Research Highlights Nature Reviews Physics | https://doi.org/10.1038/s42254-019-0084-9 | Published online 25 June 2019

IN THE NEWS collisions with the walls, which lead to clock instabilities. In addition, the mercury ion clock transition A space chronometer ticks at an approximately six times higher rate than the rubidium clock Much like marine chronometers were Laboratory developed a mercury ion transition, improving performance in essential for sea navigation, portable clock, the Deep-Space Atomic Clock an environment with high magnetic time standards will be needed for (DSAC), part of the Department of field fluctuations. The DSAC should deep space navigation. Current Defense Space Technology Program 2. therefore be 50 times more stable missions rely on communication with The clock was launched into space than the global positioning system Earth, where accurate time-keeping on board a SpaceX Falcon Heavy (GPS) rubidium clocks. is done with state of the art atomic rocket on June 24. It will spend 1 year The DSAC design stability is clocks. However, communication on board a General Atomics’ Orbital better than 2 ns per day, but its becomes slower as spacecraft go Test Bed spacecraft in low-Earth developers hope to reproduce the farther into the Solar System. orbit for in-flight operation testing. laboratory results of 0.3 ns per day. Future missions will need their own Atomic clocks are delicate devices, The team will also conduct an version of a space chronometer and so it is very challenging to make them analogue deep space navigation this is why NASA’s Jet Propulsion robust enough to withstand a launch experiment mimicking the tracking and operate in an environment with of a spacecraft in Mars orbit. large temperature and magnetic field This experiment will test the DSAC fluctuations. It took over 5 years to potential for future deep space

Credit: NASA Credit: get the current DSAC demonstration navigation. “The DSAC will be the unit ready to fly. The entire instrument first ever ion clock to fly in space and weighs 16 kg. The actual clock is promises to revolutionize the way we about the size of a large shoe box and navigate and explore deep space,” says is designed to operate continuously Todd Ely, principal investigator of the and autonomously with low power DSAC Technology Demonstration consumption. Over 10 million Mission. “We expect to learn a lot mercury ions are trapped in an ion from the space demonstration, and trap, which is sealed in a vacuum using that experience and the lessons tube. Compared with the glass learned over the years we expect to cells containing rubidium vapour be able to design a long-life (10 year used in the atomic clocks on GPS version) of DSAC in just a few years satellites, the confinement in the ion following the DSAC mission.” trap considerably reduces the ion Iulia Georgescu

Nature Reviews | Physics volume 1 | JULY 2019 | 421