Theoretical work commissioned to the National Physical Laboratory (NPL) by the European Space Agency has recently concluded that lasers capable of generating extremely short pulses — known as "femtosecond comb lasers" — could be of great help in measuring the distance between two or more spacecraft to an accuracy of just a few microns, an essential component to formation flying space missions scheduled for the next decades.
In formation flying missions, multiple spacecrafts are required to fly tens or hundreds of meters apart and gather accurate data using the entire formation as a single big sensor. The spacecrafts must be able to reach the formation and, once this is done, to autonomously control their relative position with great precision.
One example of the need for this technology is the International X-ray Observatory, a 25-meters telescope with a flexible body due to launch after 2020 that will need to be able to control position and orientation of its mirror to make sure images are well-resolved.
The LISA (Laser Interferometer Space Antenna) project, which will scout space for tiny variations in gravity called "gravity waves" with three crafts flying 5 million kms apart, is also going to need a way to assess the relative position of its components for its calculations to be carried out correctly.
While we can use sophisticated instruments down on the Earth's surface, accurately measuring distances in space becomes extremely challenging because the instruments used must be able to survive launch and satisfy a series of requisites including size, weight and power consumption.
Femtosecond comb lasers have been advanced as a possible solution to this problem. They emit light in pulses each lasting a few billionth of a millionth of a second and, by measuring the time needed for the signal to travel the distance, they make it possible to calculate how far apart two objects are to a few microns.
Further work is still needed, as the system will need to satisfy strict specifications on size, weight and power consumption and be able to withstand high levels of radiation; but once all the requisites are satisfied, we will be one step closer to answering big unresolved questions in astronomy and cosmology, including verifying the claims made by general relativity and finding out more about how the Universe developed following the Big Bang.
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