Space-based gravitational-wave lab could get new life

In light of the recent discovery of gravitational waves, the US should restore scientific support for the Laser Interferometer Space Antenna (LISA), a mission that NASA abandoned in 2011, says a report released 15 August by the National Academies of Sciences, Engineering, and Medicine.

The report , New Worlds, New Horizons: A Midterm Assessment, recommends restoring US participation so that the mission can regain the full scope of capabilities that had been planned before NASA left the project. Since then, the European Space Agency (ESA) has pursued a downsized project known as Evolved LISA (eLISA). Although the report doesn’t specify how much the US should contribute, it does call for a “significantly larger” sum than the $150 million, or 10% of eLISA’s estimated cost, that NASA is currently authorized to provide.

The report is part of a series known as the astronomy and astrophysics decadal surveys, which present the scientific community’s priorities for NASA’s astrophysics program and guide NASA’s funding decisions. The most recent decadal survey, in 2010 , had endorsed LISA but ranked it third among priorities for large astrophysics projects (see Physics Today, June 2011, page 22 ). The midterm assessment says the recent discovery of gravitational waves at the Laser Interferometer Gravitational-Wave Observatory (LIGO) strengthens the scientific case for LISA, which will offer the ability to explore the millihertz band of gravitational waves that is inaccessible from the ground due to Earth’s gravitational field and natural and manmade background noise.

“The first direct detection of gravitational waves by Advanced LIGO is a ground-breaking achievement that establishes gravitational wave astronomy as a revolutionary new probe of astrophysical phenomena,” says the report, written by a committee chaired by Jacqueline Hewitt of MIT. LISA, says the report, would enable detection of intermediate- and massive-sized galactic black holes and “unique insight into early black hole–galaxy coevolution.”

The committee also pointed to the success of the LISA Pathfinder mission, a precursor spacecraft launched last December by ESA, in proving the feasibility of key technologies needed by the larger mission (see Physics Today, July 2016, page 23 ).

As originally envisioned, LISA’s three spacecraft would be spaced about 5 million km apart in the shape of an equilateral triangle. Each spacecraft would house two free-floating cubes made of a gold–platinum alloy. The distance between the cubes in the three spacecraft would be monitored using highly accurate laser-based interferometer techniques. Passing gravitational waves would alter the distance sufficiently and trigger a detection. Since NASA’s 2011 exit from the LISA partnership, ESA has pursued the smaller eLISA mission, consisting of two shorter laser arms and reduced mission time.

Rita Sambruna, L3 program scientist with NASA’s astrophysics division, says the new report is “a strong boost for the LISA mission.” (L3 refers to the third of ESA’s large class, or L-class, missions.) NASA will discuss its response to the report’s recommendations at the NASA Advisory Council’s astrophysical subcommittee meeting in October, she says. An update of the astrophysics program’s implementation plan, which is expected to be completed in December, will include proposed tradeoffs that would impact other astrophysics missions.

MIT physicist Scott Hughes , a member of the L3 Study Team that is investigating how NASA can best contribute to ESA’s gravitational-wave mission, calls the report’s recommendations “potentially electrifying.” In an email message to Physics Today, he says the committee has “done a fantastic job noting how the LIGO discoveries have demonstrated the science potential of gravitational waves, and that the Pathfinder successes have retired a significant amount of risk which had led to concern about LISA in the 2010 survey.” But he notes the committee’s finding that preparations for the mission have been “severely impacted” by dissolution of the US LISA project.

The new report also urged NASA to proceed with caution on the Wide-Field Infrared Survey Telescope (WFIRST), the top-ranked mission in the 2010 decadal review. The multipurpose spacecraft would search for dark matter, measure cosmic acceleration, catalog exoplanets, and demonstrate technology for directly imaging and characterizing those planets. Phase A design work on the spacecraft began this year. Once that is completed, an independent assessment should be conducted on whether cost growth resulting from major mission redesigns will upset the balance of the astrophysics program, the report says. Should WFIRST consume too much of the budget, the project should be descoped, it says.

The review committee expressed particular concern that the decision to add a coronagraph, a newly developed instrument that permits the direct imaging of exoplanets by blocking the light emitted by parent stars, could further inflate WFIRST costs. “The coronagraph remains a schedule, cost, and technical risk for WFIRST,” the report says.

The WFIRST mission has evolved in other ways since 2012, when NASA obtained two 2.4 m optical telescopes that the National Reconnaissance Office determined were surplus to its needs. But the project, currently estimated to cost between $2.6 billion and $2.8 billion, has been delayed until 2025, five years beyond the schedule envisioned in the decadal survey. The WFIRST mission has benefited from strengthened congressional support, however, receiving additional funding above NASA’s budget requests in recent years.