January 12, 2018
Psyche is an SEP Mission…. What’s that mean?
Engineering intel from a Psyche JPL Chief Engineer
I’ve had a lot of people come up to me and say: “I’ve heard about the Psyche mission, but what does SEP mean?” Others have told me they’ve heard about SEP or Solar Electric Propulsion from its use on the NASA Dawn Mission to the asteroids Vesta and Ceres, but they’d sure like to know how it works.
The Psyche mission will deliver several scientific instruments into orbit around the asteroid body (16) Psyche to study its properties and try to decide if it’s a remnant proto-planetary core. Solar Electric Propulsion is how we’re getting there.
Solar Electric Propulsion is how we’re getting there.
All of the travel through the solar system is done as orbits…orbits around the Sun like the planets do, orbits around the planets like Moons and satellites do, and orbits between other orbits (called transfer orbits). To go from the Earth to Psyche you have to change the orbit of the spacecraft at Earth going around the Sun to a larger transfer orbit that rendezvous with Psyche in the asteroid belt beyond Mars, and then change the orbit again to be captured into another orbit around Psyche. If that sounds complicated (and nothing like you saw in the movie Gravity), it is.
To change an orbit requires you to change the velocity of the spacecraft, and this is where propulsion comes in. Historically we have used chemical propulsion from large rockets to produce the thrust that changes the velocity of the spacecraft. The amount of thrust you get for a given amount of propellant used is called the Specific Impulse, which is essentially equivalent to the miles per gallon you get with your car. Since we have to launch a bunch of propellant into space at huge costs per kilogram to provide the thrust we need, we’d like the specific impulse to be as high as possible. The efficiency of chemical rockets is limited by the combustion of the fuel, which determines how much energy is produced in burning each kilogram of propellant. Because the energy is limited by the chemical bonds in the propellant, we have to launch many thousands of kilograms of propellant into space to expend enough energy to change the spacecraft velocity to move around in the solar system and reach Psyche.
We get around that problem by adding energy to the propellant. The energy comes in the form of electricity from the solar cells that we normally use to power the spacecraft. We have to significantly increase the size of the solar array (the number of solar cells deployed by the spacecraft) to provide up to 20 kilowatts of power that we can channel into the propulsion system. The solar arrays get large (10’s of meters long and dwarfing the spacecraft), but similar sized solar arrays are already used by communication satellites in orbit around the earth to provide microwave signals for DirecTV and Satellite Radio. In fact, we’re going to use a communications satellite body and power supply system from SSL in Palo Alto for Psyche because they already have the right size solar array and power components on board. Neat!
So SEP, or Solar Electric Propulsion, is just what it says; we take electricity from the solar array and use it to enhance our in-space propulsion system. How do we do that? Instead of chemically burning propellants, we carry inert gases like xenon (commonly found in the atmosphere of Earth) and ionize the xenon inside the electric thruster with an electron discharge reminiscent of what happens in fluorescent lights. Then we accelerate the xenon ions by applying high voltages (300 V to over 1000 V) to the thrusters to form beams of these ions. As the ions go one way, the spacecraft goes the other. Amazingly, the specific impulse of our electric thruster is about five times higher than typical chemical thrusters, which reduces the amount of propellant we need to change the spacecraft velocity and move around the solar system by a corresponding amount. The additional mass of the larger solar array and the power supply is small compared to how much mass we save in eliminating all that chemical fuel, so Solar Electric Propulsion makes sense. While Psyche will use over 1000 kg of xenon to perform its mission, that’s still small compared to the many thousands of kilograms of propellant we would have had to carry if we used chemical thrusters in space.
Sounds easy, right? Yes, in principle, but the details can be very complicated, which is why a bunch of specialists are needed on every mission to figure it all out. The amount of thrust from electric thrusters is limited by the amount of power available and is pretty small; less than a Newton (or a small fraction of a pound). Electric propulsion can’t be used to overcome Earth’s gravity and launch spacecraft into orbit, but in space with low gravity fields and no drag this amount of thrust accelerates the spacecraft just fine. However, since the thrust is low it means in order to change the velocity of the spacecraft as much as needed, we have to thrust for years instead of minutes. SEP missions thrust for a large fraction of the time it takes to get to the target body (3.4 years for Psyche), and the thrusters have to work nearly continuously this entire time without wearing out or failing.
The intensity of solar radiation from the Sun also decreases as we move away from the Sun, so the available electrical power decreases as we move deep into space and the thrusters have to be able to throttle down to lower powers out beyond Mars. There’s lots of pointing and control of the electric thrusters that has to be done to manage the spacecraft trajectory getting to Psyche. We’ve successfully operated the Dawn mission with SEP for over 10 years, so we’re confident we can make Psyche successful too.
One of the best things about SEP is that beams of xenon ions are beautifully blue/green. So SEP missions not only work better, they look great doing it.
In my next blogs I’ll describe how electric thrusters actually work, and then what we’re doing to make sure the thrusters SSL puts on their commercial communications satellites work in deep space at Psyche.