Looking beyond LIGO

On 3 December 2015, when the European Space Agency’s (ESA’s) LISA Pathfinder spacecraft launched from French Guiana on a Vega rocket, some big questions were facing gravitational-wave astronomers. Researchers were about to find out whether getting free-falling cubes in space to stay in sync was practical. Even if the mission succeeded, no definitive proof yet existed (although the rumor mill was in full swing) that gravitational waves could be directly detected, whether in space or on the ground.

What a difference half a year makes.

Today researchers with the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced their second gravitational-wave detection , this time the result of a merger between two black holes with 14 and 8 times the mass of the Sun. These black holes are less massive but no less interesting than the already legendary black hole duo that, on 11 February, temporarily diverted the world’s attention from Donald Trump’s first presidential primary victory.

Meanwhile, LISA Pathfinder has exceeded all expectations. On 7 June researchers announced that the probe’s 2-kg test masses remained satisfyingly steady , moving almost exactly as they would in perfect free fall. Based on the results, a full-scale mission should be able to detect mergers of black holes that are millions of times as massive as the Sun. It would also spot imminent mergers of smaller black holes weeks before LIGO does , and that would give researchers a chance to study these events at different frequencies with different observatories.

The last six months couldn’t have gone much better for scientists hoping to establish the merits of a gravitational-wave observatory in space. We know that gravitational waves are detectable with interferometers—and it appears that detectable events happen more often than we thought. We know that a space-borne interferometer with million-kilometer arms can do the job. The next crucial step is getting the commitment, particularly from the US, to launch such a mission.

A lot more work needs to be done before ESA’s planned launch of a gravitational-wave observatory in 2034. A major question concerns the involvement of NASA, which had collaborated with ESA on a mission but backed out in 2011 for lack of funding.

The NASA budget currently allots $150 million to the more than $1 billion ESA project. The agency organized the L3 Study Team in December in part to determine which technology the US can best offer with that level of funding. But it’s no secret that many NASA scientists want to have a greater hand, and ESA could use the help. In a May report , ESA’s Gravitational Observatory Advisory Team urged the US to make a larger contribution. “The Committee suggests that such a mission will be more robust, and provide a greater science return per euro, if the US could consider a larger contribution, including a re-establishment of a meaningful collaboration,” the report says.

The US National Research Council’s mid-decadal report, which guides NASA’s priorities and is due for release any day now, could prove crucial to determining NASA’s future involvement. The NRC’s 2010 decadal report ranked a gravitational-wave observatory third among large projects. Now, with the recent successes in gravitational-wave astronomy, the project could get elevated to the top of the list, which could bolster NASA’s ability to secure additional funding.

Although the launch date is nearly two decades away, now is a critical time for determining the success of a gravitational-wave observatory in space. The science case is clear, and the most important questions from skeptics have been answered over the past six months. The NRC should make the mission a priority.