I found this image on the Wikipedia page for the Huygens spacecraft:

enter image description here


  • Titan orbits saturn at ~5.57 km/s.
  • The probe seemed to reach ~6.5 km/s before hitting Titan.
  • The probe's velocity vector seemed almost perpendicular to Titan.

My question is, did any considerations have to be made for such an approach where the two objects have very different velocity vectors? Were any of my above statements untrue or due to a skewed scaling on the graphic?

  • $\begingroup$ I am not 100% sure what "any considerations have to be made for such an approach" means. All spacecraft approaches are carefully studied for years or more by a group of people who think long and hard. I've discussed that this is a straight-line approach, is there something specific you'd like to see added? $\endgroup$ – uhoh Nov 20 '18 at 13:14
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    $\begingroup$ @uhoh considerations as in, why did they choose this approach? You answered that very well with the explanation of Titan's atmosphere, that I didn't know about. I just had it in my mind that this same approach on Earth may not have worked out. $\endgroup$ – Magic Octopus Urn Nov 20 '18 at 13:16
  • $\begingroup$ Neither did I! I've added a bit about the angle. I don't have the time now to make a real 3D plot, but that shouldn't be hard. $\endgroup$ – uhoh Nov 20 '18 at 13:50
  • $\begingroup$ @uhoh it's odd that Titan's inclination is 0.3 degrees but the animation makes it seem almost 30 degrees. I think the inclination shown is relative to Huygens or something, there's definitely weirdness in that graphic. $\endgroup$ – Magic Octopus Urn Nov 20 '18 at 14:12
Revised: Jan 12, 2005            Cassini Huygens Probe                     -150

 The 318-kilogram (701-pound) Huygens probe separated from the Cassini orbiter
 2004-Dec-25, beginning a 22-day coast phase toward Titan (606). It will 
 descend through Titan's atmosphere, parachuting an instrumented robotic 
 laboratory to the surface of Saturn's largest moon. 

 The Cassini orbiter contains the electronics necessary to track the probe and
 recover data gathered during its descent. On 2005-Jan-14, 45 minutes before 
 the spacecraft reaches the atmosphere of Titan, timers will activate Huygens.

 The Huygens predicted descent trajectory was provided by ESA/JPL, valid as
 of Jan 6, 2005.

I've checked Horizons at 1 minute intervals and the predicted trajectory (a few days before landing) shows a nearly straight-line to its landing site.

Checking the speed, it shows rapid deceleration followed by a gentler approach to the surface, consistent with the parachute.

Titan has 0.14 of Earth's gravity. All else equal that means the scale height is 7 times higher. While Earth's atmosphere increase by a factor of e (~2.7) every 8 kilometers, for a lower surface gravity like Titan's that's going to be every 50 or 60 km. So Titan's atmosphere increases in density much more slowly, and the low gravity accelerates Huygens above it's original incoming velocity much less.

A complex spacecraft could never do a straight-line passive entry into Earth's atmosphere and survive to land with known materials, but on Titan with so much total atmosphere to slow down in gradually, and so little gravity to fight, it seems this worked nicely!

edit: You can see that at large distance, the angle between their relative velocity and position was close to zero, but as the spacecraft closed, the angle between the two vectors increased up to 3 degrees. That's only because the spacecraft was not targeting the center of the disk but off to one side.

As the spacecraft hit the atmosphere and started to descend, the angle dropped back down close to zero again.

Data directly from Horizions, plotted with Python: https://pastebin.com/mgjbZZ4J

Horizons Huygens to Titan

Huygens to Titan

Huygens to Titan

Huygens to Titan

  • $\begingroup$ I like how you can literally see both the parachute, and the atmospheric entry. Nice plots, I guess it does make sense, I think the scaling of the original graphic may of exaggerated the vectors a bit. I didn't now about the 7x scaling as well due to lowered gravity, I had heard Titan had a "taller" atmosphere, but not by THAT much. $\endgroup$ – Magic Octopus Urn Nov 20 '18 at 13:15
  • $\begingroup$ can you paste-bin the python scripts too :)? $\endgroup$ – Magic Octopus Urn Nov 20 '18 at 14:15
  • $\begingroup$ @MagicOctopusUrn that's exactly what you'll find there. $\endgroup$ – uhoh Nov 20 '18 at 14:40
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    $\begingroup$ oh, I assumed that was just the used horizon's data pasted to the pastebin and didn't look. Whoops, that's awesome. Thank you. $\endgroup$ – Magic Octopus Urn Nov 20 '18 at 14:56
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    $\begingroup$ The relevant value for part of the question is the entry flight path angle. That was targeted to be -65°±3°, and was actually -65.6°. Not as steep as you seem to indicate. (Entry was defined to be at a radius of 3845 km.) See this reconstruction paper for more details. $\endgroup$ – Mark Adler Nov 20 '18 at 15:46

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