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If we set aside the question how the Apollo LM got into orbit around the celestial body in question, I wonder which celestial bodies other than the Moon the LM could successfully land on. Mercury for instance is very similar to the Moon, but it has more than twice its surface gravity. It would try to land on the night side of course, as it would probably melt on the dayside, but I wonder on the gravity/fuel part, if a crew would be able to brake the LM enough to make a soft landing.

I guess the LM would be able to perfectly land softly on the following bodies, other than the Moon:

  • Ceres, Pallas and Vesta in the main asteroid belt
  • Callisto and Ganymede, and without Jupiter's intense radiation belts on Europa safely too
  • Io has a higher surface gravity than the Moon, but I guess the LM would do it as it proved more efficient than expected, but one has to take into account that Io has ioquakes, volcanism, and its surface changes permanently, so landing would be very difficult but not impossible (except it would, because of the radiation belts mentioned above)
  • all the seven planetary moons of Saturn (including Titan if the LM is able to brake enough in time before entering the atmosphere at speeds that would damage it because of aerodynamic heating)
  • all five Uranian planetary moons
  • Neptune's moons Triton, Proteus and Nereid
  • all dwarf planets beyond Neptune (afawk they have low gravities and little to no atmospheres)
  • Mercury has a high gravity, so I wonder if the LM's fuel would suffice to brake it enough for a soft landing. Of course, the LM would be unable to leave again, even if it would get refueled on the surface I guess.
  • If the LM is able to land on Mercury, I think it would also be able to do so on Mars in case it was able to brake enough before entering Mars' atmosphere at too high speeds, for aerodynamic friction not to burn it up)

The LM is definitely unable to softly land on Venus and the Earth (when coming from orbit), or safely enter the atmospheres of the gas giants, as these planets have too high masses.

Did I make a mistake above? Could the LM safely enter Titan's atmosphere or not? Would it be able to land on Mercury and Mars? Could it get off Mercury into orbit if fueled completely?

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2 Answers 2

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I'll ignore the thermal, radiation, and other considerations, and consider only the general performance characteristics of the LM.

The nominal "fully automatic" descent profile for the Apollo LM required about 2080 m/s of delta-V, with a small amount of additional propellant budgeted for a manual approach and other contingencies. This is the primary limitation for landing on massive bodies.

The LM engine could throttle continuously from 10% to 65% thrust; at the end of descent, this would counter a gravitational pull of between 0.06g and 0.41g. I don't think it would be practical to pulse the engine on and off to achieve a lower average thrust. With different software, it might be possible to use the RCS thrusters for final descent; these thrusters are designed for rapid pulsed operation, and could counter as much as 0.02g.

Descent from low Mercury orbit would require quite a bit more delta-V than the LM could provide alone. If it was somehow mated to a "braking stage" that could power most of the descent, it might be able to land, but there would be little margin for error; Mercury's gravity of 0.38g is uncomfortably close to the high end of the throttleable range of the LM descent engine. Not recommended.

Mars likewise isn't possible. In addition to the high gravity, the atmosphere of Mars is just thick enough to wreck something as delicate as the LM; peak dynamic pressures would be on the order of 10 kPa.

If you could somehow get the LM down to the surface of Mars or Mercury, the ascent engine's thrust-to-weight ratio is only 0.33, just insufficient to lift off from the surface with a full fuel load. Even with a more powerful engine, it wouldn't nearly reach orbit.

Ceres, Pallas, and Vesta have the opposite problem. Their gravity is between the minimum thrust of the descent engine and the maximum thrust of the RCS. Replacing the descent engine with a scaled-down version might solve the problem, or you could time a terminal burn to come to a dead stop a short distance above the surface (say, 25 meters), and fire the RCS to slow the remaining fall; the latter strategy would require significantly different guidance and control software.

Callisto and Europa are doable. Io and Ganymede are marginal; you'd need assistance from a braking stage to provide some of the descent delta-V, but the engine would be powerful enough to perform the final landing.

Titan is out of the question. Not only is its atmosphere very thick, it has a very high "scale height", a measure of how the density of the atmosphere decreases with altitude. The LM was not designed to survive moving at speed through any atmosphere at all.

