CHARGE MANAGEMENT SYSTEMS

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.

STABLE LASER SYSTEMS

LISA uses laser light to measure tiny changes in distance between pairs of free-floating test masses inside spacecraft separated by about 2.5 million kilometers. These changes—on the order of one picometer (one trillionth of a meter)—are caused by the passage of gravitational waves, ripples in space-time predicted by Einstein's theory of relativity. To detect such incredibly small variations over such vast distances, LISA's lasers must produce light that is extremely stable in both intensity and frequency. They also need to operate reliably for years in the harsh environment of space.

NASA has developed a laser system to meet these challenges and has already achieved major technological milestones, demonstrating key performance in relevant environments. The LISA laser uses the same non-planar ring oscillator (NPRO) technology employed by ground-based gravitational wave detectors like LIGO, but in a customized package optimized for spaceflight with improved performance. The laser frequency is locked to an ultra-stable reference cavity, an upgraded version of the one flown on the NASA-German GRACE Follow-On mission. The frequency-stabilized light is amplified by a fiber amplifier system, and its output power is precisely controlled by a power monitoring system. This ensures that radiation pressure from the laser does not disturb the position of the test masses—critical for accurate measurements on the order of a picometer.

NASA's LISA laser system represents cutting-edge optoelectronics, combining proven gravitational-wave detection technology with innovations that make it possible to listen to the universe from space.

ULTRA-STABLE TELESCOPES

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. NASA, with partner L3 Harris, is working to meet those challenges.

SCIENCE GROUND SEGMENT CONTRIBUTIONS

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.