Lisa Pathfinder: Gravity quest set to go into orbit
Europe is ready to launch its most exquisite satellite mission yet.
Lisa Pathfinder is a fundamental physics experiment that will test the technology needed to detect gravitational waves - what are sometimes referred to as ripples in the curvature of space-time.
Scientists and engineers have declared the demonstrator ready to fly after more than a decade of development.
It will likely go up in November on the European Space Agency's Vega rocket.
The month marks the 100-year anniversary of when Einstein published the field equations that underpinned his Theory of General Relativity.
Gravitational waves are a direct consequence of that grand idea. And although there is plenty of evidence to suggest the Universe is awash with these ripples, so far no actual detection has been made – either here on Earth or in space.
It's not for want of trying, but the signal is expected to be very faint, even from the biggest sources.
"Gravitational waves are essentially the mechanism that carries the force of gravity through the Universe," explained Esa's Lisa Pathfinder project scientist, Dr Paul McNamara.
"They are produced by very large, violent events in the Universe – things like galaxies merging, where super-massive black holes in those galaxies come together. Supernovae, pulsars - any violent event where mass is moving. But for the space-based detectors, we're really looking at the very big things in the Universe – the super-massive; the million solar mass objects."
Confirmation of the waves' existence and their subsequent routine observation would open up a new paradigm in astronomy.
It is one that would no longer depend on traditional light telescopes to look at and understand phenomena on the sky.
"The overwhelming part of the Universe is dark and will never be visible with electromagnetic radiation, but for all we know everything in the Universe interacts via gravity. So, ‘listening’ to gravity seems like the obvious thing to do to learn about the dark side of the cosmos," said Prof Karsten Danzmann, the co-principal investigator on Lisa Pathfinder.
A new paradigm in the study of the Universe
- Gravitational waves are ripples in the fabric of space-time generated by cosmic cataclysms such as the merger of black holes and explosion of huge stars
- Extremely sensitive measurement technology is being developed to detect them, using both Earth-based laboratories and satellite systems in orbit
- Just like light (electromagnetic radiation), gravitational waves have a spectrum, and it is the lower frequencies that space-borne observations would seek
- Targets would include the truly colossal phenomena in the Universe, such as the super-massive black holes that spiral into each other when galaxies merge
- Gravitational wave detectors would enable astronomers to open a window on this "dark" activity, potentially revealing physics beyond Einstein
Gravitational waves should propagate at the speed of light, alternately stretching and squeezing space.
And despite their delicate nature, their presence ought still to be apparent to an ultra-stable, super-fine measurement system.
It is worth re-stating: Pathfinder's job is not to make a detection, merely to demonstrate the required metrology.
To do this, it will try to put two small gold-platinum blocks into a near-perfect free-fall and then track their relative movement using lasers.
The intention is to get these "test masses" following a "straight line" that is defined only by gravity.
That's easier said that done because there are plenty of forces that want to push these blocks off course.
There's the pressure of sunlight, the influence of magnetic fields, and the distortions introduced by changing temperatures. The spacecraft’s own gravity will also exert a slight tug on the blocks.
"There are phenomena like collisional molecules," explained Pathfinder’s other co-PI, Prof Stefano Vitale.
"Despite being under high vacuum, there are still residual molecules and they can hit the test masses. They’re very small but they communicate a force, and we are after forces that are less than the weight of a bacterium," he told BBC News.
"We have a performance budget book with many, many entries, but the leading ones number 10; and we think now that after all we have tested, experimented and mastered in the lab - we think we can control these forces, but obviously nothing can substitute for a final test on orbit."
The experiment has been designed such that disturbances to the blocks as small as just a few picometres should be noticed. One picometre is a fraction of the width of an atom.
A system for ultra-precise measurement
- Lisa Pathfinder's payload is a laser interferometer, which will measure the behaviour of two free-falling blocks made from a platinum-gold alloy
- Placed 38cm apart, these "test masses" are inside cages that are very precisely engineered to insulate them against all disturbing forces
- If this super-quiet environment can be maintained, the falling blocks will follow a "straight line" that is defined only by gravity
- It is under these conditions that a passing gravitational wave would be noticed by ever so slightly changing the separation of the blocks
- Lisa Pathfinder is designed to demonstrate picometre sensitivity, but the satellite cannot itself make a detection of the ripples
- To do this, a space-borne observatory would need to reproduce the same performance with blocks positioned millions of km from each other
This level of sensitivity has long been achieved in Earth-bound set-ups, but never in space.
And if scientists want to hunt the ripple signals associated with the mergers of super-massive black holes – and their low frequency means they can only be detected in space – then picometre sensitivity is what Lisa Pathfinder must achieve.
If the satellite can successfully showcase this off-world capability, it will initiate the next step: an operational gravitational-wave observatory.
Esa has essentially already committed itself to such a venture.
The space agency has said that the billion-euro mission it will fly in 2030 will investigate the "gravitational Universe". There will be a call for science proposals that fit with this theme, but in truth there is really only one contender. Certainly, only one contender will come forward with technology that has already been de-risked to the tune of 430 million euros (and that is just the cost to Esa of Lisa Pathfinder; it does not include other sums spent in the agency’s member states).
The more significant question centres on what sort of space-borne gravitational-wave observatory should be flown.
The basic architecture calls for lasers to measure picometre changes in the positions of platinum-gold blocks that are separated not by 38cm, as in Pathfinder, but by millions of km. This involves putting the blocks in different spacecraft units and flying them in formation.
In terms of the best science, the ideal scenario would be to have a trio of spacecraft with the lasers running between each of them. A three-armed laser interferometer.
But when this architecture was proposed a few years ago, the cost frightened the Americans who at that stage were going to be partners on the project – and they pulled out.
A simpler, Europe-only version was then devised that had just two laser arms. But its perceived inferior performance meant that it failed to win the support of the scientific community.
So now the mission's chief proponents are pushing to go back to three arms, and an advisory panel convened by Esa looks set also to endorse such a configuration.
Of course, the additional arm raises the price, adding perhaps another 100 million euros to the overall budget.
If the Americans came back in, it would sort that issue straightaway. But Europe will not be held hostage again by the US, and it will start from the premise that it will have to pick up the full cost of any chosen architecture.
"It's quite a long way into the future, so that really makes it a cash-flow problem. But I'm sure we could find a way," said Esa science chief Prof Alvaro Gimenez.
Assuming nothing goes horribly wrong with the Lisa Pathfinder demonstration, industry will probably be asked in 2017 to begin to spec the observatory with definition studies. A formal implementation could then follow at the beginning of the next decade, ready for that 2030 launch opportunity.