Jovian System has better colony sites than moon&mars

[marcus]

The Jovian system is a superior target for self-sustaining colonization.

Quite a bunch of PF people (Nereid, all the mentors in earshot, and plenty of others) know the facts.

It is a beautiful and interesting system consisting of a variety of different moon-size objects within comparatively easy reach of each other. Trips between Jovian moons take only a few days.

One key fact is the availability of reaction mass for propulsion.
Europa has a thick ice shell covering its liquid water oceanic layer. Missions to the Jovian system might be equipped to take on reaction mass (water or liquid hydrogen) at Europa.

Europa ice is estimated to be many kilometers thick.
A colony with abundant nuclear electric generating capacity
could hollow out habitats in the ice, far enough below the surface
to be structurally secure.

Other moons such as Ganymede and Callisto look interesting too, and appear to have water ice.

Presumably the ice on Europa is rocky---has minerals mixed in with it.
It should not be too difficult to design a chemical plant to extract the raw materials for civilization.

Personally I would not want to go anywhere as dry and barren as Mars and the moon----though Mars has some lovely desert landforms, shaped in part by wind.

Natural beauty is important and the spectacular thing Europa has is a sky with the planet Jupiter in it. Jupiter diameter is 143 thousand km and the distance from Europa is 671 thousand km.

This means the angular width of Jupiter is about 12 degrees.
Compare this with the full moon seen from Earth which is 1/2 degree wide. We are talking about a gorgeous planet 20 times wider than the full moon is in our sky. 400 times bigger in angular area, than the full moon. With more colors and cloudforms to make it interesting.
This is a very romantic sight.

Humans living in the ice-caves of Europa would probably go to the surface for their honeymoons. They might even reproduce prolifically because watching Jupiter and the other Jovian moons is so romantic.

Films made at Europa might have a good boxoffice on Earth.

The Jovian system would be a fun place to inhabit for many reasons, including the fact that the trip from one "planet" to the next takes only a couple of days. Each Jovian moon goes around the primary in a few days or a week. So transfer orbits have similar periods.

In the Solar system trips between planets take on the order of years,
whereas in the Jovian system trips take on the order of days. Plus reaction mass for propulsion is available in a not-very-deep gravity well. Energetically, water at Europa's surface is a lot more accessible than water at the Earth's surface. So as a supplier of propellant it is a good bargain to switch over from Earth to Europa.

I do not like the "moon-mars" initiative which seems to me a bad investment and destructive of human aspirations.

 

[enigma]

Some problems though are:

the multiple month long trip there or back,
the low levels of sunlight for producing food,
the large amount of radiation trapped in Jupiter's Van Allen belts. [are they called that there?]
Any ship designed to get people home from Jupiter would need to be HUGE. It's an enormous gravity well... $\mu_{Jupiter}$ = 1.26e8 vs $\mu_{Earth}$ = 3.986e5
Etc.

EDIT: fixed numbers... *&^#% website giving G with meters instead of km

 

[enigma]

I just ran numbers. If you go in, you ain't coming back out.

It takes a delta V of 19.5km/sec to escape from Jupiter's gravity at Europa's distance.

Compare with ~11.2km/sec to escape from Earth.

Using a rocket with hydrogen and oxygen as fuel and oxidizer electrolyzed from Europa's water, you'd need 80 times the mass of the rocket in propellants, regardless of size.

Ain't going to happen.

EDIT: fixed numbers

 

[marcus]

Originally posted by enigma


It takes a delta V of 19.5km/sec to escape from Jupiter's gravity at Europa's distance.

Compare with ~11.2km/sec to escape from Earth.

I believe your number is off by around a factor of three here. Not that it matters awfully.

Rough backofenvelope Europa's orbital speed is around 13.7 km/s
so escape from Europas distance out is around 19.4 km/s
The delta V is the difference-----19.4 minus 13.7

this is only 5.7 km/sec

this delta V is a factor of 3 different from the figure you quoted of 19.5

 

[enigma]

Ach. Of course you're right.

