''Atomic Clock Ensemble in Space'' (ACES) is a project led by the
European Space Agency which will place ultra-stable
atomic clock
An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwe ...
s on the
International Space Station
The International Space Station (ISS) is the largest modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA ( ...
. Operation in the
microgravity environment of the ISS will provide a stable and accurate time base for different areas of research, including
general relativity
General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...
and
string theory tests, time and frequency
metrology, and very long baseline
interferometry.
The payload actually contains two clocks: a
caesium laser cooled
atomic clock
An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwe ...
(PHARAO) developed by
CNES
The (CNES; French: ''Centre national d'études spatiales'') is the French government space agency (administratively, a "public administration with industrial and commercial purpose"). Its headquarters are located in central Paris and it is und ...
, France for long-term stability and an active
hydrogen maser
A hydrogen maser, also known as hydrogen frequency standard, is a specific type of maser that uses the intrinsic properties of the hydrogen atom to serve as a precision frequency reference.
Both the proton and electron of a hydrogen atom have s ...
(SHM) developed by Spectratime, Switzerland for short-term stability.
The onboard frequency comparison between PHARAO and SHM will be a key element for the evaluation of the accuracy and the short/medium-term stability of the PHARAO clock. Further, it will allow to identify the optimal operating conditions for PHARAO and to select a compromise between frequency accuracy and stability.
The mission will also be a test-bed for the space qualification of the active hydrogen maser SHM. After optimisation performances in the range for both frequency instability and inaccuracy are intended. This corresponds to a time error of about 1 second over 300 million () years.
The clock ensemble was expected to travel to the space station aboard a
SpaceX Falcon 9
Falcon 9 is a partially reusable medium lift launch vehicle that can carry cargo and crew into Earth orbit, produced by American aerospace company SpaceX.
The rocket has two stages. The first (booster) stage carries the second stage and pay ...
in 2021. The ACES module will be externally mounted to the ESA's
Columbus Laboratory.
with an 18-30 month expected operations phase.
See also
*
Scientific research on the ISS
The International Space Station is a platform for scientific research that requires one or more of the unusual conditions present in low Earth orbit (for example microgravity, ( cosmic) - radiation and extreme temperatures). The primary field ...
*
European contribution to the International Space Station
References
External links
ACES factsheet by the ESA (PDF)
International Space Station experiments
Columbus (ISS module)
Space science
Atomic clocks
European Space Agency
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