As a partner in ESA's Laser Interferometry Space Antenna (LISA) mission, NASA is delivering important contributions to the flight hardware, data analysis, and science support. Major contributions include the following:
LISA uses free-flying test masses as reference points from which to measure passing gravitational waves. A key requirement for LISA's performance is that these test masses are not affected by other forces. Electrostatic forces are one such force that arise when the test mass gains an electric charge due to the impact of cosmic rays on the spacecraft. If left unchecked, this charge would build up to the point where electrostatic forces would overwhelm the gravitational wave signal.
LISA will use ultraviolet light to control charge on the test masses through the photoelectric effect. This technology was successfully demonstrated first on NASA’s Gravity Probe B mission, then on LISA Pathfinder. NASA and its partners at the University of Florida are developing an improved charge management system based on UV LEDs that are smaller, lighter, less power-hungry, and more robust than the mercury-vapor lamps used on LISA Pathfinder.
LISA uses laser light to make measurements of the distance between pairs of spacecraft. In order to reach the required precision of picometers (1pm = 10-12m), the LISA lasers must carefully control both their intensity and their wavelength. In addition, the lasers have to operate for an extended period in the harsh environment of space.
NASA is developing a prototype LISA laser system to meet these challenging requirements. The LISA laser utilizes the same non-planar ring oscillator (NPRO) technology employed by ground-based gravitational wave detectors like LIGO but in a customized package that is optimized for spaceflight. The laser is stabilized to a frequency reference cavity similar to the one which flew on the NASA-German GRACE-FO mission.
The distances between the LISA spacecraft are so vast that it is necessary to efficiently transmit light from one spacecraft to another. LISA uses optical telescopes to simultaneously transmit and receive the laser light between widely-spaced pairs of spacecraft in order to deliver enough light to make the interferometric distance measurement at the required precision.
The telescopes are designed to function as afocal beam expanders with pupil relays optimized to minimize the cross-coupling of angular jitter into pathlength. Each pair of telescopes is in series with the interferometric measurement path between the LISA test masses and therefore the optical pathlength through the telescope must be extremely stable so as not to mask gravitational wave signals with the distortions of the telescope. This requires careful selection of materials and a design that is insensitive to environmental disturbances. With a primary mirror diameter of roughly 30cm (~1 foot), it is not the largest telescope NASA has ever developed, but meeting the requirements presents some unique challenges.
LISA will provide the first view of the Universe as measured in millihertz-band gravitational waves. In this band we expect to find many astrophysical sources, including some we have seen in other ways, and others not yet detected. NASA is developing data analysis algorithms which will be needed to identify and characterize individual GW sources as well as tools to help scientists utilize this information to conduct a wide array of exciting scientific studies.