You'd still need a bunch of delta V to do the orbital transfer to Earth's orbit around the Sun, which isn't a small number, though.

 

[marcus]

Originally posted by enigma


...Using a rocket with hydrogen and oxygen as fuel and oxidizer electrolyzed from Europa's water, you'd need 80 times the mass of the rocket in propellants, regardless of size.

Ain't going to happen.

What you say does not sound reasonable in light of, for example, NASAs own studies of the feasibility of manned missions to Jovian moons.
These were done in a fair amount of detail and involved
optimizing so as to make considerable use of Jupiter's own gravity
and the gravity of other moons.

I do not recall any figure like the one you offer, of a delta V for escape from Jupiter gravity at Europa distance of 19.5 km/s
or a fuel payload ratio of 80. These figures seem out of line.

Also your conclusion "aint going to happen" seems out of line compared with NASA's fairly detailed mission proposal.

Maybe not going to happen in the future we envision now, with a misdirected program. But the future changes.

One more thing, as regards the flat statement of impossibility.
At one time NASA had a nuclear rocket development program that was based not on explosion but on using a reactor to heat propellant.
For example the reactor may heat hydrogen, or water, to use as reaciton mass. I do not consider it totally certain that NASA will never re-embark on nuclear rocket development. So your calculation with chemical booster might not apply. Another case where I question
your flat statement of impossibility.

 

[enigma]

Originally posted by marcus
I do not recall any figure like the one you offer, of a delta V for escape from Jupiter gravity at Europa distance of 19.5 km/s
or a fuel payload ratio of 80. These figures seem out of line.

You were right, I flubbed up. It isn't the delta V, merely the escape velocity. That comes from the Vis-Viva equation. The fuel ratio comes from the ideal rocket equation.

It's possible to work out nuclear thermal rockets, but that improves your Isp by a factor of 3 or so. Half the mass of the ship in fuel to leave Jupiter's gravity. You still do need huge amounts of fuel to get back home, both to decelerate on a transfer ellipse, and then to decelerate again once you get to Earth. I'm not sure if it's possible to do a reverse gravity assist, or how much velocity you could bleed off from there.

Nuclear thermal rockets are a long way from operational status, unfortunately.

Another case where I question
your flat statement of impossibility.

Impossibility may be a bit harsh. Anything is possible. I'll tone my statements down to: highly unlikely anytime in the near term.

To be fair, I would love to have our current resources going into exploration missions to the Jovian moons, but bases there? well... they're just too far away from us, IMO.

 

[Jimmy]

Marcus: Europa ice is estimated to be many kilometers thick. A colony with abundant nuclear electric generating capacity
could hollow out habitats in the ice, far enough below the surface
to be structurally secure

Would tidal effects of Jupiter and the other moons on europa be a major concern for habitats built into the ice?

edit: Not trying to be negative, just curious. It is an exciting idea. I would love to have a 12 degree Jupiter in my sky.

 

[marcus]

Mission design people have gotten quite clever about using gravity assist and indeed a "reverse" assist is quite practical on homeward leg

you are welcome to put nuclear thermal propulsion out of mind and imagine only H2 and O2 chemical

for anyone who wants to try some back of envelope trials
here is Hohmann transfer ellipse stuff.

orbital speeds of Earth and Jupiter are about 30 and 13 km/s
as y'probly know
to get to Jupiter (without any gravity assist shenanigans) takes a delta V kick of about 8.5 at earth
and another (to catch up with jupiter) delta V of about 5.5

that is the ellipse starts at about 38.5 peri and
gets out to Jupiter at about 7.5 ap (so it needs 5.5 added to catch up)

but the catching up can be done with the help of the gravity and the orbital speed of the moons

have to go, back later

 

[marcus]

well I am back. don't know if this thread will go anywhere though

everybody understand Hohmann transfer ellipse?
delta V at Earth of 8.5 km/s
added to Earth's orbital 30 km/s
puts you on an ellipse with aphelion at jupiter
at which point the Jupiter system is coming at you
at 5.5 km/s

and Callisto for example has an orbital speed of 8.2 km/s and escape from the Jupiter system from Callisto's distance takes delta V of 3.4 km/s. Mission design people PLAY with the possibilities like teenagers play with weaving in and out of traffic on the freeway.

