The minibus-sized craft is equipped with four wide field cameras that can see in the near-infrared spectrum, looking for dips in light as planets pass in front of their stars.

Instead of looking for faraway planets in a small patch of the sky, TESS will survey the whole sky – dividing it into 26 slabs. Starting with the southern hemisphere, the survey will take two years to complete, covering an area 400 times greater than Kepler.

It will also be in an orbit that comes to within 100,000km of Earth (Kepler was about 10 million kilometres away) so it will be able to download a lot of data very quickly.

“In 30-minute samples, Kepler was able to look at about 170,000 stars. TESS will be able to look at 30-50 million stars in that period.”

And that means a lot more planets.

“We should be able to find 20,000 planets of all sizes ranging from Jupiter-sized planets to planets the size of Earth or even the size of Mars.”

These planets will be some of our closest neighbours, orbiting stars we can actually see when we look up at the sky.

And the closer the star, the easier it will be to see the planet’s atmosphere as it passes in front of its sun.

“If you wanted to make these kind of measurements with the planetary systems discovered by Kepler you’d need a telescope 65 metres in diameter. That’s just not practical now.”

In particular, TESS will home in on stars called M-dwarfs or red dwarfs.

“More than 75 per cent of the stars in the Milky Way galaxy are this type,” Dr Ricker explained.

“They’re a cool, red star, and from the small number of those that have been discovered by Kepler, it looks like they have about twice as many planets as stars like the Sun.”

These stars are smaller and cooler than our Sun, so it may be easier to spot habitable planets, said astronomer Tim Bedding of the University of Sydney.

While there are portions of the sky that will be viewed for an entire year, in most cases TESS will focus on different slabs of sky each month. This means many of the planets it detects will have short orbits.

“Now a few weeks to a month around the sun is really hot. But a few weeks to a month around a red dwarf could still be quite pleasant in terms of temperature, if you like the Goldilocks zone,” Professor Bedding said.

Planet hunters are more than planet hunters

While alien planet discoveries grab headlines, data from the planet hunters can tell us much more about other aspects of our galaxy and beyond.

Professor Bedding and colleagues used data from the Kepler spacecraft to discover a phenomenon known as star quakes in red giant stars. These oscillations tell us how old stars are and what will happen to our Sun.

Because TESS surveys the whole sky it will provide data on many more stars than Kepler did.

“If we can [study stars] from all directions in our galaxy we can understand how our galaxy formed and all the different parts formed in different order and how they came together,” Professor Bedding explained.

Staring at bigger patches of the sky will also make it easier to see when stars blow up, says ANU’s Dr Tucker, who studies supernovas found by Kepler.

“One of the cool things that’ve realised is we should discover some gravitational wave sources, kilonova, with TESS,” Dr Tucker said.

But the Holy Grail for Professor Ricker would be to discover something completely unpredictable.

“There are so many things that TESS may find that are related to exoplanets and phenomenon we know already,” he said.

Australian telescopes will play role in discoveries

TESS will send its data back down to Earth using NASA’s Deep Space Network in Canberra, Madrid, and California.

From there, astronomers will follow up TESS’s discoveries using ground-based telescopes as well as other space telescopes such as the future James Webb Space Telescope.

While Kepler’s planets could only be viewed by telescopes in the northern hemisphere, TESS will discover planets that can be seen in the southern hemisphere.

“One of the problems that we had with Kepler is that it looked at this really small patch of the sky, so unless your telescope was in the right position with the right instrument there you won’t see it.

One of the Australian projects that will follow up TESS’s discoveries is called Funnelweb.

It will use the UK Schmidt telescope at Siding Spring Observatory near Coonabarabran, to examine the light coming from nearby stars using an instrument called TAIPAN, Professor Bedding said.

“It has a spectrograph inside it with fibres that can position themselves called star bugs. They can move around and get themselves in the right spot where the star is,” he said.

Countdown to launch

The little satellite with the big mission is currently awaiting launch aboard a SpaceX Falcon 9 rocket.

Once blasted into space, it will eventually sit in a special orbit (red) that goes out to 250,000 kilometres then sweeps back to within 100,000 kilometres of Earth.

During one of its early orbits (green) it will be flung by gravity from the Moon (yellow) into a final position that ensures it continuously views the sky, and its solar panels always get light.

This stable 13.7 day “lunar resonant” orbit, which has never been tried before, should allow TESS to operate for well beyond two years, said Professor Ricker.

“We think that if things go the way they should and things work the way they should then we could probably operate for up to 20 years,” he said.

Kepler’s mission, in the meantime, is slowly drawing to an end.

Although about to start a new campaign, it is gradually running out of fuel and drifting further away from Earth.

But having the two craft in space together, albeit for a short time, is exciting, said Dr Tucker.

“The original view is that there would be a big gap between the two.