Ripples in the fabric of spacetime regularly zip across the universe from titanic cosmic events, such as the mergers of supermassive black holes millions to billions of times the mass of the sun. These so-called gravitational waves ought to be ubiquitous but faint, and no experiment has yet registered the disturbance caused by a passing wave. The Laser Interferometer Space Antenna was supposed to do just that. The spaceborne observatory, also known as LISA, was to be a joint mission between NASA and the European Space Agency (ESA) to detect gravitational waves and give scientists a whole new window through which to look on the universe and understand its underpinnings.
Cost overruns concerning the next-generation James Webb Space Telescope apparently helped doom the ambitious joint mission—NASA and ESA dissolved their decadelong LISA partnership in March 2011. Reports of its death may have been greatly exaggerated, however, as researchers are still fighting hard toward launch. Even scaled-back versions of the project might still have a good chance of making revolutionary discoveries, the scientists maintain.
As originally planned, LISA would have involved three identical spacecraft trailing Earth in an orbit around the sun. Each spacecraft would have targeted the other two with lasers, forming a triangle of light with sides five million kilometers long. Over the five-year mission, the laser beams would have helped detect subtle disturbances in the arrangement of the spacecraft caused by the passage of gravitational waves.
Once NASA and ESA stopped working together on LISA, the project fell off the radar. "It's probably fair to say that many people, even astronomers, think LISA was canceled," says astrophysicist Robin Stebbins of the NASA Goddard Space Flight Center, who is heading the agency's gravitational-wave mission concept study.
But each agency is actually investigating going it alone with cheaper, stripped-down missions. "The partnership may be dead, but the concept and the community and the enthusiasm is not dead," says astrophysicist Tyson Littenberg of the University of Maryland, College Park.
Moreover, improvements in our understanding of how galaxies and black holes evolve suggest these successors might only see a bit less than LISA. "In an extremely short timescale, the LISA community has really come together with a lot of studies as to what we might be able to accomplish at lower cost," says astrophysicist Sean McWilliams of Princeton University. "No one's giving up."
One scenario would scale down LISA's triangle, reducing each side to only one million kilometers in length. A smaller triangle means less propellant to set the satellites in place, saving money. Such a move would change the kinds of gravitational waves the satellites could detect—smaller sides mean sensitivity only to smaller wavelengths from smaller objects.
A downsized triangle would still be sensitive to waves from intermediate-mass black holes—those 10,000 to 100,000 times the sun's mass—which are the building blocks of the supermassive black holes seen at the heart of virtually every large galaxy. Recent astrophysics research suggests most black hole mergers involve those of intermediate-mass. So a smaller triangle could shed much light on the mysteries of how galaxies and supermassive black holes formed, according to findings McWilliams detailed January 9 at a meeting of the American Astronomical Society in Austin, Texas.
A more drastic change to the mission's architecture would be to cut off one of the legs, changing the formation from a triangle to a V-shape. Such a mission could still detect gravitational waves, but without the extra information that a third leg would provide the observatory would be significantly worse at pinpointing the location of gravitational wave sources and determining their properties. One less leg means less hardware and thus smaller satellites, which can lead to cost savings with launch.
ESA will likely pursue a strategy involving shorter sides, one less leg and a two- rather than five-year mission. The dissolution of the LISA partnership means the space agency cannot rely on NASA for launch vehicles, and given costs, ESA would have to use smaller, less-capable satellites to fit on smaller Russian Soyuz rockets. Although the European-led project, dubbed either evolved LISA (eLISA) or the New Gravitational-wave Observatory (NGO), would certainly be less capable than a full-fledged LISA, "ESA has already invested more than a half billion dollars on LISA so far, so it's unlikely to pull out," Stebbins says. If ESA goes through with the mission, NASA may participate as a minor partner.
For its part, NASA is investigating the costs and capabilities of LISA-like concepts for a Space-based Gravitational-wave Observatory (SGO) in case ESA does not pursue its own mission. Although the original LISA project would have cost about $2 billion, the total would actually run about $400 million less if NASA went it alone, avoiding some of the additional expenses that come with partnering as well as by using one of the new commercial space launch companies such as SpaceX, Stebbins says.
The problem is that ESA would have brought money into the partnership, and $1.6 billion is far too much for cash-strapped NASA. Researchers have pitched NASA a number of mission concepts with shorter or fewer arms and different orbits or lifetimes (which can be seen here), but the price tags still generally range from $1 billion to $2 billion; Stebbins says NASA is more interested in $300-million to $600-million missions right now.
ESA will make a decision on its gravitational wave mission in April, in which case it could launch in 10 years. Even if the European agency decides to not move forward this year, it could do so in the next competition between proposed missions, with a possible launch in 2028. Whereas such a mission may not be as capable as LISA, "if you were to ask the scientists working on this whether they would go forward with it or not have such a mission at all, the answer would most definitely be to go for it," Stebbins says. "We've had 400 years of telescopes that look at light, but no experience with gravitational radiation, the dominant force field of the universe when it comes to astronomy. This could revolutionize how we look at the cosmos."