You are coming into the Jupiter system at 5.5 km and there is a lot that you can do by way of "reverse gravity assist" as enigma called it. Some people are good at this and I am often amazed by their ingenuity.

Anyway I doubt you need to supply the whole arrival delta V of 5.5 from your own engine. I think you get a lot of that out of the Jovian system. And I think it is reversible. Going or coming the cost is going to be on the order of 8.5 plus a fraction of the 5.5, on the order of 10 km/s.

the problems are extremely tough. The trip is 2.7 years on the transfer ellipse (one half of it). That is already a grave if not horrendous problem right there.

But the delta V is not all that bad.

 

[marcus]

Originally posted by Jimmy
Would tidal effects of Jupiter and the other moons on europa be a major concern for habitats built into the ice?

edit: Not trying to be negative, just curious. It is an exciting idea. I would love to have a 12 degree Jupiter in my sky.

Jimmy! I didnt see that someone else had dropped in.
I bet you are right, as regards Europa. the surface looks like it breaks now and then, probably from tidal action, and liquid water comes to surface and freezes. But I don't know!
Maybe there is someone here at PF who knows what has been learned about Jovian moons.

As for the 12 degree Jupiter (or however many degree, I was making a quick estimate) I would love that too. It is a beautiful orb for sure.

I wonder if there is any practical way to harness natural temperature differences to generate power. Assume there is plenty of nuclear power but nice to be able to supplement or gradually replace that

 

[Jimmy]

Thanks for your reply Marcus. I remembered reading that the ice on Europa would crack letting water well up onto the surface. I thought it might be because of tidal forces. Wasn't sure though.

Marcus: I wonder if there is any practical way to harness natural temperature differences to generate power. Assume there is plenty of nuclear power but nice to be able to supplement or gradually replace that.

What temperature differences did you have in mind?

And in the spirit of this thread:

http://members.aol.com/jrzycrim01/images/Europa.jpg
http://members.aol.com/jrzycrim01/images/Europa2.jpg

Images captured from Celestia

How do you embed an image in a post? I thought the img tag would do it but it just displays the url.

 

[marcus]

Originally posted by Jimmy


http://members.aol.com/jrzycrim01/images/Europa.jpg
http://members.aol.com/jrzycrim01/images/Europa2.jpg

Wow

 

[marcus]

Its late and I don't know the answers anyway.

I am going to have to read something about the main Jovian moons.

regards to you,

great pictures

 

[Jimmy]

That's quite alright. I should get off my lazy backside and research this myself as well. This is a great thread and I hope it spawns lots of discussion. I enjoyed your ideas about colonizing the Jupiter system. The posts about Hohmann transfer ellipses was especially interesting to me. Anyway, I need to climb into bed myself.

As far as the pictures, thanks. I don't really really deserve any credit. Celestia did all the work.

 

[enigma]

Originally posted by marcus
Going or coming the cost is going to be on the order of 8.5 plus a fraction of the 5.5, on the order of 10 km/s.

the problems are extremely tough. The trip is 2.7 years on the transfer ellipse (one half of it). That is already a grave if not horrendous problem right there.

But the delta V is not all that bad.

Alright

If your Hohmann numbers are correct (I have no reason to doubt they are), I got ~8.6 times the mass of the craft in propellants. If you then add in the numbers for leaving Europa, say 5km/sec, you're up to 29 times the mass of the craft. From Callisto, using your number of 3.4km/sec, it's ~20 times the mass of the craft.

