- SpaceX plans to launch its Falcon Heavy rocket, the world’s most powerful operational launch system, for the third time on June 24.
- The mission will launch a NASA experiment called the Deep Space Atomic Clock (DSAC) into orbit.
- Atomic clocks are thousands of times more precise than standard watches.
- Clocks like DSAC could enable robots and crewed missions to use autonomous, real-time navigation, which is currently not feasible in deep space.
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SpaceX plans to lift off its Falcon Heavy rocket – the most powerful operational launch system in the world – for a third time on June 24.
When the 230-foot-tall rocket thunders away from Cape Canaveral, Florida, it will carry 25 small spacecraft inside its nosecone, including a groundbreaking clock designed to be the most accurate ever to work in space.
NASA created the multi-million-dollar timepiece, called the Deep Space Atomic Clock (DSAC), to keep track of time in space more precisely than any device before it – without being too heavy or big, or consuming too much energy.
The DSAC, which is the size of a four-slice toaster, is designed to keep time that’s accurate to within one-ten-millionth of a second over the span of a year.
But obsessive timekeeping is not the overarching purpose of DSAC. The project’s ultimate goal is to help robots and crewed ships navigate the solar system autonomously, without instructions from Earth. That’s something spacecraft can’t do today, but the capability would open the door for more flexible missions and could make some scientific instruments more powerful.
Todd Ely, a space navigator and leader of the DSAC experiment, said he thinks this type of clock will also be crucial for future human space explorers. Without an on-board atomic clock like DSAC, astronauts journeying to Mars might have to wait as much as 20 minutes for timekeeping signals from Earth to reach their spacecraft (since light takes a torturously long time to zip from Earth to deep-space locations). By then it might be too late.
“If we get out to Mars, the crew is going to want to know where they’re at, and they will need to know it – potentially in real-time – in case they have to make last-minute course adjustments,” Ely told Business Insider.
How the Deep Space Atomic Clock works
Modern clocks are glorified tuning forks. Many wristwatches, for example, keep time by running a small electric current through a crystal of quartz. The quartz responds by vibrating at a precise frequency, and that hum is then broken down into units that drive the advancement of seconds, minutes, and hours.
But all clocks have drift – a measure of inaccuracy – due to impurities in materials, changing temperatures, magnetic disturbances, and other factors.
This drift does not amount to much for wristwatches, perhaps 20 microseconds (or 0.002% of a second) per day, so it isn’t a problem for those of us trying to get to a meeting on time. In space, however, “the stability and precision of the clock becomes extremely important,” Ely said.
For example, NASA and other agencies keep track of a spacecraft’s location and speed by sending it a radio signal and seeing how long it takes the spacecraft to send it back. Because light travels at 670,616,629 mph, a return-trip travel time of 10 minutes would mean that a spacecraft is about 111.8 million miles away from Earth. (A similar principle also enables GPS satellites to tell us where we are on Earth’s surface.)
But this also means a clock’s daily drift of 20 microseconds can introduce an error of nearly 4 miles – which is how far light travels in that fleeting speck of time. And that could mean missing a critical window for a space maneuver, or even smashing into a planet’s surface instead of landing softly upon it.
Right now, agencies like NASA use ground-based atomic clocks to attack this problem.
Instead of using materials like quartz to generate a tuning-fork-like frequency, atomic clocks use vapors or plasmas of a carefully chosen periodic element. In the case of DSAC, that element is mercury.
The atoms are ionized to strip away one or more outer electrons, which enables them to be trapped and cooled down in a small space. A light is then shined on the ionized atoms, whose remaining electrons absorb some of that light, jump up in energy level, and then quickly fall back down. When the excited electrons release the energy they’ve temporarily absorbed, they re-emit it as light of a different and highly predictable frequency. It is this signal – the glow of excited electrons – that can be used to keep time with extreme accuracy.
The ionized atoms can bang into the walls of their container, though which can still cause drift. So atomic clocks use a number of tricks to prevent collisions, including cooling and electric fields. DSAC’s makers claim their device can prevent wall collisions altogether – which gives it such high stability and accuracy.
The atomic clocks found in GPS satellites use the element rubidium, while more accurate atomic clocks use cesium, which has about 2,300 times less daily drift compared to a wristwatch. But DSAC’s mercury ions, Ely said, are an especially “non-drifty kind of tuning fork.”
Mercury ions don’t require as much power or room, Ely said, adding that they should help DSAC beat the reliability of any atomic clock currently found on GPS satellites or other spacecraft.
“If we’re able to reproduce what we’ve seen on the ground in our testing, once DSAC is in space, it should be the most stable atomic clock in space,” he said. “We’ve actually talked to the Air Force about its potential use in future GPS satellites or other [Department of Defense]-type applications.”
How the Deep Space Atomic Clock could change spaceflight
- General Atomics; NASA
Eventually, Ely and his collaborators want to shrink DSAC into a smaller and more efficient (yet equally stable) atomic clock for future deep-space missions.
That could be a game-changer, since right now, NASA spacecraft have to wait for directions from mission control. That’s because there is no GPS for deep space: All of the best atomic clocks are located on Earth, giving navigators located there (like Ely) the best information to program maneuvers. So engineers on Earth upload instructions hours ahead of a maneuver – which means they run the risk of not accounting for unforeseen conditions or problems during the final moments of, say, a Mars landing.
Having a DSAC-like device on a spacecraft may negate the need for the vehicle to wait for Earth’s help. Instead, a spacecraft could “listen” for a broadcast of time from an atomic clock on Earth, then compare it to its local time (similar to GPS). This would enable the spacecraft to estimate its own location and speed in that moment.
Such information may permit a spacecraft to automatically compute and execute a course change if there’s an alluring target for exploration or a looming danger – things Earth wouldn’t learn about for crucial minutes due to the limited speed of light.
“You have a very powerful set of data for computing at trajectory very accurately, very robustly, and in real-time for autonomous navigation,” Ely said.
For now, though, the initial goal is to show that DSAC can keep its timing error below 2 nanoseconds per day. Ely said the team is gunning for no more than about 0.3 nanoseconds (0.00000003% of a second) per day; put another way, it’d take more than 9.1 million years for the clock to be off by 1 second.
“A good metric for us is trying to stay within that ‘don’t drift outside of a few centimeters in the course of part of a day,’ and that’s at nanosecond-or-better level of stability over a day,” Ely said, adding: “We’ve seen that kind of performance on the ground, and we expect to be able to do the same in space.”
DSAC may also help future scientific instruments map the guts of icy moons that hide oceans of water, such as Europa or Ganymede, with a precision that is not currently possible. That’s because the ability to better log subtle changes in speed turns a spacecraft into a gravity sensor, which can in turn reveal the presence of a planet’s hidden structures.
Similarly, precise timing can help turn cameras into a powerful secondary navigation sources: Since astronomers know where objects in space should be at a given time, the clock can turn images of asteroids, moons, stars, and so on into another mapping tools. This gives robots and astronauts even more confidence in navigating through space and the ability to more safely reach their intended destinations.
Perhaps more important for crewed missions, DSAC may give spacecraft multiple options for navigation.
“Let’s say you have a camera system fail on you. Well, you’re not dying – you can start relying on the radio data,” Ely said. “You haven’t lost the ability to navigate.”
Of course, that is not yet a reality – DSAC first has to work in space, then the concept has to get shrunk down to a palm-size version that consumes perhaps 40% less power.
“What we have learned in developing our prototype unit is we know how to significantly reduce the size for the next version,” Ely said.
SpaceX plans to launch DSAC between 11:30 p.m. ET on June 24 and 3:30 a.m. ET on June 25. The deployment of all 25 satellites, one of which DSAC is hitching a ride on, will take several hours. The company plans to show a live webcast of the launch via YouTube.
This story has been updated.