The extremely precise atomic clocks are used in particular for applications
such as GPS systems, requiring very precise calculations of distances based on
the travel time of the waves emitted and received by the satellites. NASA,
which recently designed its most accurate atomic clock, is preparing to test
it on a mission to Venus. The clock, the first to be used in a GPS-type
technology intended for deep space, is currently in the test phase in orbit
around the Earth.
Atomic clocks work by measuring the radiation emitted by electrons as they
move from a lower orbit to an upper orbit around atoms. They are currently
used in spacecraft, but atoms can collide with the walls of the vessel that
contains them, affecting accuracy.
NASA's Deep Space Atomic Clock (abbreviated DSAC), 25 times more accurate
than existing space atomic clocks, solves this problem by using mercury ions
as timepieces, which carry a charge and can therefore be kept away from the
wall of the clock by an electromagnetic field.
NASA launched the DSAC in 2019 to test its long-term stability and
reliability in orbit, and has now published research results showing that
its accuracy far exceeds any other clock in space. Eric Burt of NASA's Jet
Propulsion Laboratory in California and his colleagues found that the DSAC
only drifted by 4 nanoseconds overall in 23 days. Current space atomic
clocks, such as those used in GPS satellites, would show a drift of about
100 nanoseconds over the same period.
Real-time space probe navigation soon possible
Lowering the error rate, according to Burt, is critical to achieving
accurate navigation. Many space missions currently use atomic clocks the
size of a coin. These clocks are very precise but consume a lot of energy,
and their signals take longer to arrive as a spacecraft moves away from
Earth.
At the same time, current atomic clocks miniaturized for use in space are
significantly less precise than what is necessary to maintain precise
autonomous navigation. A more accurate clock on board a spacecraft would allow
it to calculate its own path instead of waiting for signals from Earth.
"This level of performance of space clocks will allow unidirectional
navigation in which signal propagation times will be measured in situ, which
will make possible near real-time navigation of space probes," the
researchers write in their paper.
A DSAC will be included in NASA's recently announced VERITAS mission to
Venus. This ship will include its own chronometer, based on a standard
quartz clock, but will carry a DSAC as a test bed for future missions. The
team is also working on a version of the atomic clock that could fit into
the space occupied by the less precise clocks currently found in GPS
satellites.
Reference:
Burt, E.A., Prestage, J.D., Tjoelker, R.L. et al. Demonstration of a
trapped-ion atomic clock in space. Nature 595, 43–47 (2021).
https://doi.org/10.1038/s41586-021-03571-7