That's a lot of fuel, but if the need is there, you just design around the need. You can plug in extra gravity assists, increasing the time. You can switch to an ion drive, but you couldn't harvest the fuel in-situ. You can go multi-stage, but that would increase system complexity, and be especially risky after a 2-3 yr minimum stationed in space without operating (on the trip there, plus mission time). It's possible (someone would have to run the numbers) that at the relative distances to the sun of the Earth and Jupiter that a bi-elliptic transfer may save fuel. It would add much time to the trip, unfortunately. Not a good alternative for manned return missions.

One really big problem I can see is you'd have to design cryogenics systems for the propellants that are built to operate for 6 years without a failure. That's a huge engineering feat. Add that to the potential for outgassing, as well as course correction and Earth re-entry delta V...

Humans in space are messy.

 

[marcus]

Originally posted by enigma

Humans in space are messy.

Oh yes, that is the main point, is it not?

I've always favored robotic space exploration over manned.
Machines do better in space and the robot probes have consistently
produced far more interesting information at less cost.

My point is that IF a decision is made at the policy level to
promote manned space ventures with extended stays (which might in itself be a wrong decision) then there are better ways to spend the money and lives----worthier manned space ventures, I mean.

I won't bother to reply to your points which seem to be general arguments against putting people into space (with all the extra equipement and fuel that entails). Because the premise is that we do that. Assuming we are to have extended stays on the Lunar/Martian surface or on some other body instead, let's compare relative merits

 

[russ_watters]

Originally posted by marcus
What you say does not sound reasonable in light of, for example, NASAs own studies of the feasibility of manned missions to Jovian moons.
These were done in a fair amount of detail and involved
optimizing so as to make considerable use of Jupiter's own gravity
and the gravity of other moons.

You have any info on that? It seems pretty far fetched. I'd be awfully surprised if the study went beyond the "gee, wouldn't it be cool if..." level.

The biggest hurdle I see is the size of the rocket due to the length of the trip. "Enourmous" doesn't even begin to describe it. Getting to the moon took only a few days and as such, life support wasn't that big of an issue. Getting to Mars (just getting there, not doing anything or coming back) takes a good 6 months and the minimum total duration of the trip is about 2.5 years. Just getting to Jupiter would take more like 5 years. A ship with provisions for supporting a crew for that length of time would be huge - something to make Clarke's "Discovery" look like a dingy.

I'm thinking $100 billion for a trip back to the Moon (Apollo style - no base), $1 trillion for Mars, and $10 trillion for Jupiter. Awful, awful ideas.

I base this all on a key assumption: existing or near-future technology. Barring a quantum leap in propulsion technology (even scaling up ion propulsion would require at least a 10 order of magnitude improvement), we'd have to haul/make hydrogen and oxygen.

 

[marcus]

Originally posted by russ_watters
You have any info on that? It seems pretty far fetched.

Just getting to Jupiter would take more like 5 years.

You are saying using Jupiter and its moons gravity is far fetched but this has already been done. Several probes have been there and taken pictures. When they go in, the paths is optimized to take advantage the system's own gravities, those of the moons as well as the primary. Gravity assists are demonstrated technology and have been used with some finesse on a number of missions.

You are wrong about the 5 years. At least according to my CRC handbook which gives Hohmann ellipse data. Half the period of the transfer ellipse Earth-to-Jupiter is about 2 and 3/4 years.

It is not 5. There is a significant difference.

this information is available to anyone who has the standard handbook of physics and chemistry. Probably also on the web.

As for the manned-mission studies done by NASA, I went down to the Engineering Library at the nearest educational institution. There is shelf after shelf of thick NASA books working out feasibility and grubby details of various missions that were considered when Manned Space was fashionable. IF they actually do revive Manned Space to a real significant extent then a lot of that that grubby detailed stuff becomes relevant again

It sure as hell was not limited to the "gee wouldn't it be cool" level. They were doing serious homework (in the years after Apollo). And that homework is sitting down at the library waiting for someone to blow the dust off.

 

[russ_watters]

Originally posted by marcus
You are saying using Jupiter and its moons gravity is far fetched but this has already been done.

Sorry, I could have pared down the quote a little better - I was referring to the study.

