In 2034, a rocket leaves a spaceport with an unusual payload: two spheres the size of a large globe, each weighing nearly a ton. No engines, no electronics. Just a mirrored surface of hundreds of corner-cube reflectors — tiny prisms that return light exactly to where it came from. This is the future space tuning fork. Its job is to resonate with the ripples in spacetime predicted nearly a hundred years ago by Albert Einstein.

A tuning fork sounds when the frequency of a sound matches its own. The GUEST mission works similarly: two satellites are placed into highly elliptical orbits (eccentricity 0.75) with an orbital period of 33.8 hours. If a gravitational wave, that same ripple in spacetime, passes by, its frequency may resonate with the orbital motion. The effect builds up day after day, like an opera singer's voice shattering a crystal. A network of laser stations on Earth tracks the spheres' positions with millimeter precision, sending short pulses and receiving reflected signals. The gravitational wave will make the orbit 'tremble' in a rhythm that stands out from ordinary disturbances — solar radiation pressure or tides.

This method opens the micro-Hertz range for the first time — from 10⁻⁸ to 10⁻⁴ Hz. That's a huge gap between ground-based interferometers and pulsar timing arrays. Here lie signals from close pairs of supermassive black holes circling each other in distant galaxy centers. Here, too, oscillations of dark matter fields might appear, if dark matter consists of ultralight particles whose existence was confirmed by Vera Rubin's observations. GUEST can test models of inflation (the rapid expansion of the Universe after the Big Bang, proposed by Alan Guth) and capture echoes of phase transitions or traces of cosmic strings — hypothetical folds in spacetime. Along the way, by measuring orbits with fantastic precision, the mission allows us to peer beyond Einstein's theory, testing modified gravity theories.

If all goes according to plan, GUEST will start operating in 2037 and gather data for decades, becoming ever more sensitive. By then, the larger LISA detector will also be in space, working at slightly higher frequencies. Together they will create the first multi-band gravitational-wave observatory, passing targets to each other like musicians passing a melody in an orchestra. Thirty years later, the tuning fork will fall silent. Its last signal will dissolve in the fire of the atmosphere, but by then, it will have told us about how black holes dance, what dark matter is made of, and how the Universe sounded when it was less than a second old.