In simplest terms, spacecraft navigation entails determining where the spacecraft is and keeping it on course to the desired destination. But it’s not as easy as just getting from Point A (Earth) to Point B (a planet or other body in our solar system.)
These are not fixed positions in space. Navigators must meet the challenges of calculating the exact speeds and orientations of a rotating Earth, a rotating target destination, as well as a moving spacecraft, while all are simultaneously traveling in their orbits around the Sun.
This method of navigation means that no matter how far a mission travels through the solar system, our spacecraft are still tethered to the ground, waiting for commands from our planet.
NASA navigators have recently built deep space atomic clock, a toaster-sized device that will allow spacecraft to safely and autonomously fly themselves to destinations like the Moon and Mars without needing to rely on that data from Earth. The clock is much like a GPS-like instrument- small and stable enough to fly on a spacecraft.
In late June, the clock will launch on the SpaceX Falcon Heavy rocket into Earth’s orbit for one year, where it will test whether it can help spacecraft locate themselves in space. If the Deep Space Atomic Clock’s trial year in space goes well, it could pave the way for a future of one-way navigation in which astronauts are guided by a GPS-like system across the surface of the Moon or can safely fly their own missions to Mars and beyond.
Jill Seubert, the deputy principal investigator, said, “Every spacecraft exploring deep space is steered by navigators here on Earth. Deep Space Atomic Clock will change that by enabling onboard autonomous navigation, or self-driving spacecraft.”
Currently, spacecraft flying beyond Earth’s orbit doesn’t have a GPS to find their way through space. Atomic clocks on GPS satellites aren’t accurate enough to send directions to spacecraft when being off by even less than a second could mean missing a planet by miles.
Instead, navigators use giant antennas on Earth to send a signal to the spacecraft, which bounces it back to Earth. Exact clocks on the ground measure how long it takes the signal to make this two-way journey. The amount of time tells them how far away the spacecraft is and how fast it’s going. Only then can navigators send directions to the spaceship, telling it where to go.
Seubert said, “It’s the same concept as an echo. If I’m standing in front of a mountain and I shout, the longer it takes for the echo to come back to me, the farther away the mountain is.”
Two-way navigation means that no matter how deep into space a mission goes, it still has to wait for a signal carrying commands to cross the vast distances between planets. This process got popular during Mars landings like Curiosity when the world waited 14 long minutes with mission control for the rover to send the message that it landed safely. That delay is an average wait time: Depending on where Earth and Mars are in their orbits, it can take anywhere from 4 to 20 minutes for a one-way signal to travel between planets.
An atomic clock small enough to fly on a mission but precise enough to give accurate directions could eliminate the need for this two-way system. Future navigators would send a signal from Earth to a spacecraft. Like its Earthly cousins, the Deep Space Atomic Clock onboard would measure the amount of time it took that signal to reach it. The spaceship could then calculate its position and trajectory, essentially giving itself directions.
“Having a clock onboard would enable onboard radio navigation and, when combined with optical navigation, make for a more accurate and safe way for astronauts to be able to navigate themselves,” said Deep Space Atomic Clock Principal Investigator Todd Ely.
This one-way navigation has applications for Mars and beyond. DSN antennas would be able to communicate with multiple missions at a time by broadcasting one signal into space. The new technology could improve the accuracy of GPS on Earth. And various spacecraft with Deep Space Atomic Clocks could orbit Mars, creating a GPS-like network that would give directions to robots and humans on the surface.