Gravitational waves are ripples in spacetime, produced when massive bodies accelerate. Different sources emit waves at different frequencies, and each detector works only within its own narrow band. The ground-based interferometers LIGO and Virgo catch waves from tens of hertz upward. The future space mission LISA targets the millihertz band. Pulsar-timing arrays operate at nanohertz frequencies. Between them lies a gap.
The GUEST concept (Gravitational Universe Exploration with Satellite Tracking) proposes to peer into exactly that gap — the microhertz band. According to the authors, this is a frequency window no existing or planned detector can reach at a meaningful level.
Two Spheres Instead of a Complex Interferometer
The technical idea is deliberately simple. Two dense, passive spheres are placed in space. Passive means they carry no active electronics, thrusters or power sources — only metal and mirrors. Their surface is covered with cube-corner retroreflectors — optical elements that send a laser beam straight back in the direction it came from.
The spheres are placed on highly elongated Earth orbits. The eccentricity is at least 0.7 (the higher the value, the more stretched the orbit), and the orbital period exceeds 33 hours.
Then the ground infrastructure takes over. A global network of satellite laser-ranging stations continuously sends laser pulses, catches the reflected signal, and computes the distance to the spheres from the flight time with high precision. This technique has long been used for geodesy and satellite tracking, so GUEST relies on existing stations rather than requiring a new ground segment.
The Orbit as a Detector
The key idea is that the orbits themselves act as a resonant detector. When a gravitational wave of the right frequency passes through the system, it slightly and periodically disturbs the motion of the spheres. These oscillations accumulate in a long series of distance measurements. The sensitivity in the microhertz band arises not from electronics but from the chosen orbital parameters — the period and the eccentricity.
That is why observation time is critical here. The minimum mission duration is 10 years, and the expected total is about 30 years. A long data series is needed to separate the faint gravitational signal from other disturbances.
Not Only Waves: A Broad Science Programme
The authors stress that the same data stream can yield science across several fields — from particle physics to geodesy.
Among the stated goals is the first coherent search for gravitational waves from supermassive black-hole binaries in the microhertz band. Also a study of the primordial gravitational-wave background in the energy-scale gap that lies between the reach of pulsar-timing arrays and LISA.
A separate direction is the search for ultra-light dark matter in a parameter region beyond other experiments, along with a search for ultra-light bosons. In addition, the mission promises order-of-magnitude-improved tests of new gravitational interactions at astronomical ranges and a substantial improvement in the absolute determination of Earth's GM — a quantity that underpins the Global Geodetic Observing System and future navigation and Earth-observation missions.
The orbits themselves act as resonant detectors of the oscillating gravitational perturbations, with the microhertz sensitivity emerging from the selected orbital parameters.
For now GUEST is a concept, laid out in the form of a white paper with motivation, scientific reach and mission description. The road to launching the spheres is long. But the proposal itself aims at a frequency window that has so far remained almost untouched.