You are wrong about the 5 years. At least according to my CRC handbook which gives Hohmann ellipse data. Half the period of the transfer ellipse Earth-to-Jupiter is about 2 and 3/4 years.

Fair enough. I pulled that out of the air. 2 3/4 years one way is still quite significant.

And I am still dubious about what these studies found. When manned spaceflight was "fashionable," they'd no doubt be willing to spend a hundred grand studying everything that popped into a mission planner's head, but I don't think that means they ever considered it a reasonable possibility. It might just mean they wanted to know.

I'm looking for this type of info. So far, I found some on MARS. It includes an estimate of $1 trillion for a single mission. THIS one says the cost of Bush I's Mars mission would have been $400 billion. That seems overly optomistic since it also says the original Apollo program cost $150-$175 billion in today's dollars (anyone think any government program ever meets its budget?). THIS one has some specific info on the early space program's costs.

 

[Janus]

Originally posted by marcus
well I am back. don't know if this thread will go anywhere though

everybody understand Hohmann transfer ellipse?
delta V at Earth of 8.5 km/s
added to Earth's orbital 30 km/s
puts you on an ellipse with aphelion at jupiter
at which point the Jupiter system is coming at you
at 5.5 km/s

and Callisto for example has an orbital speed of 8.2 km/s and escape from the Jupiter system from Callisto's distance takes delta V of 3.4 km/s. Mission design people PLAY with the possibilities like teenagers play with weaving in and out of traffic on the freeway.

You are coming into the Jupiter system at 5.5 km and there is a lot that you can do by way of "reverse gravity assist" as enigma called it. Some people are good at this and I am often amazed by their ingenuity.

Anyway I doubt you need to supply the whole arrival delta V of 5.5 from your own engine. I think you get a lot of that out of the Jovian system. And I think it is reversible. Going or coming the cost is going to be on the order of 8.5 plus a fraction of the 5.5, on the order of 10 km/s.

Okay, it doesn't quite work that way.
That difference of 5.5 km/sec only takes into account the relative velocity difference between the ship and Jupiter. Since the ship will be traveling slower than Jupiter you more or less place it in front of Jupiter and let the Planet "catch up". The thing is that as Jupiter catches up the ship will fall towards Jupiter due to Jupiter's gravity. Which means that by the time the ship reaches same distance form Jupiter as Europa is it will have a much larger relative velocity to Jupiter than 5.5km/sec.

In order to match orbit with Europa you must take into account all of the 5.5 km/sec and all but 1.4km/sec of the 5.6 km/sec gained by the fall into the Jovian system.

For a transfer from LEO to LEO (Low Earth orbit to Low Europa Orbit you need a total delta V of about 16 km/sec.

Add to this up to .9 km/sec for the broken plane manuever to match orbital inclinations of the planets.

As you said, some of this can be made up through gravity assists by fly-bys of the other Jovian moons, but to make significant inroads you would need to multiple passes. This could add quite a bit to your mission time.

 

[marcus]

Originally posted by Janus
As you said, some of this can be made up through gravity assists by fly-bys of the other Jovian moons, but to make significant inroads you would need to multiple passes. This could add quite a bit to your mission time.

You are right that it would add time. As I recall (and this was some time ago) there were multiple passes within the Jovian system. It is hard to reconstruct from memory because maneuvers using the gravity of several moons are sophisticated.

As to how much time it would add, since the typical orbit period in the system is around one week, is hard to say. One might guess a week or two.

My understanding is that gravity assist has been used in robot probe maneuvers within the Jovian system, so no big mystery about its application there.

Why don't the two of us, and whoever else wants, figure out a simple gravity assist for getting rid of some fraction of that 5 km/s. then we can see later how the pros did it, if I can ever find that article.

 

[marcus]

so we are approaching the Jovian system at 5.5 km/s
or if you prefer we are on a tangent ellipse and it is overtaking us at 5.5 km/s. But I shall think in Jovian coordinates from here on.

the first moon we use is probably Callisto
Its orbit speed around Jupiter is 8.2 km/s and escape from that distance is 11.6 km/s

So when we are in as far as Callisto we are going 12.8 km/s.
We do a flyby of Callisto while it is sunwards of Jupiter so that it parallels our 12.8 with its 8.2. We are both going in the same direction and we are passing Callisto at 4.6 km/s.

