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I found this image on the Wikipedia page for the Huygens spacecraft:
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?
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Revised: Jan 12, 2005 Cassini Huygens Probe -150
http://saturn.jpl.nasa.gov/
http://saturn.jpl.nasa.gov/spacecraft/instruments-huygens.cfm
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.
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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
