# How to achieve escape velocity from Ceres?

If I am interested in a sample return mission to Ceres using a similar propulsion technology package to Dawn, what kinds of additional propulsion would be required to land and then return?

Calculations in [1] and [2] yield that to achieve $$v_{esc}$$ you require 503m/s, and to achieve $$v_{circ}$$ 356m/s. Given the free rotation at the equator of 94m/s you need a $$\Delta$$V of 410m/s for $$v_{esc}$$ or 262m/s for $$v_{circ}$$.

My question is: aside from the large amounts of extra fuel for the return journey, what propulsion systems would be appropriate to make up this additional $$\Delta$$V. I would have thought that it is the initial 262m/s to achieve $$v_{circ}$$ that is prohibitive, as once orbiting you can continue using ion thrusters until $$v_{esc}$$ is achieved. So, what kinds of technology are appropriate for that range.

Dawns wet mass was ~1300kg and as $$g_{ceres}=0.285m/s$$ so thrust required to overcome gravity is 370N (although this would scale due to higher propellant and extra propulsion system requirements).

For asteroid mining, it seems to me that a combination of efficient ion engines with an additional propulsion system to achieve $$v_{esc}$$ is required.

## 1 Answer

A hydrazine monopropellant rocket would have a specific impulse of around 230-240 seconds and so an exhaust velocity around 2000 m/s. The rocket equation then tells us that for a delta-V of 262 m/s that means a mass ratio of about 1.13, so maybe 130 kg of hydrazine for a 1 ton payload. The advantage of a monopropellant is simplicity and reliability. Once back in Ceres orbit, the hydrazine engine could be abandoned, reducing the mass that the ion engine needs to get back onto Earth return trajectory.

You'd also need a fairly similar amount of propellant for a soft landing. The delta-Vs are much less than the exhaust velocity, so the "tyrrany of the rocket equation" doesn't really apply yet.

There are other ways to design the mission if this is too much fuel, most obviously you could have a lander and an orbiter and have the lander launch a small capsule of samples into orbit where the orbiter docks with it and brings it home. You could also shift to a more complex engine like a bipropellant, for higher specific impulse.