In The Martian, journeys to Mars are made possible through a large spacecraft known as the Hermes. Unlike the Apollo program, where each trip to the Moon required a separate spacecraft, the fictional Ares program uses the Hermes as a taxi to between Earth and Mars. Individual missions dock with the Hermes, but the Hermes simply makes the rounds between Earth and Mars over and over. While the Hermes is a work of fiction, it’s based in well established science.

The first ideas a spacecraft traveling between Earth and Mars are nearly a century old. In 1925 Walter Hohmann proposed an elliptical orbit between the two worlds. The Hohmann transfer orbit, as it came to be known, relies on Earth and Mars to be in the right positions relative to each other so that a spacecraft in a Hohmann orbit. This occurs about every 26 months, and a low delta-v trajectory. While it has its advantages, the one big disadvantage is that each Hohmann orbit has a different orientation each time. Another problem is that the orbits of Earth and Mars are not quite in the same plane, so things aren’t quite as simple as Hohmann proposed.

To have a large spacecraft that passes Earth and Mars with each orbit, you need some kind of thrust to adjust your orbit. In principle, chemical rockets could do the job, but they aren’t well suited for it. Chemical rockets are great for producing a large thrust in a short time, but a craft like the Hermes would need gradual thrust over longer periods. This can be done with ion drives, which accelerate charged particles at high speeds. In the novel, ion drives accelerate the Hermes at a constant 2 mm/s², which is enough to continually adjust the orbit to match Earth and Mars. While we don’t yet have drives powerful enough for a craft like Hermes, ion drives are being used in missions such as the Dawn spacecraft currently at Ceres.

The only real disadvantage of ion drives is calculating their trajectories. If a spacecraft is continuously accelerating, its trajectory has to be determined computationally. This posed a real challenge for Andy Weir as he was writing the book. To get realistic trajectories for the Hermes he had to write a program to calculate them, and fiddle with parameters until he got a set of trajectories that worked. You can see the resulting trajectories here.

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When you think of a rocket engine, you probably imagine one driven by a chemical reaction, that shoots a powerful flame capable of lifting a large spacecraft from the Earth’s surface. While such chemical rockets are necessary to overcome Earth’s strong gravity, they aren’t as useful in space. That’s because chemical rockets aren’t particularly efficient. So for probes like the Dawn spacecraft now in orbit around Ceres, we use another type of rocket engine known as an ion thruster.

A chemical rocket is a controlled chemical explosion. Credit: Space X

For any type of spacecraft, the key to a rocket engine is how much you can change the craft’s velocity (what’s often known as delta-v). Pound for pound, the more delta-v an engine can give a spacecraft, the more effective it is. That includes both the mass of the rocket and the mass of the fuel. A chemical engine produces thrust by a chemical reaction that superheats the propellent, which is then funneled out of the engine at high speed. Usually the propellent is the exhaust of the chemical reaction powering the rocket, so that the fuel itself becomes the propellent. It is basically a controlled explosion that can provide a great deal of thrust for a short period of time.

An ion engine takes a different approach. Rather than producing a large thrust in a short time, they produce a small thrust over a much longer time. The push given to a probe is about equal to the weight of a few coins in your hand, but it can provide that thrust for days or months at a time. In the lab, ion thrusters have operated continuously for 3 – 5 years without a failure. The only real limitation is the amount of available propellent, which is usually xenon gas.

Ion engines work by giving the gas an electric charge. This allows it to be accelerated to high speed across a potential voltage, after which it is thrust out of the engine. Since the engine is powered by electricity rather than a chemical reaction, it can derive its power by the solar panels of the probe. This, combined with their high efficiency allows ion engines to provide much greater delta-v than conventional engines. Over the course of the Dawn mission, it’s ion engines have provided a delta-v of more than 10 kilometers/second, which has allowed the probe to move into orbit around Vesta, leave that orbit, then reach Ceres and move into orbit around it. This could not have been achieved with chemical rocket engines.

There are plans to use ion engines for crewed missions, such as a journey to Mars, but for now it has allowed us to reach the largest body in the asteroid belt.

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