A return to the orangey world

By Jonathan Amos
BBC News science reporter

The first colour view of Titan's surface from the Huygens probe

It had this strange orange-yellow hue and the focussing was some way off pin-sharp, but the image returned from the surface of Titan by the Huygens probe was unquestionably one of the pictures of 2005.

Twelve months on, and you still look at it with wonder - what lies to the right or left, just out of view? What's behind?

All we got was the one shot; so, those questions will for ever be left hanging.

It seems remarkable - to this correspondent at least - that you can get any sort of picture from the surface of a moon that's more than a billion km from Earth.

Mind you, not everyone was impressed. A few newspapers remarked on the picture's mundaneness. "Is that all?" they asked; "It looks like a beach on Earth."

One columnist even suggested the public had become bored with fuzzy space images and jokingly suggested that only a Frisbee-catching dog running in the background could have piqued people's interest.

If you look at that orangey picture, look at the bottom-left - one of the pebbles is broken Prof John Zarnecki, Open University

Not for John Zarnecki, the principal investigator on the science payload sent with Huygens to investigate Titan's surface. He continues to marvel at that picture and is staggered by the probe's achievements.

"It's still not long enough for me to be able to look back in a detached way," the Open University professor told the BBC News website.

"People tend to think that it happens, you do the analysis and that's it. Not so; we're still deep in it."

'Technology tallpoles'

Indeed, the data returned by the robot probe as it floated down through the moon's thick atmosphere to land on a pebble-strewn terrain will keep researchers busy for years.

Titan lived up to expectations; the photochemical haze that shrouds its surface was finally lifted.

One year on, the European Space Agency has imagined the area around Huygens Enlarge Image

Huygens showed us a frozen world that seemed to resemble a primitive Earth - it was somehow very familiar but also very alien.

The rocks are really ice and the rain that falls in a good breeze is made of liquid methane, not water.

The probe spied channels and basins that looked as though they had been cut by rivers, and the surface itself appeared to be covered in a "wet" sand that threw up a plume of methane as the warm robot touched down.

"We actually think we hit one of those pebbles as we landed," said Zarnecki. "If you look at that orangey picture, look at the bottom-left - one of the pebbles is broken. We'll never know for sure but that might have been the one we bashed and broke in two."

However big the data return, there are always going to be more questions than answers; and the desire to go back will get ever stronger.

It won't happen for another 10 years or so. The scientific community has its eye next on Europa, the icy moon of Jupiter; but that does not mean researchers have put aside thoughts of a follow-up - far from it.

The US space agency (Nasa), for example, has teams who conceptualise future missions.

"We try to understand what can and cannot be done; what are the technological tallpoles, and from these kind of studies we can see what needs to be developed and that feeds into our programmes," explains Dr Tibor Balint, from the Jet Propulsion Laboratory in Pasadena.

Power issue

Balint's team has recently worked through the engineering challenge of putting a rover on Titan. The group envisioned a vehicle that looked much like the robot explorers despatched to Mars.

Obvious comparisons include the mast-mounted camera system and the robotic arm that can reach out to take measurements or bring back samples to an instrument carousel.

CONCEPT TITAN ROVER Designed as a low-cost option; borrows from Mars heritage To study surface composition and chemistry, weather 376kg vehicle powered by a radioisotope source Despatched singly or in a group on a much larger rocket A mission to Titan may occur at the end of the next decade

Unlike the Mars rovers' six-wheel configuration, the Titan concept would use just four large wheels.

"When you get down to the surface, you fold the wheels out, inflate them and you can have a much larger surface coverage," Dr Balint told me. "These wheels are 1.5m in diameter and over a three-year operation you could cover up to 500km."

Perhaps the biggest difference to the Mars machines, though, is the power source.

The Mars rovers have a deck of solar cells to top their batteries. Being positioned so far from the Sun and hidden under a deep column of haze, the Titan rover would need a radioisotope thermoelectric generator (RTG).

This type of generator creates electricity from heat given off by the natural decay of plutonium. The US has long experience of using RTGs in space: the 1970s Viking landers on Mars were powered in this way, as is Huygens' mothership, Cassini.

The Titan explorer would trundle over those orange pebbles looking for "lakes" of methane, and studying the surface composition and chemistry.

It is an enticing prospect but it is not the only option being considered. Another JPL team has been looking into the idea of putting a 15m-long, helium or hydrogen-filled blimp on Titan.

In the dark

A balloon can cover far greater distances than a rover and the conditions on the Saturnian moon would be ideal: a dense atmosphere, low gravity and only gentle winds (walking pace) at the surface.

Current designs call for two larger propeller engines to drive the airship forward and two smaller props at the rear to control pitch and yaw.

A gondola could be slung beneath the envelope to carry the instrument suite and a RTG that would provide that all important power.

But there is a significant challenge in operating such a vehicle so far from Earth.

"The round trip light-time is 2.6 hours; that's a lot worse than talking to the rovers on Mars," explains team member Alberto Elfes.

"In addition to that, depending on the relative positions of Titan, Saturn and Earth, you may have situations where you have blackout periods of up to 16 days or so.

"Under those conditions, you need a vehicle that is substantially autonomous, that can take care of itself."

Elfes and colleagues have tested an artificial intelligence system on a small airship over a dry lake bed in El Mirage, California. The vehicle was able to navigate itself to designated waypoints, correcting its path to take account of the wind.

Take this sophistication to another level and an aerobot sent to Titan could be left to get on with scientific observations, safe in the knowledge that the vehicle would not crash into the first hill.

Alien waves

Dr Elfes sees a Titan blimp working close to the surface, possibly firing tethered, harpoon-like probes into the ground to grab samples for study.

"You collect a small amount of material, release the [harpoon] mechanism on the surface and haul up the small, thimble full of material to be analysed onboard," he said.

Huygens was despatched to Titan by the Cassini orbiter

"We have looked at designs with 12 to 24 of these throwaway probes, which would be used as you go from one interesting site to another."

John Zarnecki favours an aerial mission but he also rather likes the idea of combining it with a mini-rover. The airship would first survey the most interesting sites and then unleash a wheeled vehicle to get in amongst those pebbles.

"I definitely want to go back," he said. "I'd love to see the lakes that must be there some times - lakes of liquid hydrocarbons. I'd love to see a lake with some gentle waves lapping on the shore.

"These physical processes are so familiar on Earth but we've never seen them in a different parameter space - one-seventh gravity with liquid methane instead of water. It would be mind-blowing."

The $3.2bn Cassini-Huygens mission is a joint venture between the US space agency (Nasa), the European Space Agency (Esa) and the Italian Space Agency (Asi).