# What are the limiting factors for interplanetary mission trajectories?

Besides the hard limit of available Delta V, and the soft limit of the time to destination, what are the main constraints when planning an interplanetary mission?

I'm thinking things like maximum time an engine can burn in one go, max number of reignitions, G limit on the payload, things that will condition what trajectory a mission can fly.

I'm specifically not after constraints on the mission itself (like needing an RTG to power an experiment or the high pressure in Venus), only delivery of a payload to a certain location.

Edit for clarification Assume some Z-Prize to be the first to land a mass of concrete on a certain body. We don't care what you send, only how to bring it there.

There's plenty of additional constraints on interplanetary trajectories. Of course the ones you mentioned first are the most driving constraints.

Concerning the others you mentioned maximum number of reignitions is a very valid constraint depending on the engine used. I would think that the G limit is not that much of a factor, as it's safe to assume the highest loads by far occur during launch and not during the actual transfer.

Here are a few more I could think of:

# Thermal limits

Usually, the satellite is designed for a specific thermal environment, which means that some trajectories are excluded because they pass through eclipse for too long or too often. Especially in the case of electric propulsion, eclipses are usually avoided as much as possible, since EP needs a lot of power, which requires sunlight for the solar array to generate power. Furthermore the high thermal loads introduced by going traveling through an eclipse can be something that needs to be avoided.

# Attitude Control System Limits

The attitude control system has hard limits on the torque they're able to produce (and for how long, with regards to momentum dumping This means that some trajectories or manoeuvres will not be possible since they require the spacecraft to rotate too quickly or for too long. Apart from the actual trajectory being flown which requires rotation of the spacecraft in order to point the thrusters in the required direction, things such as solar array orientation and communication with the earth may also require torques that are outside of the limits of the reaction wheels.

# Orbit Determination

A precise estimation of the position of the satellite is needed for determining correcting manoeuvres. This is not done continuously but intermittently. When the position is being determined the thruster is usually turned off as to eliminate the uncertainties resulting from misalignment, actual thrust level vs. nominal thrust level,... This makes it possible to determine the state of the satellite much more accurately.

Radiation is generally one of the bigger concerns as well. Going through the van-allen belts kills things. This especially true if your using a low thrust trajectory to spiral out of Earth orbit into interplanetary space.

• While radiation is a concern, and the Van Allen belts are worth considering, there aren't any trajectories that make that much difference in risk factor within the belts. There's a lot of variability in radiation dose in interplanetary space, though, which the question alludes to in the "soft limit". Commented Mar 14, 2019 at 2:22
• Actually radiation in interplanetary space is low by comparison to going through the van allen belts. Since any interplanetary mission will go through them, they do limit what can be flown Commented Mar 14, 2019 at 13:42
• That radiation would affect you regardless of what trajectory you fly, so it is not a factor when planning delivery. It only affects what you can send, but not how. I've added a clarification to the question. Commented Mar 14, 2019 at 15:55
• No. Radiation goes up with time. This increasing radiation exposure sets mission lifetime which in turn sets what type of propulsion system you can use and what types of trajectories are possible to fly before component failure. Commented Mar 15, 2019 at 2:48
• You're absolutely correct that radiation goes up with time. The few hours spent passing through the Van Allen belts don't vary much with different trajectories (assuming trajectories that at least reach Earth escape velocity), so, unless you spend months with a Hall thruster slowly spiraling out from LEO and crossing the belts over and over again, they're a problem, but essentially the same problem for all flights. However, different trajectories to get to other planets, even with similar engines, can change flight time and radiation by 2-5 times. Commented Mar 15, 2019 at 4:15