NASA to test atomic clock to keep space missions on time

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The DSAC is more accurate and stable than any clock previously flown (Image: NASA)

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If you thought the Apple watch was something to write home about, take a look at NASA's Deep Space Atomic Clock (DSAC). This miniaturized, ultra-precise mercury-ion atomic clock is described by the space agency as "orders of magnitude more stable than today's best navigational clocks," and is smaller and more accurate than any that's been previously sent into space. In 2016, it will fly on a test mission to demonstrate a technology that NASA sees as key to a number of high-priority Earth-orbit and deep space missions.

The Age of Exploration with its wooden sailing ships and picking weevils out of hardtack may not seem much like today's era of space exploration, but the two have much in common. For one thing, both depend heavily on clocks. In the old days, one of the key problems of navigation for centuries was how to calculate a ship's longitude. In essence, doing so required knowing the difference in time between the ship and Greenwich, England, which is the position of zero longitude. To do this required the navigator to have a chronometer of great accuracy, and one that could work as well on a tossing ship as another could on shore.

Spacecraft have a similar problem. Navigation, and other tasks, rely on very precisely timed radio signals and this requires ever more accurate atomic clocks. The latest of these is the DSAC.

Assembled at NASA's Jet Propulsion Laboratory in Pasadena, California, the DSAC is designed to not drift more than 1 nanosecond in 10 days. In addition, it's smaller, lighter, more accurate and more stable than previous clocks flown in space.

The DSAC is unusual in that it has no need of consumables, so it's suitable for very long-duration space missions. It uses mercury ions, which have a hyperfine transition frequency of 40.5 GHz. This allow the clock 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, such as temperature and magnetic variations. The result is not only an atomic clock of great accuracy, but also a very rugged one.

NASA says that the DSAC is more than the ultimate Mickey Mouse watch. It also has a number of applications that could greatly improve many space technology fields. For example, GPS navigation is based on precisely timed doppler radio signals from a constellation of satellites orbiting the Earth. These currently depend upon a two-way radio link with ground control to remain accurate because GPS satellites cannot carry accurate enough clocks to properly time their navigation signals. The DSAC would allow a GPS satellite to use one-way-based communications because it wouldn't need any time signals from Earth to remain accurate.

This fact would also help deep space missions. Since a spacecraft equipped with a DSAC doesn't need two-way communications for navigation, it would mean less work for NASA's Deep Space Network, which could effectively track twice as many craft as today. This would also allow for more ambitious missions, such as using aerobraking to place spacecraft into Mars orbit. With current technology, hitting the window that would keep the arriving craft from skipping back into deep space or burning up would be very difficult, but much less risky with the DSAC to help with the calculations.

Another application for the DSAC would be on the planned Europa flyby mission. The clock would not only allow for precise navigation, but would allow scientists to determine the nature of the subsurface ocean on Europa by using the clock to track the spacecraft's trajectory and make a gravitational profile of the Jovian moon.

But before any of this can happen, the DSAC has to be put through its paces, which is the reason for the upcoming test flight. The purpose of the mission is to show that the DSAC is not only accurate, but that it remains so in the changing temperatures, gravitational forces, and other stresses that occur during a space mission.

NASA says that the clock will be installed in a spacecraft to be built by Surrey Satellite Technologies US of Englewood, Colorado, and will be launched in 2016 atop a SpaceX Falcon 9 Heavy rocket as part of US Air Force's Space Test Program (STP)-2 mission. During its year-long mission, it will demonstrate one-way-based navigation and its ability to use GPS signals for precision orbital navigation. After its initial shake down, NASA will then monitor the clock by telemetry to determine its performance over long durations.

Source: NASA

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