If the vagaries of weather and rocket science do not intervene, the most ambitious search for alien worlds around the brightest stars in the sky will begin on Monday with the launch of Nasa’s newest planet-hunting spacecraft.

After final preparations at the weekend, the Transiting Exoplanet Survey Satellite, or Tess, is on course for take off as early as 6.32pm local time (11.32pm UK) from Cape Canaveral in Florida, the first opportunity mission controllers have to launch in a window that remains open until June.



The lofting of hardware high into space often calls for a holding of breath, but for those who have ploughed time and money into the $200m (£140m) Tess space telescope there is an extra frisson. Theirs is the first Nasa mission to hitch a ride on a Falcon 9, a rocket made by Elon Musk’s SpaceX company, and which was certified for such missions only in February.



Barring any mishaps, such as a “rapid unscheduled disassembly” – a glorious euphemism Musk used to describe the explosion of a Falcon 9 rocket stage in 2015 – Tess will be flung into a highly elliptical orbit around Earth that has never been attempted before. The space telescope will swing as far out as the moon as it scours the heavens for planets, then swoop back towards Earth to beam home its data. Each orbit will take nearly 14 days.

“Right now, everything is go for a launch on Monday,” said Stephen Rinehart, the Tess project scientist at Nasa’s Goddard Space Flight Center in Maryland. “Putting stuff up in space is not without risk, but at this point there is nothing more we can do. We think we’ve got a spacecraft that is ready to kick ass once it’s up in orbit.”

Tess comes hot on the heels of Nasa’s Kepler space telescope which transformed scientists’ understanding of planets beyond the solar system. When Kepler launched in 2009, astronomers knew that alien worlds circled faraway stars, but had little idea of their number and sizes. As of last month, Kepler had found more than 2,300 “exoplanets”; a similar number await confirmation.

Based on Kepler’s observations, astronomers now believe that the Milky Way is home to at least two billion potentially habitable planets where conditions are neither too hot nor too cold for life-sustaining water to flow. But for all Kepler’s success, it observed only a fraction of the sky and most of the stars it studied are extremely faint, making it hard for astronomers to look more closely at the planets that swing around them.

This is where Tess comes in. The space telescope will spend two years observing 200,000 of the brightest stars in the sky. Most will be no more than 300 light years away. Like its predecessor, Tess is designed to spot alien worlds by detecting the subtlest of shadows they cast as they move across the face of their parent stars.



Bill Chaplin, the professor of astrophysics at Birmingham University, will use Tess data to look at how stars contract and expand as seismic waves surge through them. His calculations will help astronomers confirm the masses and ages of the new planets they find. “When you look up at the night sky, we’ll know about the planets that are around the stars you can see with the naked eye,” he said.

Mission scientists hope to spot 500 Earth-sized planets and perhaps 20,000 new worlds in total. Astronomers will then draw on ground-based telescopes, and soon-to-launch space-based observatories such as the James Webb Space Telescope scheduled for 2020, to look more closely at the planets.

With those follow-up measurements in hand, scientists can work out the planets’ masses and densities, and perhaps even what gases fill their atmospheres. It is no simple task, but if water, methane and other key molecules are found, scientists can start to talk about whether life could, or did, gain a foothold on the distant worlds.

“There are some people on the mission who are very, very, very keen to find Earth-sized planets in the habitable zones of their host stars and that would be absolutely fabulous,” said Rinehart. “But the data on all these planets is interesting, because they help us form a picture of how planetary systems form and evolve. It’s going to be a game-changer in our ability to study planets.”

In 2028, the European Space Agency plans to launch Ariel, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey, to study the atmospheres of 1,000 exoplanets ranging in size from Jupiter to not much larger than Earth. “Tess will provide hundreds of exoplanets, especially super-Earths, which will be optimal targets for atmospheric characterisation with Ariel,” said Giovanna Tinetti, principal investigator on the mission at UCL.

But first Tess must reach its intended orbit. Should the Falcon 9 release the space telescope on the correct trajectory, mission scientists will spend 60 days running tests on its systems and sensors, calibrating the cameras and ironing out any bugs. The first batches of data are expected to land in June, but researchers will work on the information for months before making results public.

“My real hope is that we start finding things that we didn’t expect,” said Rinehart. “If we knew what all the answers were before we launched the mission, why the hell would we fly the mission?”