So we do a hyperbolic flyby around Callisto.

To understand what we are trying to do think of how it would be if Callisto were much denser so that we could swing around it in a hairpin turn, reversing direction. then we would be at Callisto distance (where circular orbit velocity is 8.2, but we would be going 4.6 km/s in the contary direction. So the net is 3.6 km/s in the same direction as Callisto, let's say 4 km/s. Since circular orbit velocity at that distance is 8.2 we would be falling inwards toward the planet, in an elliptical orbit around Jupiter.

This would have gotten us into Jupiter orbit essentially without expenditure of fuel and in one flyby maneuvre.

But Callisto is not dense enough that you can do a hairpin at 4.6 km/s. So we need a second encounter with a Jovian moon. This is where good mass data and mission design expertise comes in, neither of which I have. But perhaps you or someone else can supply some.

In effect a craft can go into the Jovian system like a pinball goes into a bunch of bumpers and cancel part or all of its unwanted speed relative to Jupiter by a series of flybys.

So if one eventually wants to get to Europa then one would not necessarily think of going directly there, but might well consider first passing either Callisto or Ganymede (by now this kind of indirection must be almost second nature among mission designers)

Does anyone have recent data on the masses of the Jovian moons and escape velocities. What I have is a bit out of date. Let's consider next using Ganymede for the first encounter, instead of Callisto

 

[enigma]

Escape velocity for any object at any distance from it is:

$$\sqrt{\frac{2*\mu}{r}}$$

Where $\mu$ is G times the mass of the object, and r is distance from the center in km.

The calculations necessary are enormously difficult... certainly more than I would care to do for fun.

 

[marcus]

Again we are approaching the Jovian system at 5.5 km/s
(or it is catching up with us)
this time we focus on Ganymede.
Its orbit speed around J is 10.9 km/s and escape
from its distance from J is 15.4

Again we square 15.4 and square 5.5 and add them and take square root
(as before). I get 16.3 km/s.
The mission is timed so that Ganymede is sunwards of J so that it is going "backwards" at 10.9 km/s.
Our 15.4 is roughly in the same direction (this is just an approximate calculation) and so we are passing Ganymede at a speed of
about 5 km/s.

If we could whip around Ganymede in a hairpin turn so that we reversed direction our new speed would be in the same direction as Ganymede but only 5.9 km/s instead of Ganymedes 10.9.
So our new orbit, instead of being a circle like Ganymede's, would be an ellipse and we would be falling in towards the inner moons, Europa and Io et cetera.

But again, Ganymede is not dense enough for a tight turn. It is the most massive of the moons and has, I believe the highest escape-from-surface velocity. This is what you need for a tight turn. So a Ganymede flyby might be the most useful way to get introduced to the Jovian system. Since I don't have the best data on masses of the Jovian moons I will just leave it there---hope someone else can
take it further.

Anyway I'm skeptical that you need a whole bunch of reaction mass to take care of that 5.5 km/s with which you encounter Jupiter. I think engaging with the Jovian system at that end is more a matter of finesse. Glad of any comments!

 

[marcus]

Originally posted by enigma
Escape velocity for any object at any distance from it is:

$$\sqrt{\frac{2*\mu}{r}}$$

Hello enigma! Glad you are on board or at least somewhere off in space watching

I have already taken the escape velocity formula into account in the above discussion. A useful equivalent form is to say that it is equal to the circular orbit velocity multiplied by the square root of 2.

So the circular orbit velocity at Callisto distance is 8.2 and this means that escape from Jupiter from Callisto distance is 1.414 times that or 11.6 km/s.

This allows us to calculate how much speed an inbound craft will pick up if it is aiming for a Callisto flyby.

