SpaceX ITS: Reaching the Moons of Jupiter

Flying to Titan will be easier than flying to the large Moons of Jupiter. As strange as that claim might sound, it’s based on the simple fact that an atmosphere as thick as Titan’s is a boon to space-travelers trying to shed excess speed. Plus Titan is held in its orbit by a lighter planet – Jupiter masses 318 times Earth, while Saturn masses 95 times.

To launch into a parabolic solar trajectory requires a delta-vee of at least 8.75 km/s from LEO, as already mentioned in the Titan discussion. If we arrive at the orbit of Callisto and have to shed sufficient speed to come to a halt relative to Callisto (orbiting Jupiter at 8.2 km/s) then the delta-vee is 21.65 km/s. To land on Callisto softly requires fighting its gravity to touchdown. The minimum is 1.73 km/s, but if we use the Apollo experience as a guide, we’ll need about 10-20% more. Call it ~2 km/s.

Using ITS Tankers as stages, it’s pretty easy to compute the number required for a given Spaceship payload and delta-vee. Even with 0 payload, not even 10 stages gets 19.4 km/s delta-vee. A Hohmann trajectory, which takes ~1,000 days to get to Callisto, can be done with a single Tanker as a booster, if we’re carrying just 100 tons. However landing – an extra 2 km/s delta-vee – requires an extra booster. ALternately a Tanker can be sent separately to supply the landing propellant on arrival, as the dry-mass of the Tanker is less, so there’s less fuel required for a given delta-vee.

Three orbits can be contemplated – the Hohmann transfer, 1,000 days and lowest delta-vee; an Elliptical arc, which halves the time, and the aforementioned Parabolic orbit, which launches at Solar escape velocity from Earth’s solar orbit. Boosting from LEO, then braking into the L2 Point about 50,000 km up from Callisto’s surface, before touching down, the total delta-vee, with a 15% increase for the landing is as follows:

Hohmann (1,000 days): LEO Boost = 6.2 km/s; Braking into Orbit = 4.7 km/s; Landing = 2 km/s; Total = 12.9 km/s

Elliptical (500 days): LEO Boost = 7.2 km/s; Braking into Orbit = 8.4 km/s; Landing = 2 km/s; Total = 17.6 km/s

Parabolic (400 days): LEO Boost = 8.8 km/s; Braking into Orbit = 12.9 km/s; Landing = 2 km/s; Total = 23.7 km/s

Aerocapture in the atmosphere of Jupiter into a transfer orbit to Callisto, or a higher orbit to reduce the delta-vee, does equalize the encounter phase of the mission. For example, if the initial capture orbit has an apojove (furthermost point) that’s 5 times Callisto’s orbital radius, the total delta-vee after aerocapture is just over 4 km/s. The down-side is that 77 days get added to the journey. Directly braking into Callisto orbit after aerocapture requires a delta-vee of 6 km/s.

Is aerocapture really possible in such hostile conditions? Firstly the speed at which the vehicle encounters the atmosphere is nearly 48 km/s, much higher than any re-entry conditions encountered at Earth, Mars or Titan. This isn’t a show stopper necessarily, as the H2/He mix of Jupiter has quite different heating properties to the N2 of Earth/Titan and the CO2 of Mars. Also the required braking is ~2-4 km/s, rather than all 48 km/s. Secondly the vehicle has to survive passage through the radiation belts of Jupiter. As an interplanetary vehicle needs radiation shielding against solar storms, this isn’t a killer either, but does complicate the electronic shielding required. Third, the dynamics is potentially different, since the vehicle is carrying more propellant for the final braking phase than if it was just landing on a planet.

To safely aerocapture a vehicle massing ~750 tons will take some careful design and lots of simulation. I’d suggest a remotely operated Tanker would be the first vehicle to attempt the manoeuvre, after heavy simulation. A possible technological advance is the Magnetoshell Aerobraking system, which uses a magnetic field and plasma to interact with the neutral atmosphere flowing around the re-entering vehicle. Whether it’s up to the task is a question I’ll need to pose to its creator, David Kirtley.