LISA (astronomy)

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For other uses of this acronym, see LISA.

The LISA is the Laser Interferometer Space Antenna experiment. It was planned to be launched in 2015 or later, but in February 2006, the orientation of the NASA budget toward the exploration initiative caused the development phase of the mission to be infinitively deferred together with the other missions in the Beyond Einstein program.

A selection of one mission in this program is expected to take place in 2008.

LISA is intended to measure gravitational waves by using laser interferometry over astronomical distances. It will use three spacecraft arranged in an equilateral triangle to form the arms of a giant Michelson interferometer with arms about 5 Gm (5 million kilometers) long. A Michelson interferometer consists of an L-shape with two long arms, each terminated with a mirror, and a corner station containing a light source, beam splitter, and combiner. When a gravitational wave disturbs the space-time field between two of the spacecraft, small differences in the relative lengths of the arms are measurable.

Since forming the arms in any case requires three widely separated spacecraft, the whole setup is made up of three identical spacecraft, each at the corner of a pair of arms, forming three complete interferometers; each arm is part of two interferometers. This requires that the arms be at 60 degrees to each other rather than the ideal 90 degrees, but it is easier to design and test only one type of spacecraft, and the redundancy helps to validate the data.

To eliminate non-gravitational forces such as light pressure and solar wind, each spacecraft is constructed as a zero-drag satellite. The end of each interferometer arm is defined by the mirrored surface of a free-floating internal "proof mass", and the surrounding spacecraft uses very precise thrusters to keep itself centered on the proof mass.

The entire arrangement, being ten times larger that the orbit of the Moon, will be placed in solar orbit, at the same distance from the Sun as the Earth, but trailing it by about 20 degrees. The mean linear distance between the constellation and the Earth will be 50 million kilometers.

The main goal of LISA is to study gravitational waves in detail. In this effort the LISA mission will test Einstein's theories about gravitational waves. Although most physicists believe that they do exist — there is indirect evidence by the increase of the orbital period of pulsars, such as the famous [[PSR 1913+16]] — they have never been directly observed. The main reason for this is that their effect is extremely small. Observing them requires two things: a very large event generating the gravitational wave — such as a collapsing black hole — and extremely high detection sensitivity. The LISA instrument should be able to measure displacements with a resolution of 20 picometers over a distance of 5 million kilometers, yielding a strain sensitivity of better than 1 part in 10^20.

There are other gravitational wave telescopes already in operation on Earth, but their sensitivity at low frequencies is limited by practical arm lengths and interference from moving masses on the planet (e.g., logging operations and highway traffic around the telescope).

A single satellite ("LISA Pathfinder") is due to be launched in 2009 to validate the design and configuration of the definitive mission in 2012–2013.

The mission is sponsored jointly between NASA (which will provide the launcher, the three spacecraft, and about half of the science payload) and the European Space Agency (providing the propulsion modules, and half of the science payloads through national contributions) under the Beyond Einstein program.

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