Celestia Forums

I just downloaded the current orbital elements for the Cassini-Huygens Saturn probe from NASA's Horizons ephemeris server and created an ssc from them. The elements seem to be reasonably accurate, since Celestia shows the probe almost colliding with Saturn in July of 2004.

"Cassini 2002-10-10" "Sol"
{
Class "spacecraft"
Mesh "galileo.3ds"
Radius 0.01
EllipticalOrbit
{
Period 12.44131
SemiMajorAxis 5.36921504
Eccentricity 0.728031108
Inclination 0.821640702
AscendingNode 127.187606
ArgOfPericenter 167.738633
MeanAnomaly 94.9254870
Epoch 2452557.50
}
Albedo 0.5
}

I don't have a 3ds model of the probe, so I specified the Galileo model that's included with Celestia. Generating the correct orientation so the antenna points back toward Earth is left as an exercise for the reader

But if someone does manage to do that, please share!

Here are the orbital elements for 2000-01-01, when the probe was midway between the orbits of Mars and Jupiter, showing the orbit as it was before the gravity assist at Jupiter. It's nice to see the two orbits intersect behind the planet, with the probe in almost the same place in both orbits! Of course, neither orbit shows the correct position when the probe was near Jupiter, since Celestia doesn't include any gravitational effects.

"Cassini 2000-01-01" "Sol"
{
Class "spacecraft"
Mesh "galileo.3ds"
Radius 0.01
EllipticalOrbit
{
Period 8.045222
SemiMajorAxis 4.01506026
Eccentricity 0.784576797
Inclination 0.713669599
AscendingNode 144.749737
ArgOfPericenter 131.989887
MeanAnomaly 20.650847
Epoch 2451544.50
}
Albedo 0.5
}

selden wrote:Here are the orbital elements for 2000-01-01, when the probe was midway between the orbits of Mars and Jupiter, showing the orbit as it was before the gravity assist at Jupiter. It's nice to see the two orbits intersect behind the planet, with the probe in almost the same place in both orbits! Of course, neither orbit shows the correct position when the probe was near Jupiter, since Celestia doesn't include any gravitational effects.

I just created .xyz files with daily positions of Galileo and Cassini starting from launch date . . . The elliptical elements are a good approximation for the Jupiter-Saturn cruise phase, though.

I used JPL's HORIZONs system to create the .xyz files:

http://ssd.jpl.nasa.gov/horizons.html

The telnet interface is quite easy to use, if anyone's interested in doing some more spacecraft trajectories.

--Chris

Great!

I'm rashly assuming you'll be including them in the next pre-release

If you want to try them out now, they're right here:

http://www.shatters.net/~claurel/trajectories/

Place the .xyz file in Celestia's data directory and add the line:

SampledOrbit "cassini.xyz"

To the object description. And make sure you change the parent body to Sol, as the trajectories are all in heliocentric coordinates. The data isn't perfect yet, and there are occasional jumps. I suspect that there's a problem in my translation program, but I'll need to investigate further.

--Chris

Chris,

Using your xyz files, the probes seem to be following quite reasonable paths.

Unfortunately, the orbit drawing routines seem badly confused. Below is a screenshot of the galileo probe for 1989. I expected to see the orbit swirling around the inner plannets and heading out to Jupiter, but instead it looks like a squished bow-tie near Jupiter's orbit.



aha. This happens when I have several objects in the same .ssc file.
Galileo's xyz in a .ssc file by itself almost looks reasonable. The orbit routines seem to be insisting on drawing a closed orbit, however. Note the line from Jupiter's future location back to the current(?) location of Galileo
(or maybe they're connecting it back to the initial point. it's hard to tell.)



Here's the second .ssc file

"Galileo xyz" "Sol"
{
Class "spacecraft"
Mesh "galileo.3ds"
Radius 0.01
SampledOrbit "galileo.xyz"
Albedo 0.5
}

and here's the viewpoint info from favorites.cel

"Galileo xyz" {
isFolder false
parentFolder ""
base [ 0.003287800591656431 0.0005529944625383384 -9.18621273917751e-005 ]
offset [ 7.815970093361102e-014 -1.459889067273457e-014 1.031509946902709e-015 ]
axis [ 0.944346 0.236355 0.228794 ]
angle 1.77837
time 2447846.659788231
selection "#0/Galileo xyz"
coordsys "ecliptical"
}

I just tried simulating Cassini's orbit using Starry Night Pro 4 and the orbital elements for 10-10-02. The simulation shows the spacecraft arriving at a periapsis of 240,483 km from Saturn on July 11, 2004. The distance sounds about right, however the date is off by 10 days. Does the Horizons site publish projected orbital elements for the day of arrival (July 1, 2004)?

Peter,

Horizons' database includes whatever spacecraft trajectory information is provided to them by the relevant project management. In the case of Cassini, they have information up through the end of the offically funded project in 2008. It certainly includes the SOI in 2004. The reliability of the orbits after SOI have to be questionable, however. Although what they have is sure to be gravitationally accurate, my understanding is that the final decisions about many of the details of the trajectory have not yet been made.

The periapsis you mention is rather high. According to the most recent NASA document that I've been able to locate online (which is dated August, 2001), it should be at altitude of about 20,000 km above the cloudtops i.e. a pariapsis of about 80,000 km. See http://www.jpl.nasa.gov/news/fact_sheets/cassini.pdf

Using Horizons' heliocentric trajectory coordinates for Cassini, Celestia predicts a minimum altitude of 18,827 km on July 1, 2004 at 2:38 UTC.

I've put a summary of what I've found on my "spacecraft" web page at http://www.lns.cornell.edu/~seb/celestia/spacecraft.html#3.4.1

Oh good. I'll check Horizons for the orbital elements on 07/01/04 and plug those numbers into the orbit editor for SN Pro 4.0. I suspect that the reason that my periapsis is over 200,000 km instead of 78,800 km is that I was using the orbital elements from 10/10/02 instead of 07/01/04. It's also possible that Starry Night rounded off some of the significant digits in the orbital element list. I'll check that as well.

The last time that we had the pleasure of witnessing a spacecraft arrival at one of the gas giants was December 7, 1995 when Galileo and its probe cruised into the Jupiter system. The mother ship skirted by the south pole of Europa and came within a few hundred kilometers of Io before slipping behind the planet and firing its main engine. I did a simulation of that event using the old Microsoft Space Simulator. I uploaded the scenario to the Compuserve file library so that anyone could follow the ship during its final 10 hours of flight before periapsis. It was so cool because I had all these school teachers writing thank you notes to me for helping their students appreciate the live events as they were unfolding. The teachers ran my sim all day while classes were in session so that the students could glance toward the computer monitor whenever they wished and see the planet and moons in the same proportion as they would if they were on the real ship. The more curious walked up to check out the radial and tangential velocity of the ship relative to Europa, Io, and Jupiter as well as the current distance from those bodies. A lot more spacecraft information is on the net now than there was in 1995, however I might refine the Cassini simulation and do it again in 2004 just for fun.

Oh good. I'll check Horizons for the orbital elements on 07/01/04 and plug those numbers into the orbit editor for SN Pro 4.0. I suspect that the reason that my periapsis is over 200,000 km instead of 78,800 km is that I was using the orbital elements from 10/10/02 instead of 07/01/04. It's also possible that Starry Night rounded off some of the significant digits in the orbital element list. I'll check that as well.

The last time that we had the pleasure of witnessing a spacecraft arrival at one of the gas giants was December 7, 1995 when Galileo and its probe cruised into the Jupiter system. The mother ship skirted by the south pole of Europa and came within a few hundred kilometers of Io before slipping behind the planet and firing its main engine. I did a simulation of that event using the old Microsoft Space Simulator. I uploaded the scenario to the Compuserve file library so that anyone could follow the ship during its final 10 hours of flight before periapsis. It was so cool because I had all these school teachers writing thank you notes to me for helping their students appreciate the live events as they were unfolding. The teachers ran my sim all day while classes were in session so that the students could glance toward the computer monitor whenever they wished and see the planet and moons in the same proportion as they would if they were on the real ship. The more curious walked up to check out the radial and tangential velocity of the ship relative to Europa, Io, and Jupiter as well as the current distance from those bodies. A lot more spacecraft information is on the net now than there was in 1995, however I might refine the Cassini simulation and do it again in 2004 just for fun.

Peter,

Unfortunately, the "elliptical" orbital elements at the time of SOI are hyperbolic in both the Solar and Saturnian coordinate systems. Celestia really doesn't handle those yet: it insists on being provided a period, which is, of course, undefined. *sigh*

But, of course, the xyz trajectory is probably more appropriate anyhow, since the spacecraft will be experiencing forces due to both gravity and its own propulsion system. Celestia does not (yet) include a display of the velocities and accelerations involved. Perhaps one of the developers could be persuaded to augment the HUD appropriately, but I suspect that may also depend on a better interpolation algorithm to supply the values.

Selden,

That's where Starry Night Pro and Orbiter offer something extra. It would appear that Starry Night Pro's orbit editor can accept hyperbolic orbital elements. The orbit editor in SN has a special category called "Pericentric" which I believe is applicable to orbits having an eccentricity greater than 1. I'm not sure about Orbiter however its Scenario editor may also has this capability. Starry Night's display panel doesn't provide velocities and accelerations of the spacecraft, however Orbiter does. That's why I'm hoping to work out the Cassini simulation in Orbiter.

Peter,

(I'm sure you're aware of the following points, but I thought I'd make some comments at least to clarify the issues for others who may not be so familiar with astrodynamics.)

Don't forget that the orbital elements provided by Horizons are strictly for the reference epoch you've specified. Also, they're strictly Keplerian. As a result, they can't show what will happen as a result of accelerations due to anything other than the gravitational attraction of the central reference body. In other words, they can't show the orbit changing due to Cassini's propulsion system being applied to slow the spacecraft for Saturn Orbital Insertion.

In contrast, Horizons' cartesian vectors can include either or both the position and velocity vectors (as well as light travel time to a specified observatory), although not accelerations. At the moment, Celestia only uses txyz position information. This is one reason for needing a more appropriate interpolation algorithm which can, among other things, compute the accelerations that are causing changes in a trajectory.