LISA Pathfinder Surpasses Expectations

The European Space Agency's LISA Pathfinder mission paves the way for a future space-based gravitational-wave detector.

An artist's conception of LISA Pathfinder in space.
ESA–C.Carreau

LISA Pathfinder is now open for business and working better than researchers had hoped.

The announcement came June 7th during a press conference held at the European Space Astronomy Centre in Madrid, Spain, and was published today in Physical Review Letters.

“LISA Pathfinder's test masses are now still with respect to each other to an astonishing degree,” said Alvaro Giménez (ESA) in a press release.

LISA Pathfinder won't detect gravitational waves. Instead, the mission is a technology-validation platform, set to overcome technical challenges and prove that the full-scale Laser Interferometer Space Antenna (LISA) mission is possible.

Launched on the morning of December 3, 2015, from Kourou, French Guiana, LISA Pathfinder reached its halo orbit around the L1 Lagrangian point 1.5 million kilometers (900,000 miles) sunward of Earth on January 22, 2016. The two identical cube-shaped, gold-platinum test masses were released on February 15th and 16th, and science operations began on February 22nd. Each tiny cube, only 46 millimeters (1.8 inches) on a side, has a mass of 2 kilograms (4.4 pounds).

The actual (red, blue, and gray lines) versus the predicted (black lines) performance for LISA Pathfinder and a future LISA mission. The gray line is uncorrected, the red line shows the data corrected for the centrifugal force, and the blue line shows the data after an additional correction for spacecraft motion.
ESA / LISA Pathfinder Collaboration

The idea behind LISA Pathfinder is to place a detector in a very "quiet" free-fall environment. On Earth a gravitational wave detector such as the Laser Interferometer Gravitational wave Observatory (LIGO) must contend with earthquakes and local disturbances while seeking to isolate minuscule vibrations caused by a passing gravitational wave. In space, the test masses only have to contend with the impact of stray gas molecules in the vacuum surrounding them.

An expanded view of LISA Pathfinder.
ESA / ATG medialab

The full-scale LISA mission will have three 1-million-kilometer-long arms. LISA Pathfinder shrinks one of these arms down to 38 centimeters (15 inches) and places it inside a single spacecraft. The collaborative effort involved 40 space companies and 14 countries.

While LIGO operates in the 100-hertz range, LISA will hunt for lower-frequency oscillations in the range of 0.1 mHz to 1 Hz, which can come from exotic events such as the merger of super-massive black holes millions of times the mass of our Sun. To this end, engineers needed LISA Pathfinder to see relative motions of the two test masses on the picometer scale, around a trillionth (that's a million of a millionth) of a meter. The first results from LISA Pathfinder exceeded this, reaching femtometer-scale measurements, where a femtometer is a million billion times smaller than a meter — five times better than expected. For comparison, the charge radius of a proton is about 0.85 femtometers.

Researchers added that "noise" from collisions between stray gas molecules and the test masses is dying down, as the spacecraft continues to vent the vacuum chamber out to space. The two test masses are now nearly motionless with respect to each other.

“The results astonished us. They surpassed anything we predicted,” says Harry Ward (University of Glasgow) in reaction to the announcement, adding that there is now “no technical impediment to a full LISA observatory.”

LISA versus LIGO

The first direct detection of gravitational waves occurred late last year on September 14, 2015, courtesy of the newly inaugurated Advanced LIGO. But LIGO's twin detectors use a 4-km-long baseline; a space-based interferometer such as LISA will feature a triangular baseline a million kilometers on a side, sensitive to long duration and low-frequency gravitational waves.

Set to head to space sometime in the early 2030s, LISA has been an on-and-off proposal. NASA pulled out of the original LISA project due to budget cuts in 2011. The ESA-led Evolved or eLISA proposal was born later the same year. The eLISA detector will consist of three free-flying spacecraft, each a million kilometers apart.

Unlike LISA Pathfinder, the eLISA constellation will follow 20° (about 50 million km) behind the Earth in a trailing heliocentric orbit inclined 0.33° relative to the ecliptic plane. The large baseline will enable researchers to determine which direction a gravitational wave comes from, pinpointing the source to an area of about an arcminute across on the sky, or about 1⁄30 the diameter of a full Moon.

“The things I look forward to with LISA are the things we don't know yet,” said LISA Pathfinder project scientist Paul McNamara (ESA). “It's the things that we don't have theories for that get us excited. Every time we open a new window to the universe, we see things we don't expect.”

This announcement comes 101 years after Einstein's publication of his general theory of relativity, the framework that enabled the prediction of gravitational waves. The indirect detection of gravitational waves seen in the timing glitches of the binary pulsar PSR 1913+16 won Russell Alan Hulse and Joseph Taylor, Jr., the 1993 Nobel Prize in Physics. Last year's direct detection by Advanced LIGO of gravitational waves from the merger of stellar mass black holes opened the window of gravitational wave astronomy on the universe. LISA Pathfinder, and eventually eLISA, will give astronomers a unique tool to examine the cosmos on the gravitational wave spectrum.

The launch of LISA Pathfinder from Kourou.
ESA / Stephane Corvaja

LISA Pathfinder will continue to operate for at least one year if all goes to plan. But already, the mission has demonstrated the feasibility of a full-scale LISA mission, something to watch for as part of a future large-class (L3) mission in ESA's Cosmic Vision program.

Follow the LISA Pathfinder mission on Twitter as@ESA_LPF.

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