Galileo mission fact sheets (in case yr interested)

[marcus]

this 9-page NASA fact sheet about the
circa 1995-2003 Galileo mission at Jupiter
is pretty informative

http://www.jpl.nasa.gov/news/fact_sheets/galileo0309.pdf


also there's this longer document with several chapters
here are links to chapters 6 and 8

Chapter 6 has a diagram of arrival day
which included an Io flyby and
the dropped probe parachuting into the atmosphere
and a JOI burn at perijove (the first orbit was a long 7 months)

Chapter 8 has a petal diagram for the first dozen or so orbits

http://www.jpl.nasa.gov/galileo/tour/6TOUR.pdf

http://www.jpl.nasa.gov/galileo/tour/8TOUR.pdf

this document says the main (400 Newton) engine was used 3 times
on the way there (ODM)
at first perijove (JOI)
at first apojove (PJR)
there were a dozen or so smaller (10 Newton) engines

there were close moon flybys at each of the subsequent 35 or so orbits except for one orbit (number 5) which happened when
Jupiter was behind the sun

lot of interesting description of the mission and the science results
cant begin to summarize. You may have already seen similarly detailed source material on Galileo, I had not.

 

[marcus]

Here's a sample table from the first source
looks like there were two orbits (both at conjunction
when J was behind sun) when they didnt get a moon flyby
Incidentally the thing was supplied about 500 watts by two
thermoelectric generators heated by plutonium isotope decay.
the mission was extended a couple of times because
things kept working, so the original dozen orbits turned
into some 35 orbits. by that time the power supply was down
to around 400 watts. Here's the table on page 4 of the
fact-sheet

"Close encounters by the Galileo orbiter"

Orbit Target Date Altitude
--------------------------------
0 Io Dec. 7, 1995 897 km (558 mi)
1 Ganymede June 27, 1996 835 km (519 mi)
2 Ganymede Sept. 6, 1996 261 km (162 mi)
3 Callisto Nov. 4, 1996 1136 km (706 mi)
4 Europa Dec. 19, 1996 692 km (430 mi)
5 none
6 Europa Feb. 20, 1997 586 km (364 mi)
7 Ganymede April 5, 1997 3102 km (1928 mi)
8 Ganymede May 7, 1997 1603 km (996 mi)
9 Callisto June 25, 1997 418 km (260 mi)
10 Callisto Sept. 17, 1997 535 km (333 mi)
11 Europa Nov. 6, 1997 2043 km (1270 mi)
12 Europa Dec. 16, 1997 201 km (125 mi)
13 none
14 Europa March 29, 1998 1644 km (1022 mi)
15 Europa May 31, 1998 2515 km (1562 mi)
16 Europa July 21, 1998 1834 km (1140 mi)
17 Europa Sept. 26, 1998 3582 km (2226 mi)
18 Europa Nov. 22, 1998 2271 km (1411 mi)
19 Europa Feb. 1, 1999 1439 km (894 mi)
20 Callisto May 5, 1999 1321 km (821 mi)
21 Callisto June 30, 1999 1048 km (651 mi)
22 Callisto Aug. 14, 1999 2299 km (1429 mi)
23 Callisto Sept. 16, 1999 1052 km (654 mi)
24 Io Oct. 11, 1999 611 km (380 mi)
25 Io Nov. 26, 1999 301 km (187 mi)
26 Europa Jan. 3, 2000 351 km (218 mi)
27 Io Feb. 22, 2000 198 km (123 mi)
28 Ganymede May 20, 2000 809 km (502 mi)
29 Ganymede Dec. 28, 2000 2338 km (1452 mi)
30 Callisto May 25, 2001 138 km (86 mi)
31 Io Aug. 6, 2001 194 km (120 mi)
32 Io Oct. 16, 2001 184 km (114 mi)
33 Io Jan. 17, 2002 102 km (63 mi)
34 Amalthea Nov. 5, 2002 160 km (99 mi)
Planned Encounter
35 Jupiter Sept. 21,2003, impact

 

[marcus]

how they killed 5.5 km/s using very little delta-vee

One interesting thing to me is how little actual delta-vee was involved in the Jupiter Orbit Injection (JOI).

