Gravitational Waves Get New Focus From NASA

NASA has decided to resume technology development for a space-based facility to detect gravitational waves in cooperation with the European Space Agency (ESA).  ESA is planning to launch such a mission in the 2030s.  Funding constraints led NASA to curtail planning for a Laser Interferometer Space Antenna (LISA) earlier this decade and its role in ESA’s mission was expected to be minor, but dramatic advances in the field have altered the landscape.  A recent report from the National Academies recommended that NASA reconsider its role and the agency has done just that.

Paul Hertz, Director of NASA’s Astrophysics Division in the Science Mission Directorate, told a NASA Advisory Council (NAC) subcommittee yesterday that the agency has agreed to increase its participation in ESA’s L3 gravitational wave mission to 20 percent, the maximum ESA will allow.  The L3 mission is expected to be launched in 2033 or 2034.  Over that period of time, Hertz said, NASA will spend approximately $300-350 million.

The first direct detection of gravitational waves was made in February 2016 using the National Science Foundation’s (NSF’s) ground-based Laser Interferometer Gravitational-wave Observatory (LIGO).  LIGO consists of two instruments, in Louisiana and Washington, that listen for the extremely faint sounds from “ripples in spacetime” from the collisions of massive objects like black holes.  They were predicted by Albert Einstein in 1915, but are so difficult to find that it has taken until now for scientists to obtain unambiguous evidence.  The discovery by Ronald Drever, Kip Thorne, and Rainer Weiss made them contenders for the 2016 Nobel Prize in Physics, although they did not win this year.

As a ground-based instrument, though, LIGO cannot look in all parts of the electromagnetic spectrum. To search in the millihertz band, a space-based facility is required.

LISA was one element of a NASA strategy released in 2003 to study the structure and evolution of the universe called “Beyond Einstein.”   A 2008 report from the National Academies of Sciences, Engineering and Medicine concluded that the technology was not ready to pursue such a mission at that time.  ESA agreed to build a technology demonstrator, LISA Pathfinder, with the idea that LISA would follow in due course as an equal NASA-ESA partnership.

ESA’s LISA Pathfinder was launched in December 2015 and is operating well.  In the meantime, however, budget constraints in NASA’s Astrophysics Division, due in part to overruns on the James Webb Space Telescope, caused the agency to terminate its gravitational wave technology development effort.   ESA continued its plans, however, and in 2013 officially made gravitational waves the focus of its next large science mission, L3.  A March 2016 ESA report outlines the concept for an L3 mission in which NASA would play only a minor role. It does not have all the capabilities that were envisioned for LISA.  (The ESA program is sometimes referred to as eLISA where “e” is for evolved.)

NASA science priorities are guided by Decadal Surveys conducted by the National Academies in each of NASA’s science disciplines.  The most recent Decadal Survey for astrophysics, New Worlds New Horizons (NWNH), was completed in 2010.   It ranked LISA as the third priority primarily because of the technology development required, but said the issue should be reconsidered once the LISA Pathfinder results were known.

By law, NASA is required to contract with the Academies for a “mid-term assessment” for each of the Decadal Surveys half-way through the relevant decade to ascertain NASA’s progress in meeting that Decadal Survey’s recommendations.  The mid-term assessment of NWNH was completed in August.   Chaired by MIT’s Jackie Hewitt, one of the study’s recommendations was that NASA reconsider its participation in ESA’s L3 mission based on the LIGO discovery and the success of LISA Pathfinder.

A space-based observatory can “explore the source-rich millihertz band that is inaccessible from the ground,” Hewitt’s report stated.  NASA should reinstate support for gravitational wave research so the U.S. science community can “be a strong technical and scientific partner” in ESA’s program and “NASA and ESA together should rethink their strategy” for LISA.

NASA has quickly followed that recommendation.  Hertz told the NAC Astrophysics Subcommittee yesterday that he informed ESA last month at the 11th LISA symposium in Switzerland that NASA is willing to participate at the 20 percent level.  For its part, ESA has accelerated its planning efforts, with the call for mission concepts now set for this month instead of next year, Hertz added.

The Astrophysics Subcommittee will hear about options NASA is considering for its role in L3 as its meeting continues today.

The agency is establishing an L3 Study Team to prepare a report to be considered by the next astrophysics Decadal Survey in 2020.  It will still have to compete with other astrophysics priorities at that time.

Update:  The original version of this article, written before the 2016 Nobel Prize in Physics winners were announced, mentioned that the three scientists who discovered gravitational waves might win this year’s prize.  Subsequently, three British-born scientists, who work at U.S. universities, were awarded the prize instead for revealing “the secrets of exotic matter.”

Update, October 2017:  Three scientists who detected gravitational waves did win the 2017 Nobel Prize in Physics: Rainier Weiss, Barry Barish and Kip Thorne.