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I was actually wondering which planets in our solar system would allow for probe landing VIA parachute-based technology and which would have a high likely-hood of not destroying the probe before it even got near the surface. Gas planets are obviously excluded in this, but I was wondering which planets could actually use an aerocapture, drogue chutes or parachutes to reduce incoming velocity.

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    $\begingroup$ Related: what-if.xkcd.com/30 $\endgroup$ – Hobbes May 10 '18 at 5:54
  • $\begingroup$ @Hobbes that's an amazing graphic! Holy cow, that was enough to be an answer! XKCD has a nack for explaining things to people. $\endgroup$ – Magic Octopus Urn May 11 '18 at 14:00
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Once again, apologies for the length, but there are a lot of interesting aspects to this.

There's a big difference between aerocapture and parachute-assisted descent, so I'll treat those separately. Aerocapture is using aerodynamic drag in a body's (planet's or moon's) atmosphere to go from a hyperbolic approach orbit to a captured orbit around that body. It doesn't involve descent to the surface. More at the end.

Obviously parachutes work well at Earth and Titan—they did, and do. Here on Earth parachutes are used frequently, no questions there. At Titan, the Huygens entry probe and lander went all the way to a soft landing on the surface, on a parachute (https://en.wikipedia.org/wiki/Huygens_(spacecraft); https://www.esa.int/esapub/bulletin/bullet92/b92hassa.htm; oddly enough, the Wikipedia article has much more specific information than any of the ESA sites I found in a quick search). It survived the landing in fine shape, even after jettisoning the initial main parachute and opening a smaller one to shorten the time required for the descent, which was still 2-1/2 hours. You just have to ensure that the parachute materials don't get brittle and break at temperatures as low as the tropopause, ~70 K (http://sci.esa.int/cassini-huygens/55222-science-highlights-from-huygens-1-profiling-the-atmosphere-of-titan/).

Mars is different. Where the surface atmospheric mass densities at Earth and Titan are ~1.2 and 5.5 kg/m^3, respectively, at Mars it's more like 0.02 kg/m^2 (https://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html), not enough to support descent all the way to a soft landing. Successful landing craft to date have used aeroshells to decelerate from hyperbolic (and hypersonic) approach speeds to low supersonic speeds, ~Mach 2, then parachutes for deceleration from supersonic to subsonic speeds, then retropropulsion with rocket engines for deceleration from there to speeds allowing landing with landing legs, airbags, etc.

At JPL I saw Mars mission architects and engineers looking at using parafoils at Mars. Parafoils can give a glide ratio of ~3:1, allowing potentially pin-point landings. With proper control inputs you can get them to land the payload at significantly slower than their nominal stall speeds (reference: my work on the Genesis Mission parafoil [https://www.jpl.nasa.gov/missions/genesis/], and my experience as a skydiver!). But apparently the JPL Mars people rejected the idea, because I haven't seen any advanced mission concepts using it. That rejection might have been premature. I watched a small group looking at a simulation of opening a parafoil with an attached lander at Mars. Upon opening it assumed the normal shape and position relative to the payload, except that its glide slope was ~70°, only ~20° from straight down! They saw that and said, "Nope!", ended the simulation, and went on to other things.

That behavior is precisely what you'd expect if the parafoil is going too slow. They dive to pick up speed. Those folks should have let the simulation run until it either reached normal flying speed or impacted the surface. That said, the flight speed for a given wing loading (system mass divided by the surface area of the wing, in this case a parafoil) is proportional to 1/√(rho), where rho is the atmospheric mass density. At Mars, all else being equal, that means you expect flight speeds almost 8 times as fast as at Earth, for the same wing loading. Even at 1/10 of stall speed this would be considered a crash. You'd have to go to much lower wing loading, which means a much larger parafoil, and that could wind up taking too much mass. To me, the jury is still out.