The effect of the MOONs' gravity is comparatively small until you get near them.

 

[marcus]

We are doing a very rough-and-ready back-of-envelope style of mission design here. Anyone who wants to can try it and get a rough approximate idea of the potential of gravity assist in the system.

If someone can supply a recent table of masses and orbit radii for the main moons it would help. ("semi-major axis" not radius of course)

For now here are the main data I'm using. Here are circular orbit speeds for the four big moons (from my old handbook)

Io 17.3 km/s
Europa 13.7 km/s
Ganymede 10.9 km/s
Callisto 8.2 km/s

I sure wish someone with more recent data would update these.

From these circular orbit speeds one can readily estimate what escape speed from Jupiter is, starting at that distance. Just multiply by squareroot 2.

 

[Nereid]

A few things about the Galilean moons ...

A nice site is JPL's Galileo one; it has all kinds of goodies about Jupiter, the inner moons, and the mission, including this, packed with facts about the moons:
http://galileo.jpl.nasa.gov/moons/moons.html

Snippets:
- all Galilean moons have synchronous rotation, so Jupiter will appear to stand still in the sky (or be below the horizon) on all moons.
- Io in particular is in a fierce radiation zone (like the van Allen Belts, as enigma said), and there's a flux tube connecting it with Jupiter, carries millions of amps?
- the tidal locking is what keeps each of the inner three in its orbit (look at the periods - 1.77, 3.55, 7.15 days)
- tidal heating gives Io its spectacular volcanos (surely one of the more awesome sights), and also Europa its ocean; jury's still out re relative importance of tidal heating for Ganymede to keep its ocean
- IMHO, Callisto may be a better place to start - not as deep in the well, undifferentiated (so likely to have more rocks and metals around)
- Galileo used gravity assist to a considerable extent to change orbits (still needed rockets, of course); with four moons there's a lot of choice

 

[enigma]

Originally posted by marcus
Hello enigma! Glad you are on board or at least somewhere off in space watching

This allows us to calculate how much speed an inbound craft will pick up if it is aiming for a Callisto flyby.

How's that?

This really isn't a simple problem. You pick up or bleed off speed because the center of gravitational attraction is moving away from you as you pass it. I haven't gotten into the nuts and bolts of gravity assists in a full year of orbital dynamics and space navigation classes, much less jumping around from moving moon to moving moon, all of which are moving about the moving Jupiter reference frame. I just checked, and my (very in-depth) textbook doesn't cover the topic either.

I don't know how much detail the CRC you looked at has, so I'll just post the full form of the energy equation, just in case (not meaning to insult if this is old-hat). That way you can do away with circularizing the orbital velocities.

$$\epsilon = \frac{V^2}{2}-\frac{\mu}{r}=-\frac{\mu}{2a}$$

That relates velocity to distance from Jupiter. It will have some (small) effect due to the eccentricity of the orbit, particularly for Callisto and Ganymede. You also get escape velocity by plugging in infinity for a.

I also found http://www.the-planet-jupiter.com/moons-facts-sheet.html website which gives some of the orbital parameters, but not enough to place each moon's position relative to Jupiter or each other (it's missing argument of perigee, right ascension of the ascending node, and a reference true anomaly).

 

[marcus]

Originally posted by enigma

...
I also found http://www.the-planet-jupiter.com/moons-facts-sheet.html website which gives some of the orbital parameters, ...

thanks! turns out my figures were not so bad
agree with yours to the indicated accuracy, it looks like

http://www.the-planet-jupiter.com/moons-facts-sheet.html

source is NASA Goddard
Here's what they give for orbit radius thousands of km, and for siderial period days

Io 421.6 ******** 1.769138
Europa 670.9 ******* 3.551181
Ganymede 1,070 ******* 7.154553
Callisto 1,883 ******* 16.689018

those are practically the same as what I used, except for rounding, so the average orbit speeds should come out about the same

 

[marcus]

Here's what they say for moon mass and radius, so we can get the escape-from-surface velocities

Io 893.2 ****** 1821.6
Europa 480.0 ******** 1560.8
Ganymede 1481.9 ******* 2631.2
Callisto 1075.9 ******* 2410.3

units are E20 kilograms for the mass, and kilometers for the radius

Someone want to post the escape velocities? It gives a handle on how much use you can make of gravity assist from that moon.

