Jonathan Amos takes a look around the 'ice explorer'
"I think I was stunned to start with," recalls Duncan Wingham.
"There is always a statistical chance a launch will fail; you know that. But you never think it will be you."
Cryosat's chief scientist remembers all too well the painful moment he realised the space mission he had proposed and spent six years building was lost.
The satellite's Russian rocket had malfunctioned just a few minutes after blasting clear of an old Soviet missile complex and had ditched in the Arctic Ocean.
Designed to make unprecedented measurements of the thickness of Arctic sea-ice, Cryosat had ended up in charred pieces hundreds of metres below the very subject it was meant to monitor from above.
The mission's failure in October 2005 was a massive disappointment also to the European Space Agency (Esa).
The organisation had changed its practice of launching large, multi-instrumented satellites in favour of building small, low-cost platforms aimed at getting rapid answers on a number of issues of pressing environmental concern.
Cryosat was meant to be the trailblazer, the first in this new series of "Earth Explorers".
"It was a very hard time for everyone, especially Duncan," said Esa's director of Earth observation, Volker Liebig.
Arctic ice in retreat
"I am proud we managed to get the permission of the Council of Esa in just four months to rebuild Cryosat. It's an absolute record."
And four years later, Cryosat-2 is ready to take the second chance.
Currently in the IABG test centre in Ottobrunn near Munich, Germany, the re-born spacecraft will soon be packed up and despatched to Kazakhstan.
Mission managers are targeting a launch in February 2010 from the Baikonur cosmodrome on a Dnepr rocket.
Broadly speaking, Cryosat-2 is a facsimile of Cryosat-1, but there is one significant difference. This time, it has a back-up system for its sole instrument - its radar altimeter.
This should make the mission more robust, extending its lifetime well beyond what the original might have achieved.
But if version two of Cryosat is all but the same as version one, the same is not quite true of the satellite's objectives. These have shifted somewhat.
The intervening years have witnessed a dramatic retreat of Arctic sea-ice in summer months, far ahead of what the majority of climate computer models had forecast.
"When we proposed the mission, what we wanted to do was test the theory that Arctic ice was declining," explains Professor Wingham.
Prof Wingham details the mission's aims
"Well, we're now in a situation where that is known; and the question has changed to 'what are the consequences of the decline of the ice?'"
One such consequence is the potential for an ice-free ocean to "spin up": for its surface waters to start moving faster or in different directions because the wind can act on them more easily.
This could have implications for circulation patterns beyond just the Arctic basin - it could affect sub-Arctic waters, in the Norwegian and Greenland Seas, and ultimately the North Atlantic.
In other words, the climate impacts felt in the Arctic could start feeding back further south.
Cryosat will begin to test this idea, says Professor Wingham.
"Cryosat is the perfect equipment to do this because not only is it going to measure ice thickness, it is also going to measure changes in the sea-surface height, from which we can deduce what the wind is doing to the ocean," he explained.
HOW TO MEASURE ICE THICKNESS FROM ORBIT
Cryosat's radar has the resolution to see the Arctic's floes and leads
Some 7/8ths of the ice tends to sit below the waterline - the draft
The aim is to measure the freeboard - the ice part above the waterline
Knowing this 1/8th figure allows Cryosat to work out sea-ice thickness
The altimeter on Cryosat follows a fine tradition of Esa radar instruments that goes all the way back to the agency's original remote sensing mission, ERS-1, in 1991; but the ice explorer has significant enhancements.
By sending down rapid pulses of microwave energy and counting the time to the echoes' return, the altimeter can measure accurately both the distance to the top of the Arctic sea-ice floes and the water in the cracks, or leads, which separates them.
The Pine Island Glacier transports huge volumes of ice to the ocean
The difference in height is what sailors call the freeboard. If you know that number, it is then a relatively simple calculation to work out the full volume of ice above and below the waterline.
Professor Wingham's team at University College London has already pioneered this approach using data from Esa's ERS-2 and Envisat platforms.
But the altimeter on Cryosat will see finer detail and its "vision" will cover more of the Arctic. Its orbit will leave just a 400km-wide circle at the northern and southern poles that is out of sight of the instrument.
And Cryosat has a second trick. By listening to the radar echoes with an additional antenna offset from the first by about a metre, its instrument can sense much better the shape of the ice below, returning more reliable information on slopes and ridges.
This is important for the study of Greenland and Antarctica where past missions have struggled to discern the action at the edges of the ice sheets - the very locations where some of the biggest, fastest changes have been taking place.
Exhilaration of success
Just this month, Professor Wingham's UCL team reported rapid thinning of the Pine Island Glacier (Pig), one of the huge drainage glaciers in the West Antarctic.
ERS/Envisat data suggested that close to 100m had eroded from the top of the Pig since 1995. This revelation has taken many, including the London scientist, by surprise; and such thinning has major implications for sea-level rise.
Antarctic glaciers like the Pig carry a volume of ice that would push up the world's oceans by tens of centimetres were they to let go completely.
The Dnepr launcher is a converted intercontinental missile
"We simply have no way today of modelling this behaviour, so if we don't keep flying these satellites and measuring this behaviour we won't know what is going on," said Duncan Wingham.
Cryosat-2 will fly to orbit on a different type of rocket to the one that failed in 2005.
The choice is not driven by fear of further disappointment, but simply by the issue of availability - the satellite is ready but the previous launcher is fully booked until 2012.
This time, Cryosat will go with a Dnepr, a curious vehicle that is popped out of a tube by the explosive release of compressed gas before lighting its first-stage motors.
"There's a heart-stopping moment when the Dnepr just hovers above its silo before igniting," said Richard Francis, the Esa project manager on Cryosat-2 and its previous incarnation.
"Every launch is a nervous time, but it will be so exhilarating when Cryosat-2 finally makes it into orbit and we get the first contact with it - the thing we didn't get the last time."
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