Using a standard transfer ellipse one gets to J with a 5.5 km/s difference in speed to make up. If you don't do something, the Jove System will barrell on past you at 5.5 km/s

It would be expensive to burn rocket to get that 5.5 km/s, so they don't do that. Instead they let the craft fall in close to J and made a comparatively small burn.

A lot of people are familiar with this but I will say it (if I get some detail wrong maybe someone will help out).
At Io distance circular speed around J is 17.3 making escape just under 24.5. If you enter with 5.5 and fall to within that distance, you will be going

\sqrt{5.5^2 + 24.5^2}

which is only 25!
so to catch hold of the jovian system all you need to
burn off is ONE km/s.

this gets it down to 24 km/s at perijove, and you are now in an elliptical orbit around J.

I don't know the exact details but this gives a rough idea. Their actual first pass was within 4 Jove-radii (they were closer than Io actually) and they did the JOI burn shortly after "watching" the parachute probe go down. It must have been on the order of 1 km/s.

Then at the top of the first ellipse they needed a small burn to fatten the ellipse out---add a little angular momentum so that the next perijove would not be so close into Jupiter. In the fattened ellipse the perijove distance was 11 Jove-radii, instead of 4. The whole first orbit took 7 months.

After that, they say they didnt use the 400-Newton engine (at least for the next dozen loops, maybe never AFAIK) so they apparently navigated by gravity assists, from flybys of the moons,
and they quickly got their orbits to have periods like a month or two. You can see this from the table, which lists the orbits by number and gives a key date in each orbit. Often orbits seem to have taken something on the order of a month.

 

[marcus]

what delta-vee do you actually need to get to J

the standard Earth-to-Jupiter transfer ellipse seems to involve two boosts
the first by about 8 km/s and the second one 5.5 km/s

because the perihelion speed is 8 km/s faster than the Earth goes
and the aphelion speed is 5.5 slower than Jupiter goes

in another thread we were discussing the total delta-vee cost of going to J and you might say from this that the leg-out is going to cost 13.5

but it doesn't need to because you can use Jupiter's gravity and do the JOI with only ONE km/s at that end. After that there are plenty of opportunities for close flybys with the moons and a chance of getting moon-gravity-assist.

Essentially, every time you fall past a moon you gain or lose energy. If you pass "in front" of the moon you lose energy and
if you pass "astern" of it (in the moon's wake, so to speak) you gain energy. Angular momentum also changes.

Now if someone is for some reason opposed to imagining humans going to the Jovian System they could well jump up at this point and say that this kind of JOI is only good for robots. Because if you are stingy with the engine and "save gas" then the first loop takes 7 months and the people don't want to wait 7 extra months they want to land right away somewhere. Also I calculated it where the first pass is at Io distance and there is a bunch of radiation at that distance, which is surely a consideration. But let's not address too many issues at once. Let's just see what the delta-vee cost is
when we are allowed a 7 month loop and all that gravity assist.

What I'm wondering now is about the 8 km/s cost at our end, on the outbound leg. Suppose we are down in Low Earth Orbit going what?
Maybe 7 km/s? Does anyone want to help out and estimate the delta-vee cost of doing two things at once----getting loose from Earth gravity, from the altitude of Low Earth Orbit, and also at the same time getting going in the same direction as the Earth but 8 km/s faster.

this is to get on the transfer ellipse with its perihelion speed of 38 km/s.

My guess is that starting from LEO it only takes about 8, or just a little more, to do that.

So as a rough estimate the delta-vee cost from LEO to Jupiter orbit would be around 9 or 10 km/s
Something like 8 or 8.5 at this end
and 1 or 1.5 at that end.

If this is majorly out of line please help out with corrections!