Science: Next stop : Pluto

Life there is unlikely, but a fly-by will tell us plenty, says the Pretoria boffin helping the mission.

'When people hear I'm an astronomer, the first question they ask is: 'Do you think aliens exist?' Let's see, there are 100 billion stars in our galaxy and 100 billion galaxies, so yes, I do think there has to be life out there."

Dr Henry Throop splits his time between "blowing up stuff" at the National Aeronautics and Space Administration in the US and lecturing at the University of Pretoria. On his desk is a yellow yo-yo.

He moved to South Africa two years ago when his wife came to work at the US embassy. Last year he shivered in the snow at one of the South African Astronomical Observatory telescopes in Sutherland so he could see Pluto pass in front of a distant star.

Throop co-discovered Pluto's smallest moon, Styx, in 2012. When he isn't looking for new celestial bodies or hunting asteroids, he's tracking a $700-million spacecraft, the Nasa probe New Horizons, which he helped build. It roared into the sky in 2006 and in July next year it will reach its destination: Pluto.

New Horizons is about the size of a VW Polo and the fastest craft humans have ever hurled into space, travelling at just under 50000km/h. "The trick is to build the smallest spacecraft you can and put it on the biggest rocket you can buy," says Throop.

The probe will speed past Pluto. Landing would be too risky. "We don't know the surface, and there isn't enough fuel to slow down." As it zooms past, it'll snap photos.

"We have one chance to do it right," says Throop. "If we screw it up, we won't get another. We had three years to design this spacecraft, build it, and fly it. The only thing that matters is that it gets to Pluto." And that is not guaranteed. "A dust speck is enough to puncture the propellant tanks."

Pluto was demoted to "dwarf planet" in 2006, but size isn't everything and Throop still calls it a planet. "We're going out there, no matter what someone else calls it. Pluto is like a time machine. We'll see what our solar system looked like when it formed, four-and-a-half billion years ago."

Throop is trying to recreate the compounds found in DNA to grasp just how life emerged in the early solar system. "The chemistry of life is far too complex for us to explain yet. There are giant holes in our understanding."

The search for life is not just a search for little green men, but for a little green moss. The closest we came was when a Mars meteorite, which crashed in Antarctica, was discovered in 1984. Scientists peered at it under an electron microscope and found what could be fossilised bacteria. The jury's still out.

"We're going to keep looking," says Throop, by studying the only life we know of, which is on Earth. While life-forms with the ability to thrive in extreme environments continue to be discovered, one thing they all need is liquid water. "Much as I'd love to cross that off, so far we can't," says Throop. "Because all life we've found on Earth requires it. As far as we know, the search for life in the universe is the search for liquid water."

The Curiosity rover has been driving around on Mars for two years. Among other things, it was sent to solve a mystery.

"Several billion years ago, you would have been able to drink the water on Mars," Throop says. But now the liquid water is gone, either lost to space or frozen below the dusty ground. "Mars is dry and dead. It's time to go someplace else. We've gone there enough times."

No, they don't expect to find life on Pluto (follow #PlutoSci on Twitter). "Although there was a study that suggests Pluto has an ocean below its surface, so who knows?"

Throop says most people ask the same questions: "Are we alone? How big is space? And, oh, is Pluto a planet? That's why we astronomers do this, because we are curious, and we want to tell other people what we find out about the world around us." He picks up the yo-yo from his desk. An aid to help him ponder the gravitational fields of celestial bodies? "Nah," he says. "It's just for fun." LS