# Are rovers useful on low gravity, low atmosphere bodies?

Are rovers useful for exploring low atmosphere, low gravity objects? Are there more appropriate exploratory vehicles being proposed?

If I am interested in taking geological samples from a region that is many kilometers wide and has rugged terrain, would it be appropriate to use a rover or just to use the probes propulsion to move to new sample locations?

I'm interested in very low g (1-10% of Earth's)

Both flying and hoping options work, see for example https://en.wikipedia.org/wiki/Hayabusa2 to 162173 Ryuguto which has deployed hopping/rolling sub probes and is scheduled to hover in to take surface samples later in 2019. As gravity decreases the traction available for wheeled travel falls and the probability of vertical or beyond vertical terrain increases, making hopping/jumping/flying traversal both more useful and more achievable in terms of energy expended.

Gravity at 162173 Ryugu is has around 1/80,000 of a G.

A probably useful data point is the 0.05-0.02G region (bodies in hundreds of kms radius) at which the gravity of a body will start to pull the surface into a sphere with more flatter/firmer surfaces amenable to wheeled roving and escape velocity is no longer something that is achievable by accident.

A body with very low gravity has no atmosphere anyway. But rovers did work on the Moon too, not only on Mars.

A rover should be substantially slower than the escape velocity of the small body to be useful. There should be no danger of escaping the body just accidentally.

Lets look at some small bodies for examples.

Pluto's surface gravity is 0.62 m/s^2 and the escape velocity is 1.21 km/s.

Ceres 0,29 m/s^2 and 0.51 km/s, Vesta 0.25 m/s^2 and 0.36 km/s.

For constant density, the escape velocity and the surface gravity of a body scale linearily with its radius and the diameter.

The diameter of Vesta is about 525 km, a similar body with a diameter of about 105 km would have 0.05 m/s^2 and 0.07 km/s. A rover with a peak velocity of 10 m/s would be still much slower than the escape velocity of 70 m/s. No danger for accidental escape at a gravity even less than 1 % of Earth's gravity.

The velocity for a very low orbit is a bit lower, the factor is the square root of 1/2, so if escape velocity is 70 m/s the orbit velocity is about 50 m/s. No danger of an accidental orbit.

• With regards to the rover speed needing to be lower than escape velocity. Would it not also be useful to compare velocities to that of a circular orbit? Because say it is traveling with enough velocity to achieve circular orbit and accidentally hits a ramp, the rover is as useless as if it hits escape velocity? Commented Jan 6, 2019 at 12:15
• The difference between escape velocity 11.2 km/s and orbit velocity of about 8 km/s of Earth is not that big. A factor of the square root of 2, half the kinetic energy, that is the relation between escape velocity and orbit velocity.
– Uwe
Commented Jan 6, 2019 at 12:36
• I think poor traction will be a major problem for rovers in very low gravity long before accidentally reaching orbit becomes a problem. Commented Jan 6, 2019 at 22:43
• @Capeboom for the purposes of that sentence, yes that's correct. But for realistic up/down/bumpy terrain you'll have problems at much lower speed.
– uhoh
Commented Jan 6, 2019 at 22:44