It takes much more fuel to descend at low speed than at high speed (because it takes longer, and gravity is trying to pull you down faster the whole time, so you have to expend propellant fighting it), so it's not practical to brake to a safe speed above the atmosphere of Mars or Titan and descend slowly.

Mimas, Enceladus, and Tethys, Uranus's moon Miranda, Neptune's Proteus and Nereid and many other minor planets and small moons, are probably landable using the RCS; Dione, Iapetus, and Rhea and the other 4 major Uranian moons are similar to Ceres, Pallas, and Vesta, with surface gravities in the no-go range between the RCS and descent engine capabilities.

Triton and Pluto have just a trace of atmosphere (on the order of 1/1000 of 1% of Earth's air density); I believe the LM could survive that; the peak dynamic pressure would be negligible, on the order of 50 Pa. Pluto's surface gravity is right at the lower limit of descent engine throttling, but with careful timing it's doable.

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    $\begingroup$ That would be similar to the Falcon 9's (ill-named) hover-slam maneuver. I'm not certain that the LM's radar altimeter could manage the timing correctly at an unprepared landing site, but I suppose it's not out of the question. I'll edit. $\endgroup$
    – Russell Borogove
    Commented May 10, 2021 at 19:16
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    $\begingroup$ “The LM engine could throttle continuously from 10% to 65% thrust” This confuses me, what happens above 65%? $\endgroup$
    – Michael
    Commented May 11, 2021 at 5:09
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    $\begingroup$ @Michael Nozzle erosion, apparently, between 65% and 95% or so. I don't know if this is due to turbulence in the combustion chamber at intermediate throttle settings, or some other factor. The descent program held full throttle until the computer calculated it was time to go to 50% or lower throttle, then skipped over the forbidden throttle zone and began dynamically controlling the throttle to aim for near-zero velocity at zero altitude. $\endgroup$
    – Russell Borogove
    Commented May 11, 2021 at 5:16
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    $\begingroup$ @Giovanni The new suggestions only address the very last part of the landing; they don't significantly change the high speed atmospheric issues. Triton's and Pluto's atmospheres are thin enough (around 1/1000 that of Mars, 1/100000 that of Earth) that the LM could land (added to the answer). I believe you're correct on the Uranian moons. $\endgroup$
    – Russell Borogove
    Commented May 11, 2021 at 14:53
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    $\begingroup$ @Michael I looked for more info on the throttle restriction and couldn't find much detail. "The initial design attempted to provide variable thrust near the maximum rated thrust; however, excessive problems with mixture ratio control and throat erosion caused the selection of a fixed throttle point maximum thrust setting..." Possibly the changing mixture ratio produced a more corrosive exhaust in the 65%-90% range. $\endgroup$
    – Russell Borogove
    Commented May 11, 2021 at 15:29
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The LM had a landing stage and an ascending stage. Fuel of the ascending stage could not be used for landing, the ascending stage had no legs. But all resources (oxygen, water and batteries) for a stay on the Moon were located in the descending stage. The ascending stage had only very small storage of oxygen, water and electricity.

If landing was aborted, the landing stage was separated and the ascending stage used during separation. This maneuver was called "fire in the hole".

So landing was possible only for slightly more gravity than the lunar gravity.

The gravity of the Moon is 1.62 m/s2, the escape velocity 2380 m/s.

Io: 1,796 m/s2 and 2376 m/s The difference between Io and the Moon is so small that landing seems possible.

Mercury : 3.70 m/s2 and 4.3 km/s. The ratio of the squares of the escape velocities of Mercury and Moon is 3.26. ( the energy needed for a certain speed is proportional to the square of the speed). So even the fuel for both ascent and descent stage is not enough for a landing on mercury

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  • $\begingroup$ My question focuses on the landing stage of course. Do you think the LM could handle Io's gravity (0.183g)? $\endgroup$
    – Giovanni
    Commented May 10, 2021 at 16:00
  • $\begingroup$ Fuel from the ascent stage could actually be routed through the rcs to provide additional delta-v. Not sure when they would overheat, but some of that fuel could have been burned during a landing. $\endgroup$
    – Innovine
    Commented May 10, 2021 at 19:04

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