 

[enigma]

Originally posted by marcus
Here's what they say for moon mass and radius, so we can get the escape-from-surface velocities

Io 893.2 ****** 1821.6
Europa 480.0 ******** 1560.8
Ganymede 1481.9 ******* 2631.2
Callisto 1075.9 ******* 2410.3

units are E20 kilograms for the mass, and kilometers for the radius

Someone want to post the escape velocities? It gives a handle on how much use you can make of gravity assist from that moon.

$\mu$'s [km^3/sec^2]

Io: 5957
Europa: 3201
Ganymede: 9884
Callisto: 7176

Gives surface escape velocities of [km/sec]:

Io: 2.55
Europa: 2.02
Ganymede: 2.74
Callisto: 2.44

I still don't see how you're going to back out gravity assist potential without knowing the geometry of the trajectories...

 

[marcus]

Originally posted by enigma


...surface escape velocities of [km/sec]:

Io: 2.55
Europa: 2.02
Ganymede: 2.74
Callisto: 2.44

thanks! these agree except possibly in the second decimal place
with ones I was just now thinking of posting:

Io: 2.56
Europa: 2.02
Ganymede: 2.74
Callisto: 2.45

Glad for the agreement.

Your question about the possibilities for gravity asst. at the Jupiter end of the trip.
Obviously the full potential depends on the configuration of the system at the time of entry and the specifics involve geometric detail. I'm interested in quick rough guesstimates.

As Nereid already indicated in this thread, considerable use has already been made of gravity in the Jovian system and opportunties are good because there are a lot of massive objects (close together and going different speeds). The 1995 Galileo mission probably used gravity assist in the system a lot, not to mention 3 or 4 others that have been thru that neck of the woods.

From the rough guesstimates already done, I can see why the 5.5 km/s that you enter the system with might not be too hard to shed gravitationally. And for that matter, if your destination is Callisto, say, a good bit of the escape energy at Callisto's distance might also be shed.

(Once out of the Earth's well) I'm guessing that the delta V cost of getting into orbit around Jupiter is mainly the initial 8.5 km/s kickoff at our end. Too sleepy to think more about it tonight tho.

 

[meteor]

What about Titan, the satellite of Saturn, that will be reached in January 2005 by The Cassini-Huygens probe?
http://saturn.jpl.nasa.gov/index.cfm

Titan has plenty of methane lakes, and I personally think that they will find life there. Is Titan profitable like a base for a human colony?

 

[marcus]

Originally posted by meteor
... methane lakes, and I personally think that they will find life there...

Meteor, I can't respond to the issue of "profitable" (either scientifically or economically).

I tend to think of the colonization urge as a healthy species urge, like sex, building houses, art and music. Things that
healthy people naturally want to do.

I would find living on Mars or the Moon depressing. But then I don't appreciate the state of Texas either.
I wouldn't like Titan becasue of the permanent fog and extreme cold---a most dreary possible climate.

A liquid water phase (like Europa's) is intrinsically around zero Celsius, a temperature I can understand. I would not mind a zero
Celsius ice cave environment as long as there was plenty of electric lighting. the prospect is a bit like Antarctica or Greenland, under the ice. Reminds me of where they built that neutrino telescope AMANDA 2.

But a liquid methane phase (like Titan's) is pretty scary for me to contemplate. And the atmosphere is so opaque one would never see the stars.

The question you need to ask, I think, is not "Would you like to do Science there?"
but
"Would you like to raise children there?"

In space, Science is something done via robotic extensions, surrogates, probes.

Titan could be of considerable scientific interest, and need to be investigated on scientific grounds. But it does not strike me as attractive for colonization.

What other large moons does Saturn have and what are they like, do you know?