A Sydney University team is building a low earth orbit telescope on a shoestring with the mission to find plants “next door,” at Alpha Centauri.
“We have a small telescope because we couldn’t afford a big one,” says Professor Peter Tuthill from Sydney University’s Institute for Astronomy and School of Physics. “So we’ve had to do lots of tapdancing,”
His team is now building the telescope which is due to be launched next year. It faces daunting challenges, not the least of which is the orbit.
When the multi-billion dollar Cassini spacecraft turned its advanced camera back towards the central solar system after passing Saturn, Earth was just a few pixels big at 1 hour 21 minutes light speed away.
So how can astronomy hope to find an Earth-sized planet 4.4 light years away at Alpha Centauri?
We can’t see an Earth-sized planet. But we can measure its pull on its host star.
“We’re trying to get this tiny signal from what’s really not an ideal orbit with its varying heat load.
“We’ve invented a whole bunch of technologies, including (our) fancy diffractive pupils, which I think would apply to calibration of any optical system in space,” Tuthill told the Australian Space Discovery Centre in the South Australia capital, Adelaide.
“We’ve invented technologies to keep things very thermally stable, and these can be applied by anybody flying a spacecraft.
Tuthill says Alpha Centauri helped out.
It is actually a cluster of three stars (two sun-like and one red dwarf), and each offers a stable reference point against which to measure the movement of the others.
“It’s a crazy, crazy difficult, tiny angle to measure,” he says. “It’s a bit like measuring a football as far away as Singapore moving by about the width of one human hair”.
Such sensitivity is an immense technical challenge. But so is countering the distortions caused by minute movements in the satellite, the camera, and the mirror itself.
The solution, says Tuthill, was to break the captured starlight into a “messy” diffraction pattern. So if the telescope bends, the pattern also moves. But it stays the same fingerprint-like pattern that captures the star’s movements.
“So the error won’t affect me anymore,” he says. “I’ve made the error and the signal a common mode. “So this is the secret sauce that powers my TOLIMAN mission.”
(Telescope for Orbit Locus Interferometric Monitoring of our Astronomical Neighbourhood.)
But another engraving mark was needed to extract a spectrum from the starlight. Even at such a vast difference, shifts in its temperature – such as a flare – have an impact. This must be measured to compensate for its effect on the telescope.
“If you carve the right shape into your mirror, you get this magical outcome where you preserve the message that we need – our fingerprint registration of where the star is – in the middle. And the spectrometer side lobes are perfectly diffraction limited.”
The upshot?
“We can find the stellar temperature, we can find the effective wavelength – and we can lock the calibration of our entire telescope,” Tuthill concludes.
“A lot of the technologies we are building into TOLIMAN, have legs,” he says.
“I’m really excited about where we’ve gone with the resources we have. Maybe some of these innovations might propel new initiatives and next-generation performance from tomorrow’s spacecraft”.
One example is the optical analysis software Deluxe. It can detect and measure the tiny warps and distortions NASA could not eliminate from the James Webb Space Telescope’s mirrors. This can then be used to refine its images further.
Another is the need to efficiently and effectively transmit the TOLIMAN camera’s trove of data back to Earth. That means optimising any data compression to avoid losing the minute signals that would reveal an earth-size planet.
And while TOLIMAN’s mirror system corrects for distortion, it must be given the best start-point possible. That’s why the camera will be fitted with tiny patch heaters and sensors to actively adapt to shifting heat loads.
“If we can make it as stable as possible before we even need to implement this fancy diffractive pupil, then we’re going to be in the black,” Tuthill states.
Finally, the satellite must be able to keep the camera pointed precisely at the Alpha Centauri.
“We need to get Hubble-like pointing accuracy out of a CubeSat,” he adds. “So to make it point, we have our own onboard pointing system consisting of a set of piezos (actuators) around the centre of mass. That’s another innovation we’ve had to come up with in Sydney to make this mission possible.”
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