and the outer reaches?

Timing. Mars just isn't in the right place to be used for gravity assist very often. If it were, I can't think of a reason to not use it. But there are more times when Venus is in a good position. Also, Venus-Venus-Earth gains you a LOT more delta-v than just going by Mars. I'd say probably 5x more.

The problem is not whether one orbit is between two others, but where the planet is in the orbit when you want to try to get elsewhere. Such timings happen a lot less often than one might think. At least, for the things we are trying to do now.

jmkkkj
2004-Feb-22, 04:05 AM
One possible explanation for NASA not having used Mars' gravity to propel a probe to the outer planets is that Mars might not have been in the right alignment to do so. You see, some times Mars is on the opposite side of the Sun from Earth. For example, lets suppose that NASA wants to send a probe to Uranis while Earth and Uranis are on one side of the sun and Mars is on the other. Mars cannot be used for gravitational assist because it is on the opposite side of the Sun, it's in the opposite direction from Uranis in spite of the fact that its orbit is closer to the Sun than Jupiter's or Saturn's. Mars is simply not "on the way" to Uranis. For that mater, Jupiter and Saturn might not be on the way either! That's why NASA's launch windows are so critical... so that they CAN use gravity as means for propulsion.[/i]

clemmentine
2004-Feb-22, 06:23 AM
thanks for all the responses.


Rc2000 wrote:

Remember, search engines are your friend.

yes. search engines found me plans for a Neptune orbiter and also took me here. should have found this nice place years ago. I guess I've been living under a rock.


sorry, I wasn't being clear on the Mars gravity assist question. I was wondering why we haven't used or planned to use Mars gravity assist to get to Jupiter, Saturn, Uranus, Neptune, and Pluto. like ToSeek said, it could be that it's just not convenient for the other missions, and it probably is the case for the 6 missions we have already to the outer planets (Pioneer and Voyager were light enough to get to Jupiter directly, and Galileo and Cassini were too heavy to go to Mars). could we theoretically use it? or is it that Mars is too small to make a Mars gravity assist practical for a mission to the outer planets?

thanks parejkoj. I forgot that VVEGA gives you more delta-v than MGA. but wouldn't the flight time be greater for a VVEGA versus an MGA? let's take Cassini's trajectory to Jupiter (ignore the Jupiter-Saturn part for the moment). that's a VVEGA trajectory to Jupiter in a little over 3 years. what if Mars was in position for a gravity assist to Jupiter, how long would it take if we used that MGA to Jupiter instead?


Rc2000 wrote:

Clemmentine, I should not read and post before breakfast. I just saw that you already had been looking and reading across the internet about what you're looking for, but couldn't find exactly the right thing.

no problem. this board is far far more dynamic than all the other boards I have been on.


kenneth rodman wrote:

WE ACTUALLY used gravity assit to launch one of our earlier probs to jupiter.
the probe went around venus, then around earh thenaround venus one more time before hurling towards its destnation if I remember correctly.

Pioneer 10 went to Jupiter directly.
Pioneer 11 and Voyager 1 went to Jupiter directly then used a JGA to get to Saturn.
Voyager 2 went to Jupiter directly, JGA to Saturn, SGA to Uranus, then UGA to Neptune. thus the 'Grand Tour'.
Galileo was tied to the Shuttle, but after Challenger, the Centaur upper stage was no longer allowed, so it took a VEEGA trajectory to orbit Jupiter.
Cassini went to orbit Saturn via a VVEJGA trajectory.


ToSeek wrote:

Neptune is a livelier and more interesting planet than Uranus.

Jason Thompson wrote:

But it is true that Neptune is far more interesting in many ways.

at least some of us think Neptune is more interesting. I was blown away in '89 when I first saw the beautiful blue orb that Neptune is. I agree that Neptune has more visual appeal than Uranus. remember that when Voyager 2 got to Uranus in '86, one of its poles is in the direction of the Sun, so one hemisphere is in constant sun light while the other is in constant darkness. I suppose that may be part of the reason why Voyager 2 saw a plain featureless Uranus. but..

spectacular Uranian storm patterns (http://www.spacedaily.com/news/decadal-02a3.html)

note: I'm now having problems getting to Space Daily; you may need to get that out of Google's cache.


Jason Thompson wrote:

Still, maybe that's why I find it so intriguing. And besides, it's still got the unusual axial tilt, the rings, the odd moons (especially Miranda).

to me, because of its moons, the Uranus system as a whole is just as interesting as the Neptune system. Uranus has got 5 interesting moons to show while Neptune's only got Triton. in my mind, the Neptune system come out even with the Uranus system mainly because of Neptune's prettier face.


ToSeek wrote:

Actually, NASA just doesn't want to deal with all the jokes that will result from announcing that they're sending a probe to Uranus.

LOL! I bet that's the real reason why Uranus was passed over in favor of Neptune.

jokes aside, anybody know of reasons other than the ones in that Space Daily link?


a Neptune orbiter will not be cheap. I was just wondering if it will be reasonable without using new technologies like NEP and aerocapture. NEP is like SEP used on DS1, but it's nuclear instead of solar. it is supposed to get us out to Neptune faster than chemical rockets. and because it arrives at Neptune faster, it can't break into orbit with just regular rockets (hence the need for aerocapture). aerocapture is the use of atmospheric drag to slow a spacecraft down in order to enter orbit.

- clemmentine

AK
2004-Feb-22, 08:41 AM
I have only this link to contribute to this discussion: a proposed Neptune orbiter:

http://www.lpi.usra.edu/meetings/outerplanets2001/pdf/4095.pdf

Sounds great to me. The Neptune system has always been my primary interest. Fascinating planet.

TriangleMan
2004-Feb-22, 05:43 PM
LOL! I bet that's the real reason why Uranus was passed over in favor of Neptune.

jokes aside, anybody know of reasons other than the ones in that Space Daily link?
I think there are a handful of JPL/NASA employees who frequent this board, perhaps they can ask around and give us an idea. :)

clemmentine
2004-Feb-22, 10:08 PM
Huge storms hit the planet Uranus (http://science.msfc.nasa.gov/newhome/headlines/ast29mar99_1.htm)

- clemmentine