# Can you use a light sail to achieve a survivable landing anyplace in our solar system?

Current answers at the new question Can you launch a probe to the Moon using a laser propelled craft? points out getting to the moon is not the problem, landing on it is.

This answer suggests that you can achieve escape velocity from Deimos using a bike and a ramp. If you can launch a bike presumably you can land it safely.

In theory you could use a light sail to put yourself in orbit around Deimos. You should also then be able to use it to lower your orbital speed so you touch down.

Is it possible to use a light sail to achieve a touch down survivable for a human? (Not sure what this would be, I saw several limits, but not sure how to translate that into something here)

• Jun 12 '16 at 10:35

A light sail in orbit around the sun can reflect light in a way to increase or decrease the orbit.

A light sail in orbit around another body is much more constrained. I haven't seen that you could easily manipulate such an orbit in a way that keeps it roughly circular. But if so, it would be the same as any other low-thrust engine. It would give you the ability to change the orbit over time, but not provide any significant landing thrust.

So the minimum speed you would have for impact with Deimos would be the same as the speed of a circular orbit at the surface, with higher points better. Lets be optimistic and say that above the mean radius of $6.2\text{km}$, there's a hill with an altitude of $0.4\text{km}$ (wild overoptimistic guess). The mass of the moon is more than $1.4 \times 10^{15}\text{kg}$. That makes the landing speed: $$v = \sqrt{\frac{GM}{r}}$$ $$v = \sqrt{\frac{(6.67\times 10^{-11} \text{N m}^2\text{kg}^{-2})(1.4\times10^{15}\text{kg})}{(6.6\text{km})}}$$ $$v = 3.8\text{m/s}$$

If the craft were small enough, you could probably design a landing/impact system that could cope with such a speed. But if the only propulsion is a sail, it will have virtually no maneuverability. Hitting a sandy plain at that speed and smacking the face of a boulder at that speed will probably have different outcomes.

• Google tells me that 3.8 metes per second is 8.5 miles per hour (13.6 KPH) my bicycle should easily be able to handle that. Jun 10 '16 at 17:51
• That is true, but something using a solar sail will be really light, and thus might not be able to handle much impact velocity... Jun 10 '16 at 18:56
• To a first approximation, a 4m/s impact is what you get if you drop something while standing up. Would you be happy dropping your probe onto a concrete or gravel path from that height? If so, great... Jun 10 '16 at 19:20
• @Andrew with enough balsa wood, anything is possible :) space.stackexchange.com/a/18862/12102
– uhoh
Jun 19 '17 at 3:24