Planning for Interference

NASA/JPL

The dream of finding life on an alien Earth-like world is hampered by a number of technical challenges. Not the least of which is that Earth is dwarfed by the size and brightness of the Sun. We might be able to discover evidence of life by studying the molecular spectra of a planet’s atmosphere as it passes in front of the star, but those results might be inconclusive. The way to be certain is to observe the planet directly, but that would take a space telescope with a mirror 3–4 times that of Webb.

Back in 2002, NASA proposed just such a mission. Called the Terrestrial Planet Finder (TPF), the proposed mission would launch an array of Hubble-sized mirrors connected by a frame. Observations from each mirror would be combined through a process known as interferometry to create a large virtual mirror. It’s similar to the way radio observatories use arrays of antennas to create a kilometers-wide virtual dish. The mission was ambitious, but within a few years, Congress decided the mission would be too expensive and risky, and by 2011, the mission was canceled.

But hopes of a similar mission are still alive, fueled in part by the proposed Laser Interferometer Space Antenna (LISA) for space-based gravitational wave astronomy. In 2015, the European Space Agency launched LISA Pathfinder as a feasibility test. Rather than having detectors on a single rigid structure, LISA Pathfinder tested whether an array of free-floating detectors could move together in high precision. The mission was a success and proved that the LISA mission was feasible. And if you can have a free-floating array for gravitational waves, why not have one for optical and infrared light?

That’s the idea behind a new paper on the arXiv.¹ The authors propose a test mission similar to LISA Pathfinder. Named SILVIA, or the Space Interferometer Laboratory Voyaging towards Innovative Applications, the mission would launch three small satellites into low-Earth orbit. As with LISA Pathfinder, the SILVIA mission wouldn’t make any astronomical observations. Instead, the mission would test whether the three satellites can move in unison with optical precision. The test would use laser alignment to move the three satellites into a triangle array 100 meters on a side and keep them in alignment for an extended period.

With masses of only 100 kg, the satellites would be easy to launch. Alignment would be maintained by low-power microthrusters and laser interferometry, which are established technologies, so the engineering challenges should be minimal. The authors even sweeten the deal by demonstrating that if SILVIA maintained optical precision, it would be sensitive enough to see the effects of gravitational waves, further supporting the LISA mission.

If the mission can gain funding, it could launch in the early 2030s. If it proves successful, we probably want to reconsider the Terrestrial Planet Finder mission.