Ice mission returns for second go

[Cees Binkhorst]

REPLY TO: D66 at nic.surfnet.nl

En 6 jaar later (februari 2010) alsnog.
Best wel uitzonderlijk.

Als het deze keer wel lukt zal het zorgen voor gegevens waarop vertrouwd
kan worden (exact, geen schattingen).

Groet / Cees

By Jonathan Amos
Science reporter, BBC News

"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.

"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.

Wider goals

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.

"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 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.

"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."

Jonathan.Amos-INTERNET at bbc.co.uk
Story from BBC NEWS:
http://news.bbc.co.uk/go/pr/fr/-/2/hi/science/nature/8257557.stm

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