Coasting on Fumes in the Wake of the Earth

The first automobile to leave Earth orbit was Elon Musk’s Tesla Roadster. If we imagine that space will one day be filled with spacefaring personal cars, one thing to consider is how to refuel! Perhaps one day Musk will build a network of Supercharger Stations around the inner Solar System, but at the moment, Terran spacecraft need to carry enough fuel for their entire journey.

Right now, NASA’s Kepler spacecraft could use a refueling stop. Kepler follows the Earth in its orbit around the Sun, and is currently about one sixth of an orbit behind us. Launched in 2009, the platform has helped researchers confirm more than 2,500 exoplanets. In its ninth year of operation, Kepler has exceeded its original three-and-a-half year mission duration by several years, but its fuel tank is getting close to empty.

Most of the energy for spacecraft operation is in the form of electricity generated by solar panels. Electricity powers the electronics on the vehicle, including computers, communications equipment, and some attitude control hardware. Attitude control is the science of making sure the spacecraft is oriented, or pointed, in the correct direction.

One type of hardware used for fine attitude control is called a reaction wheel, which is primarily a circular mass that rotates. Because the angular momentum of a spacecraft will stay constant, changing the rotational speed of the reaction wheel makes the bus (main body) rotate in the opposite direction.

One might imagine that since the reaction wheel uses electricity from the solar panels, that it could run indefinitely. Unfortunately, most spacecraft require many attitude adjustments in one direction around a single axis, which requires continuously moving one or two of its reaction wheels in one direction. As these adjustments occur over time, the speed each reaction wheel turns continues to increase, until one or more of the wheels reaches its maximum rotation velocity and “saturates,” at which point the wheels can no longer function to provide additional adjustments in the saturated direction.

When a vehicle’s controls are saturated, the spacecraft must use a second system to perform a momentum dump to remove collected momentum from the reaction wheels. In the case of Kepler, de-saturation is achieved by firing its thrusters. Thrusters function by expelling material from the vehicle at high velocity, which imparts a force in the opposite direction on the vehicle. You may have felt a similar effect when using a high pressure hose, as your hand feels a force in the opposite direction that the water sprays.

Creating rotation uses multiple thrusters. Imagine a clock with an upward pointing thruster at 3:00, and a downward pointing thruster at 9:00. When both are activated, the clock will rotate in a clockwise direction. While thrusters are more versatile than the reaction wheels, they require physical fuel to operate, and when the tank hits empty, it will no longer be able to orient its sensors—that’s when it would be nice to have a refueling station!

Running out of fuel for thrusters designates the end of the mission for many spacecraft, and Kepler is no exception. What’s unique is how the Kepler vehicle will end its mission.

In the case of Earth orbiting vehicles, operators save a bit of fuel for a final maneuver to ensure that it will be out of the way of other orbiters, in order to avoid collisions in space. For objects in low Earth orbit—less than 1,000 kilometers (600 miles) in altitude, such as cubesats—atmospheric drag will eventually slow the craft until it falls out of orbit. For objects in higher orbits—such as television satellites in geosynchronous orbit, which lies about 36,000 kilometers (22,000 miles) above Earth’s surface—the final maneuver typically pushes the satellite higher into a graveyard orbit that is unused by any practical vehicle.

Kepler actually orbits the Sun, in a horseshoe orbit linked to Earth. Because it follows a trajectory that will not interfere with any other vehicle, it does not need to save fuel for a final terminating maneuver, and can continue performing its planet hunting mission until its fuel is completely spent.

Once Kepler goes inactive, it will remain in its current orbit, out of the way of other spacecraft. Because it is travelling slower than the Earth, orbiting the Sun every 372.5 days, our planet will eventually “catch up” with the slower moving vehicle. When this happens, Earth’s gravity will accelerate Kepler, such that its orbit becomes faster, and chases Earth. This pattern of acceleration and deceleration is called a horseshoe orbit because the shape of the smaller object’s path looks like a horseshoe.

It is possible that one day we will have the technology to refuel and upgrade our vehicles in space. Until then, we’ll keep launching new vehicles with more advanced sensors to explore our solar system and beyond.

Want to look for exoplanets in Kepler’s data yourself? Google has recently released code you can use as a starting point!

Want to explore orbital mechanics? Try the space simulation game Kerbal Space Program! (You won’t discover any exoplanets, but you’ll probably have fun.)