Artist’s impression of the LIFE space mission concept consisting of 4 collector spacecraft and 1 … [+]
These days almost every other week, there’s a new media flap about some newly discovered potentially habitable planet orbiting a nearby star. Trouble is, most of these exoplanet detections are indirect and researchers end up speculating about their mass, their makeup, their atmospheric compositions and whether they could ever harbor life.
But a group of planetary scientists and astrophysicists, led by a professor at ETH Zurich, has largely resurrected an idea for a flotilla of optically linked, free-flying mid-infrared telescopes that could find heretofore undiscovered earth mass planets in their star’s habitable zone. From there, using a technique known as nulling interferometry, the telescopes would electronically null out the parent star’s light so that the planet’s thermal emission would be detectable in the mid-infrared.
This would enable the team to characterize planetary systems and detect atmospheric biosignatures from hundreds of nearby extrasolar planets.
The concept, known as LIFE (The Large Interferometer For Exoplanets) is very similar to two missions from the 1990s, NASA’s Terrestrial Planet Finder (TPF-1) mission and the European Space Agency’s (ESA) Darwin mission. NASA and ESA eventually scrapped both in large part because, at that time, they were simply too technically challenging.
Twenty years later and LIFE is trying to capitalize on technology advancements that would make the project possible and at comparatively lower cost than NASA and ESA’s earlier proposals. Last month at the EPSC in Granada, Spain, the team made a presentation which in fact was a pitch to garner interest in the project. And the LIFE team will be discussing their proposal at an international interferometry conference in Pasadena late next month.
NASA’s original plan for its TPF mission would have used four 3.5-meter telescopes that would operate entirely in the mid-infrared. From there, four free-flying spacecraft spread over distances of between 75 meters and 1 kilometer were to relay their data to a fifth spacecraft that would beam it back to Earth.
Problem was, the technology to successfully operate such free-flying spacecraft to the precision needed to meet the goals of these missions proved to be too difficult. And at that point, the planetary science community hadn’t yet reaped data from missions like NASA’S Kepler telescope which detected thousands of new extrasolar planets circling other sunlike stars.
The LIFE team admittedly used the earlier mission outlines as a starting point for their new proposals. But they think they are now ready to begin new technology development that would lead to a full-fledged mission as early as late next decade.
The team’s current mission concept is four free-flying collector spacecraft and a fifth spacecraft acting as beam combiner that also relays the data back to Earth, Sascha Quanz, associate professor of physics in the planetary habitability group at ETH Zurich, the LIFE concept’s principal investigator, told me in his office just outside the city. The size of the primary mirrors of the collector spacecraft is assumed to be in the range of 2 to 3.5 meters, he says.
The final aperture size will eventually also depend on the overall throughput of the system (in other words, photons that end up on the detector, says Quanz. The better the overall throughput, the smaller the primary mirrors on the collector spacecraft can be, he says.
In contrast to every other method planet-hunting used today, LIFE would allow for direct imaging of the system in its totality and the ability to focus in on a given planet within a system.
Quanz says the project is now being funded by ETH ZURICH, but the big money will have to come from the major space agencies once their lab work is finished in some three to four years.
The life mission itself, says Quanz, would entail a minimum of six years split into roughly a 2.5 year search phase to detect new planetary systems and then 3.5 years for in-depth characterization of a subset of these newly-detected planets.
The LIFE team hopes that the advance of technology needed in the positioning of the spacecraft can achieve what is known as interferometric nulling. As ESA notes, as light from a distant star hits two optically linked telescopes, the beam from one telescope is delayed by half a wavelength.
This means, ESA says, that when the rays are brought together, wavelength peaks from one telescope line up with wavelength troughs from the other and so are cancelled out, leaving no starlight. Light from a potential planet orbiting the star, in turn, enters the telescopes at an angle and when the photon beams are combined, notes ESA, the planet’s light is reinforced rather than cancelled out.
At the time of the NASA and ESA initiatives, this was deemed too technologically challenging to be done with free-flying spacecraft in space. But Quanz insists that ESA and NASA also failed to adequately consider sensitivity. The number of photons you’re going to get from the planet is really low, but it’s really mid-infrared thermal emission that you’re interested in, he says.
In other words, LIFE will key in on the thermal emission from the atmosphere of the planet itself, rather than the star’s light reflecting off the planet’s atmosphere.
Artist’s Conception of NASA’s original Terrestrial Planet Finder (TPF-1) mission
But achieving this will still be no small feat.
To detect methane, ozone (a natural byproduct of oxygen) and nitrous oxide, we need to suppress the stellar light, says Quanz. But we also need to make sure that the whole setup is sensitive enough to detect only a few photons from the planet per hour, per square meter, he says. this is not off the shelf technology, says Quanz.
What about NASA’s Webb telescope; can it directly detect earth mass planets?
Webb does not have the spatial resolution to get close enough to the stars to probe for small terrestrial planets in the habitable zone, says Quanz. That’s why a mission like LIFE is needed, he says.
In the intervening two decades since NASA and ESA were pursuing this type of interferometer, there’s been great leaps in optical photonic chips, says Quanz. This, in turn, could allow for the mission to use a much less massive payload for LIFE’s central beam combiner and data collector, he says.
If some of the bulk optics elements can be replaced with photons of only a few microns in size, you would have less of a payload and could launch at less cost and be less prone to failure, says Quanz. LIFE’s beam combination spacecraft, in fact, could be shrunk to the size of a shoebox, he says.