Parachutes would work quite well at Venus until they melt somewhere before reaching the surface, where the temperature is ~735 K, or ~865 F. Various landers have used parachutes for deceleration high in the atmosphere, where temperatures aren't so hellish, but after a while they release those parachutes and complete the descent using drag plates that seem to work just fine. (ex: http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?1977SSRv...20..283C&defaultprint=YES&filetype=.pdf ; https://en.wikipedia.org/wiki/Venera) It could be that future materials research might find a material for a Venus parachute that would withstand the harsh conditions yet is light enough and flexible enough to use for a parachute.

Aerocapture is hypersonic in and hypersonic out, just slowed enough to change from what would have been a permanent, hyperbolic departure to a captured orbit. You don't want dense atmosphere for that, it's too much deceleration. You want densities corresponding to pressures in the few-millibar to tens of microbar range, depending on atmospheric composition, planet radius, and required ∆V (https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20060012088.pdf, https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20060007561.pdf). NASA is considering aerocapture at Venus, Earth, Mars, Titan, and all the giant planets except Jupiter (too darned fast!). Plus, and this is speculative, there might even be some things you could do at Triton or Pluto!

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    $\begingroup$ Why can't you aerocapture at Jupiter? Yes, you're going very fast but that simply means you want to stay higher. Is the atmosphere too unpredictable to safely aerocapture? $\endgroup$ – Loren Pechtel May 10 '18 at 3:07
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    $\begingroup$ It's a matter of dissipated energy. The typical approach trajectory has a V∞ of ~7 km/s, and that leads to a speed at the entry interface of ~59.7 km/s, with a kinetic energy of ~1.78 GigaJoules per kilogram of mass. A long, 180-day orbit at Jupiter with a periapsis at the entry interface has a periapsis velocity of ~59.2 km/s, kinetic energy of ~1.75 GJ per kg; difference of 31.9 MegaJoules per kg the aeroshell has to dissipate. Current-technology heat shield materials can't handle that. If you stay higher, you go in and out of the atmosphere before you get enough deceleration. $\endgroup$ – Tom Spilker May 10 '18 at 3:43
  • $\begingroup$ Those kinetic energy numbers are irrelevant because you're not shedding anything like that velocity. Juno only had to shed about 500 m/s for capture and it could be done with even less (lower periapsis and you only need capture on the first pass.) $\endgroup$ – Loren Pechtel May 11 '18 at 0:33
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    $\begingroup$ (59.7 km/s) - (59.2 km/s) = 0.5 km/s = 500 m/s. No different from what you're quoting. And at non-relativistic velocities, there's no arguing with Ek = mV^2/2: the difference in kinetic energies is indeed 31.9 MJ/kg. Juno JOI was 542 m/s, from ~58.5 km/s. Specific kinetic energies pre- and post-JOI are then ~1.712 GJ/kg & ~1.680 GJ/kg, with a difference of 31.6 MJ/kg. Juno's main engine had to remove that energy, averaging ~0.26 m/s^2 for 35 minutes. Rough check: 645 N thrust X 58.3 km/s (avg velocity) X 35 min = 30.4 MJ. Pretty close! $\endgroup$ – Tom Spilker May 11 '18 at 6:39
  • $\begingroup$ And I thought the answer was technical these maths... wow. $\endgroup$ – Magic Octopus Urn May 11 '18 at 13:38
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Earth, Mars, Titan: Atmosphere where aerocapture and parachutes might be useful, and the surface conditions aren't too brutal. Mars's atmosphere is on the order of 1/100 the density of Earth's, so a parachute landing attempt probably has to be assisted by retrorockets or large landing bags. Titan's atmosphere is somewhat denser than Earth's, so parachutes should work fine.

Venus: Very thick atmosphere. Even a small parachute will make for very gentle descent, but it's very hot and your probe won't last long at the surface -- the record for lander survival after touchdown to date is 127 minutes.

I believe none of the other planets or moons have enough atmosphere to provide for aerocapture -- Triton, e.g., has about 1/70000 the pressure of Earth's atmosphere; Pluto even less.

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