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."