The European Space Agency’s LISA Pathfinder mission has successfully demonstrated the technology needed to build a space-based gravitational wave observatory, with sophisticated electronics from University of Birmingham at the heart of the instrument.
Results based on just two months of science operations show that the two test masses at the heart of the spacecraft are falling freely through space under the influence of gravity alone, unperturbed by other external forces, to a precision more than five times better than originally required.
In a paper published today in Physical Review Letters, the LISA Pathfinder team show that the test masses are almost motionless with respect to each other, with a relative acceleration lower than 1 part in ten millionths of a billionth of Earth's gravitational acceleration, g.
The successful demonstration of the mission’s key technologies opens the door to the development of a large space observatory capable of detecting gravitational waves emanating from a wide range of exotic objects in the Universe.
Hypothesised by Albert Einstein a hundred years ago, gravitational waves are oscillations in the fabric of spacetime, moving at the speed of light and caused by the acceleration of massive objects.
They can be generated by astronomical phenomena such as supernova explosions, neutron star binaries spiralling around each other, or pairs of merging black holes. Even from these powerful objects, however, the fluctuations in spacetime are tiny by the time they arrive at Earth, smaller than 1 part in 1020.
Sophisticated technologies are needed to register such minuscule changes, and gravitational waves were only directly detected for the first time in September 2015 by the ground-based Laser Interferometer Gravitational-Wave Observatory (LIGO).
This experiment saw the characteristic signal of two black holes, each with approximately thirty times the mass of the Sun, spiralling towards one another in the final 0.3 seconds before they coalesced to form a single, more massive one.
The signals seen by LIGO have a frequency of around 100 Hz, but gravitational waves span a much broader spectrum. In particular, lower frequency oscillations are produced by even more exotic events such as the mergers of supermassive black holes.
With masses millions to billions of times larger than the Sun's mass, these black holes sit at the centres of massive galaxies. When two galaxies collide, the black holes at their centres eventually coalesce, releasing vast amounts of energy in the form of gravitational waves throughout the merger process, and peaking in the last few minutes before their final coalescence.
To detect these events and fully exploit the newly inaugurated field of gravitational astronomy, it is crucial to open access to gravitational waves at low frequencies between 0.1 mHz and 1 Hz. This requires measuring tiny fluctuations in distance between objects placed millions of kilometres apart – something that can only be achieved in space, where an observatory would also be free of the seismic, thermal, and terrestrial gravity noises that limit ground-based detectors.
LISA Pathfinder was designed to demonstrate key technologies needed to build such an observatory.
A crucial aspect is placing two test masses in freefall, monitoring their relative positions as they move under the effect of gravity alone. Even in space this is very difficult, as several forces – including the solar wind and pressure from sunlight – continually disturb the test masses and the spacecraft.
Thus, in LISA Pathfinder, a pair of identical, 2-kg, 46-mm gold-platinum cubes, separated by only 38 cm, fly, surrounded, but untouched, by a spacecraft whose job is to shield the cubes from external influences, adjusting its position constantly to avoid hitting them.
“Exceeded our most optimistic expectations”
LISA Pathfinder was launched on 3 December 2015, reaching its operational orbit roughly 1.5 million km from Earth towards the Sun in late January 2016. The mission started operations on 1 March, with scientists performing a series of experiments on the test masses to measure and control all of the different aspects at play, and determine how still the masses really are.
“The measurements have exceeded our most optimistic expectations,” says Paul McNamara, LISA Pathfinder Project Scientist.
“We reached the level of precision originally required for LISA Pathfinder within the first day, and so we spent the following weeks improving the results a factor of five better.”
These extraordinary results show that the control achieved over the test masses is essentially at the level required to implement a gravitational wave observatory in space.
Today’s results demonstrate that LISA Pathfinder has proven the key technologies and paved the way for such an observatory, to be implemented as the third “Large-class” (L3) mission in ESA’s Cosmic Vision programme.
A Birmingham success story
Professor Mike Cruise, from the School of Physics and Astronomy at the University of Birmingham said, “The team at the University of Birmingham who designed and built the phasemeter electronics have made a central contribution to the great success of the LISA Pathfinder mission. The Birmingham Phasemeter, which is the electronic heart of the instrument, is performing perfectly and the European Space Agency is planning to extend the mission by a further six months to make the maximum use of this extraordinary mission.”
“The team of six scientists and engineers working on LISA Pathfinder at the University of Birmingham have contributed to the success of LISA Pathfinder by their skills and experience in building sophisticated space electronics. The Birmingham Gravitational Wave Group now looks forward to the next step of the LISA mission and the new window it will open on the Universe.”
Notes for Editors
'Sub-femto-g free-fall for space-borne gravitational wave detectors: LISA Pathfinder results' is published in Physical Review Letters.
The results were presented today during a media briefing at the European Space Astronomy Centre (ESAC) in Villanueva de la Cañada, Madrid, Spain.
The LISA Technology Package payload has been delivered by several national funding agencies and ESA, in particular: Italy (ASI); Germany (DLR); the United Kingdom (UKSA); France (CNES); Spain (CDTI); Switzerland (SSO); and the Netherlands (SRON). LISA Pathfinder also carries the Disturbance Reduction System payload, provided by NASA-JPL.
Science operations involving the full LISA Technology Package will last until late June, followed by three months of operations with the Disturbance Reduction System.
For further information, please contact:
ESA Media Relations Office, Tel: +33 1 53 69 72 99, Email: firstname.lastname@example.org
For the University of Birmingham, and interview requests with Professor Mike Cruise, please contact Luke Harrison, Media Relations Manager, University of Birmingham on +44 (0)121 414 5134.
Images for media
LISA Pathfinder in space [artist's impression]
LISA Pathfinder animation
About the European Space Agency
The European Space Agency (ESA) provides Europe’s gateway to space.
ESA is an intergovernmental organisation, created in 1975, with the mission to shape the development of Europe’s space capability and ensure that investment in space delivers benefits to the citizens of Europe and the world.
ESA has 22 Member States: Austria, Belgium, the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Luxembourg, the Netherlands, Norway, Poland, Portugal, Romania, Spain, Sweden, Switzerland and the United Kingdom, of whom 20 are Member States of the EU.
ESA has established formal cooperation with seven other Member States of the EU. Canada takes part in some ESA programmes under a Cooperation Agreement.
By coordinating the financial and intellectual resources of its members, ESA can undertake programmes and activities far beyond the scope of any single European country. It is working in particular with the EU on implementing the Galileo and Copernicus programmes.
ESA develops the launchers, spacecraft and ground facilities needed to keep Europe at the forefront of global space activities.
Today, it develops and launches satellites for Earth observation, navigation, telecommunications and astronomy, sends probes to the far reaches of the Solar System and cooperates in the human exploration of space.