ESA to launch mission to study elusive gravitational waves
By David Szondy
December 8, 2013
Mark your calendars for 2034, because that is when science is set to get a whole new spectrum to play with when the European Space Agency (ESA) launches its eLISA mission. Consisting of a constellation of three spacecraft flying in precise formation, eLISA will study gravitational waves in a manner that may one day revolutionize our understanding of the Universe.
Gravitational waves were first predicted in Einstein’s General Theory of Relativity almost a century ago. They’re basically similar to sound, but instead of waves pushing through air, they are ripples in the very fabric of space and time. Though predicted and sought for decades, none have been witnessed directly yet. They’re believed to be produced by all manner of phenomena, such as merging black holes, massive stars and planets pulling at one another, and even the Big Bang itself.
What makes gravitational waves so interesting to scientists is that, unlike light, electromagnetism, and other forces, gravitational waves move without interference. Dust, gas, and glare mean nothing to them, meaning that astronomers could use them to look farther out into space and farther back in time than was previously possible.
The eLISA team says that being able to detect and study gravitational waves would also open up new insights into dark energy, the relics of the early universe called cosmic strings, compact stellar-mass binaries, quasars, the structure of the Milky Way, and help produce detailed history of black holes. This is particularly important because astronomers believe that all bright galaxies have supermassive black holes at their centers, so understanding black holes is necessary for understanding the evolution of galaxies themselves. Gravitational waves could even shed light on the Hubble constant, which describes the expansion of the Universe, and allow for new tests of general relativity.
The problem with gravitational waves is that detecting ripples in space-time requires extremely sensitive equipment, which would make the touch of the finest needle look like being hit by an asteroid. The preferred method is a laser interferometer, which involves making a laser beam interact with itself over long distances. The pattern resulting from the interaction provides scientists with a tool for measuring extremely small displacements. However, it’s also a technique that requires very long distances and near-absolute stability.
Earth-based attempts to detect gravitational waves haven’t amounted to much because the baseline for the interferometer can’t be very large, there are all sorts of tremors and vibrations getting in the way, and having the detector sitting in the middle of Earth’s gravitational field is a bit like trying to observe stars by setting up an observatory on the Sun.
In contrast, the eLISA mission is composed of a constellation of three spacecraft that form a high precision Michelson interferometer floating in outer space with a baseline of one million kilometers (620,000 mi). The interferometer, or gravitometer, works by detecting how the length of the baselines change in infinitesimal increments as the ripples of gravity stretch and compress space-time.
The international consortium of scientists and engineers behind eLISA call it a “sciencecraft” because the payload and the spacecraft had to be specially designed so that one would not interfere with the operation of the other. Though it won’t launch for twenty years, the design of the interferometric measurement system, the telescope, and the gravitational reference sensor have been settled for a decade.
eLISA will orbit the Sun at a Sun-Earth Lagrange Point trailing 20 degrees behind the Earth, where the gravitational forces of Sun and Earth balance out, allowing objects to remain on station. The spacecraft maintain their positions in a near-equilateral triangle between one and five million km (three million mi) apart by performing a “cartwheel” orbit about a common center. The spacecraft can be kept at a constant distance from Earth or allowed to drift as far away as 70 million km (44 million mi), which is the eLISA’s communications limit.
Inside each thermally stable spacecraft are “test masses.” These are 46-mm (1.8 in) cubes made from a dense non-magnetic gold-platinum alloy that are floating free in their sealed vacuum chambers. This may seem odd in outer space, but the spacecraft emits gases from time to time and the test masses need protection from these. In addition, UV lights shine inside the chamber periodically to free electrons and keep the chamber electrostatically neutral in the event of a cosmic ray bombardment.
The clever bit about eLISA is that the whole system is “drag-free.” What this means is that the spacecraft moves itself to keep the test masses centered at all times. Each chamber has capacitive sensors that monitor how the test masses shift relative to the spacecraft and the laser interferometer measures how they shift relative to each other. If the masses leave their null positions, micro-propulsion thrusters make the spacecraft follow them until they recenter.
The measurements are done using a 20-cm (8-in) telescope that beams a Nd:YAG laser along the baseline arm. On an optical bench, the received light interferes with that of a reference laser. This interference allows the systems to calculate the minute movements of the test masses with tremendous sensitivity. There’s even a system to virtually eliminate laser “noise” that might upset the measurements.
Using the laser interferometer, the spacecraft can use the test masses to measure distance between the satellites to within less than a picometer – that’s less than 1/31 the size of a helium atom. This allows eLISA to detect gravitational waves between 0.1 mHz to 100 mHz and it can determine frequency, phase, and polarization. In addition, eLISA can see the entire sky and can resolve and distinguish overlapping signals.
The first target for eLISA will be binary compact stars, which will act as a sort of calibration benchmark because of their known positions and periods. This will allow for extrapolation and more confident future measurements.
"This mission will enable us to study the universe in a completely new way – we’ll be 'listening' to it as well as looking at it," says Physicist Professor Tim Sumner, who leads the work on eLISA at Imperial College London. "Over the centuries astronomy has grown to cover more and more of the electromagnetic spectrum, seeing more colors if you like, whether visible light, infrared, X-rays or submillimeter. With gravitational waves, we’ll have a totally different way of collecting information. It’s as though we've been watching a television with the sound off, and now we’re going to be able to turn the sound up and have a much clearer sense of what’s happening. The possibilities are mind-blowing. We’ll be able to get to grips with black holes; see how gravity works more precisely than ever before; and potentially even see what happened in the seconds after the Big Bang."
In order to test the eLISA technology, ESA is sending up the LISA Pathfinder (LPF) in 2015 on a six-month mission to test the systems that will be used in eLISA, and to study the effectiveness of optical measurements, any stray forces in the spacecraft, and the limits of the technology.
eLISA is classed by ESA as its “L3 mission” (L for Large) and will follow the 2028 launch of ESA’s L2 mission, which will be an advanced X-ray observatory.
The video below outlines the eLISA and the related Athena mission.