NASA switches on rugged Deep Space Atomic Clock
NASA has confirmed that a new atomic clock currently in low-Earth orbit has been switched on. Launched atop a SpaceX Falcon Heavy rocket on June 25, 2019 at 2:30 am EDT from the Kennedy Space Center in Florida, the Deep Space Atomic Clock (DSAC) is a miniaturized, ultra-precise mercury-ion atomic clock that is smaller, more accurate, and more rugged than any other atomic clock previously sent into space.
About the size of a toaster, the DSAC was confirmed as activated by NASA engineers on August 23 after weeks of in-orbit testing before powering up. Its purpose is to help solve the problem of how to build a spacecraft for deep space missions that can autonomously handle its own navigation problems with minimal help from Earth.
As on Earth, navigation in space is based on having clocks that are as precise and accurate as possible – and that means an atomic clock. Unfortunately, previous generations of such timepieces have either been too large, heavy and dependent on consumables to fly in space, or weren't as accurate or rugged enough to be practical for deep space missions.
Because of these limitations, missions to the Moon and beyond have relied on two-way radio communications by NASA's Deep Space Network or other deep space tracking systems. A precisely timestamped signal generated by an atomic clock is transmitted from Earth, received by the probe, and retransmitted home. The time for the round trip is measured against the timestamp and the distance to the spacecraft can be calculated.
This is very time-consuming, reduces the number of craft that a network can serve, and isn't suitable for probes that have to handle very complicated orbital maneuvers in a short time when it takes minutes or hours for signals to travel to and from Earth. To open the way to more ambitious deep space missions, the Jet Propulsion Laboratory (JPL) in Pasadena, California developed DSAC.
The DSAC's main selling point is that it doesn't rely on consumables to work – a desirable trait for deep space missions. Instead, it uses mercury ions with a hyperfine transition frequency of 40.5 GHz. This allows the DSAC to set the frequency output of a quartz oscillator to a near-constant value by confining the ions with electric fields, which are protected in turn by magnetic fields and shielding to keep out interference, including temperature and magnetic variations.
The end product is an atomic clock that is both highly accurate and very rugged. According to NASA, the DSAC is 50 times more accurate than the atomic clocks used in GPS satellites – losing just one second in 10 million years.
The DSAC has already been extensively tested on Earth and will now be monitored over the next year at the nanosecond level to determine how well it works under actual space conditions.
"The goal of the space experiment is to put the Deep Space Atomic Clock in the context of an operating spacecraft – complete with the things that affect the stability and accuracy of a clock – and see if it performs at the level we think it will, with orders of magnitude more stability than existing space clocks," says navigator Todd Ely, principal investigator of the project at JPL.