Earlier last week I was talking to one of my undergrads about the different subspecialties in astrodynamics and I’ve decided to distill that conversation into the following blogpost.

Trajectory Design

The best-known sub-specialty of astrodynamics, and the one that I’m in, is trajectory design. We answer questions like how do we get from Earth to Mars, or how can we rendezvous with another spacecraft. We also answer questions like, what orbit should we put a satellite into so that it is over a certain spot every N hours, and what should the trajectory of the entry, descent, and landing phase of a lander look like.

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Dynamical Astronomy

One of the smallest and least well-known branches of astrodynamics has an overlap with the field of astrophysics. To design orbits correctly we need to understand the gravitational field of a planet and how it deviates from being a perfect point source. This becomes even more important with small bodies like asteroids where the gravity fields fluctuate greatly. This has led to astrodynamicists participating heavily in the investigation of the formation of asteroids. Additionally, the same tools we use to investigate the motion of spacecraft are the same as those that govern the movement of Kuiper belt objects under the gravitational attraction of Neptune and the Sun.

Finally, knowing, the dynamics behind planetary science helps us model the planets, their movement, and their precision/nutation. All of these are essential if we want to design a mission to a specific region of a planet years in advance.

Attitude Dynamics

For scientific instrumentation to function correctly they often need to be pointed at a relatively close to their target. Hubble’s FOV is only about 0.05 degrees, which is tight. Figuring out how the spacecraft is pointed, and how to get the spacecraft to point a certain way is the specialty covered by attitude dynamics. The difficult part of attitude dynamics is that not only are the dynamics nonlinear, so are the kinematics. Because of the intermediate axis theorem, if you spin a spacecraft around the wrong axis of inertia, you can start to flip around another axis. Veritassium has a great video about that here.

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Estimation

It may sound a bit trite, but to get somewhere you first need to know where you are. Astrodynamicists who work on estimation focus on determining the states of spacecraft and other celestial bodies. Most work on spacecraft they can actively control and communicate with, while others focus on spacecraft that they only observe passively. The latter are usually focused on space debris and increasing space situational awareness to minimize the risks of collisions. With the rise of mega-constellations, we need space situational awareness more now than ever before.

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Astrodynamics + Reality

The final main subject in astrodynamics isn’t new at all, it’s just combining the other fields with reality. Designing a trajectory to a Lagrange point is one thing, designing the same trajectory when taking into account mission requirements is another. For example, let’s say you have found the Earth to Mars mass optimal trajectory for a solar-powered rover, but the arrival date corresponds with the beginning of the Martian dust storm season. Now you have to find the mass optimal trajectory subject to the martian weather cycle, and one that’s amenable to the rover builders, and one that’s early enough for the scientists who already want the data. After you’ve gone through the process and found a trajectory that satisfies everyone, the launch provider has a problem with their launch vehicle which won’t be fixed until the launch window closes (See icon launch and Europa clipper on SLS). If you didn’t plan for this, you may be set back a whole two years until a similar launch window opens back up. It’s because of difficulties like this that I separate out Astrodynamics + Reality.

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