A robotic lander on a science mission, especially to an airless body, would like to avoid disturbing the surface it is landing on with rocket exhausts. MSL Curiosity used a Sky Crane to somewhat mitigate exhaust materials polluting the landing ground and the rover itself. Spirit and Opportunity were landed with airbags. For a stationary lander this would be even more important since the landing spot is the only within reach for physical sampling.

Landing on a low gravity asteroid or comet might allow for other techniques, and not allow for airbags to come to rest at a chosen place, if not even bouncing off. Rosetta's Phillae lander used a harpoon and grabbing landing legs, that however failed at the intended landing spot. Even when the mechanics works as intended, this seems risky at a body the composition of which is largely unknown and variable in place and over time.

Would it suffice to use heated or pressurized cold gas helium as exhaust gas during the final approach to the surface, since it is a chemically non reactive noble gas, and light so that it quickly evaporates? It would still blow away surface dust. Would a controlled crash landing with shock absorbing landing legs be an option?

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    $\begingroup$ not a Nobel gas, but a noble gas... $\endgroup$
    – Hobbes
    Mar 3, 2019 at 12:00
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    $\begingroup$ @Hobbes Heh, sorry! I'm Swedish so advertising Alfred Nobel is instinctive to me. $\endgroup$
    – Tombola
    Mar 3, 2019 at 13:53
  • $\begingroup$ Actually, I was logged in with an old user name. I'm LocalFluff posting this question. $\endgroup$
    – LocalFluff
    Mar 3, 2019 at 13:56
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    $\begingroup$ The problem with helium is that it leaks: Tank walls are a bit porous to the tiny helium atom. Helium as a cold gas propellant works fine for short-term missions, but not so fine for missions that span many years. $\endgroup$ Mar 4, 2019 at 0:28
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    $\begingroup$ I think the "controlled crash" idea has merit. Especially if, as you mention, the target is a low gravity object. It may be that in the real world (rotating target, autonomous guidance and control systems) this is too challenging with current landers. Would love to hear from anyone who has experience with the planning side of these types of missions as I'm sure they've considered the option. $\endgroup$
    – ben
    Mar 5, 2019 at 1:49

3 Answers 3


Any spacecraft that uses reaction mass for propulsion (pretty much everything that isn't theoretical) is going to somewhat contaminate the landing area. If you're using an inert fuel such as Helium or Xenon, you can at least prevent it from reacting chemically with the landing site, but it will still disturb surface features.

In a low gravity situation with a smaller vehicle mass; you might be able to kill your horizontal and vertical speed using thrusters while far away, use reaction wheels to control its orientation, and have (relatively) large shocks to absorb the impact.

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    $\begingroup$ In very low gravity environment, what about physically ejecting ballast with a spring to remove most of the approach speed? The ejected reaction mass might be harpoons with instruments for subsurface measurements. What constrains how much kinetic energy could feasibly be stored in a spring? $\endgroup$
    – LocalFluff
    Mar 5, 2019 at 12:15

Wikipedia says that Ryugu (for example) has a mass $M_{ryu}$ of 4.5×10¹¹ kg and a radius $R_{ryu}$ of about 450 meters. With G ~ 6.674 × 10⁻¹¹ m³ kg⁻¹ s⁻² that makes the surface gravity about 1.48 × 10⁻⁴ m s⁻².

1. Hover at 200 m with angled thrusters

You could use either big ion thrusters using a noble gases angled at +/- 45 degrees to bring you within ~200 meters of the surface without much ion sputtering. Unlike cold gas thrusters, the ion beams accelerated to say 100,000 eV would have a very narrow emission angle, roughly the square root of the ratio of the ion plasma thermal energy to the acceleration energy for a design optimized for the task.

$$ \theta \approx \sqrt{\frac{k_B T_{ion}}{Ve}} \approx \sqrt{\frac{1}{100,000}} \approx 0.2° $$

1 eV corresponding to about ~10,000 K sounds pretty hot for an ion temperature, this is probably a conservative number. See for example JPL/Descanso Fundamentals of Electric Propulsion: Ion and Hall Thrusters Dan M. Goebel and Ira Katz If the ion beam spread out due to self-propulsion, you might start with a wide exit aperture and try to play tricks with attraction to a central electron beam which you need for spacecraft charge neutrality.

For an $m=$600 kg satellite like Hyabusa 2 at this range, each thruster would need a thrust of

$$T = \frac{\sqrt{2}}{2} G\frac{m M_{ryu}}{(R_{ryu}+200)^2} \approx 45 \text{mN} $$

which is only about half the thrust of one of DAWN's three main ion thrusters.

2. Drop from 200 m landing on dissipative legs or stilts

Shutting off the thrusters the spacecraft would fall from it's 200 meter hovering altitude towards the surface. We can get the terminal velocity by conserving energy $\Delta T + \Delta U = 0$.

The kinetic energy gained at impact would be:

$$\Delta T_i = -\Delta U = m M_{ryu} G \left( \frac{1}{R_{ryu}} - \frac{1}{R_{ryu}+200} \right) \approx 12.3 \text{Joules},$$

and so the velocity at impact is

$$v_i = \sqrt{\frac{2 T}{m}} \approx 20 \ \text{cm/sec}.$$

As suggested by the OP in the question, you could soak that up with the landing gear. You could do that with some combintation of thing like:

  • viscous friction using some passive dashpots
  • springs equipped with a fast-acting latch at zero velocity
  • linear motor/generators with dynamic braking in each leg (convert energy electricity and dump in resistors) my favorite!

You could also add one or three thrusters (noble ion or not) on the backside of the spacecraft pushing you into Ryugu, in order to cancel any possible recoil if you latched your springs too soon or your legs didn't all touch at the same time or there was a more complex landing involving shifting rocks or gravel. You'd use three instead of one to cancel angular momentum resulting from the legs hitting at different times.

This is why my favorite option is the linear motor/generator with dynamic braking in each leg. You can wait until you have something like three point contact before starting active deceleration. Servo control can be really smart and fast with modern electronics and inertial sensors.

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On any reasonably low gravity world you can use the skycrane concept and keep your rockets far from your touchdown site. Without a really low-g world the booster is going to crash somewhere nearby, though, as the fuel needed to boost it back into orbit would be major. (Not so much in terms of what it would have to carry, but what it would do to your launch weight.)

  • $\begingroup$ Do you mean like, when landing on an airless body with more substantial gravity such as the Moon, having the landing engines on long booms on either side of the surface science instruments, pointing the exhausts away from it? $\endgroup$
    – LocalFluff
    Mar 5, 2019 at 12:12
  • $\begingroup$ @LocalFluff Long booms need rigidity. You're much better off connecting the engines to the probe with a cable like they did with curiosity. On an airless world you'll do the equivalent of an ullage burn to straighten the cable before you start the landing burn. $\endgroup$ Mar 5, 2019 at 15:37

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