eLISA - Hunting waves in space
- An article by
- Peter Aufmuth
The possibility of extremely large structures, and the absence of Earth tremors - seismic vibrations - these are two convincing reasons for sending a gravitational wave detector into space. The concept of such a space-based gravitational wave detector, LISA (Laser Interferometer Space Antenna), has been studied jointly by the European and US space agencies ESA and NASA for 20 years. Due to a shortage of money on the NASA side, ESA is currently studying a variant of the original LISA concept, called eLISA (or, sometimes, NGO), that ESA could fund on its own, should NASA not be able to come up with a financial contribution. eLISA, the "evolved Laser Interferometer Space Antenna", is an interferometric detector in space, with laser arms one million kilometres in length.
For eLISA, one "mother" and two "daughter" spacecrafts will be brought into an orbit around the Sun, which is similar to the Earth's orbit. The satellites will fly in a near-equilateral triangle formation, with a constant distance of one million kilometres between, following the Earth along its orbit at a distance of around 50 million kilometres. The configuration is sketched in the following image, which is not so scale; the eLISA triangle itself is drawn larger than life, and if the satellites were drawn to scale (each is three metres in diameter), they would not be visible in the image at all:
[Bild: AEI/Milde Marketing/Exozet]
The triangle's orientation changes during each orbit. This fact enables researchers to determine the direction of gravitational waves reaching eLISA.
An orbiting interferometer
eLISA works similarly to a so-called Michelson interferometer. However, in contrast with a Michelson interferometer, it does not split light into two portions, sending each to be reflected back by a distant mirror. Instead, the mother satellite sends a laser beam to each of the two daughter satellites. Because of the great distance, the laser beam that reaches a satellite is much too weak to reflect back. That's why the satellites do not contain mirrors - instead, they actively send back laser light in response to whatever light reaches them. Comparison of the original laser beam and the response is used to monitor changes in the distance between the satellites. In this way, distance changes can be measured with a precision of the order of fractions of a picometre (1 picometre = 10-12 metres). For this, it is of crucial importance that the free falling test masses in the satellites remain undisturbed by external influences such as Solar wind - the stream of particles constantly being emitted by our Sun.
One of eLISA's tasks is to listen to signals from the hot early phase of our universe, corresponding to a cosmic time of tiny fractions of a second. However, eLISA's primary objective is the detection and examination of gravitational waves emitted by the supermassive black holes that reside in the centres of many galaxies. In addition, eLISA will measure the signals of thousands of compact binary star systems in the Milky Way.
eLISA is the result of decades of development in the area of laser interferomtry, propulsion and sensor technology. The new technologies are due to be tested in space by the LISA pathfinder mission in 2014, well before the mission commences in 2022. In both missions, the Max Planck Institute for Gravitational Physics plays a leading role.
More information about LISA can be found on the project's websites: