Watching Earth's convulsions from space
One of the most impressive applications of satellite technology is the now-routine observation made from space of earthquakes.
I've posted previously about the "damage maps" [4MB JPEG] that can be made from radar data.
Relief workers use them to guide their activities in a shattered city, such as in Haiti's Port-au-Prince at the beginning of the year. But radar satellites are also used to study the shifting geological faults that give rise to these catastrophic tremors.
One of the most fruitful areas of research in the past 20 years has come from the use of Sar Interferometry (InSar).
The 4 September quake in New Zealand had its epicentre about 55km north-west of Christchurch
It involves combining at least two radar images of the same location on the Earth's surface in such a way that very precise measurements can be made of any ground motion that has taken place between the acquisitions.
In the case of quakes, these will be "before" and "after" images.
The European Space Agency's Envisat is a huge Earth observation spacecraft
It enables scientists to produce something called an interferogram. Somebody once described these maps to me as looking like the rainbow effect you see on the surface of bubbles. They're certainly very pretty.
The coloured bands, or fringes, represent ground movement relative to the spacecraft. This gives researchers a remarkable overview of how rocks have shifted, and allows them to see phenomena that no amount of leg work at the surface could achieve.
At volcanoes, likewise, interferograms can show the mountains "breathing" as surface rock is pushed from below by rising magma.
Scientists have only just started to pick over the information, so there is not yet much to say, but interferograms for previous quakes have given important insights into the true causes of those events and the consequences for future seismic hazard. Professor Barry Parsons is affiliated to the Centre for the Observation and Modelling of Earthquakes and Tectonics (Comet) at Oxford University:
"If you take the Bam earthquake that occurred in 2003, there is a very well-mapped fault - the geologists had been there and known about it for a long time. And immediately after the earthquake, they all rushed out and had a look at it, and they couldn't see very much. It turned out that when we looked at the radar, the fault that had moved in the earthquake was not that fault - it was a completely unknown fault and it was buried; it was not visible at the surface before."
InSar has become something of a European speciality. Although conceptually devised as a technique in the US, it was only really with the launch of the European Space Agency's ERS satellites in the 1990s that this branch of science really took off.
Today, the key spacecraft for InSar, certainly in a European context, is the mighty Envisat.
The Bam interferogram revealed the quake was caused by a previously unrecognised fault
This eight-tonne behemoth is responsible for producing many interferograms, including the Bam image you see on this page (the NZ image comes from Japan's Alos sat [update:25/09/10]), and its data supplies an army of geophysicists right across the Esa member states, and indeed the rest of the world.
But Envisat, launched in 2002, is entering its dotage. And to eke out its remaining thruster fuel to keep it operating until a replacement capability is launched in 2013, Envisat's orbit is going to be allowed to drift slightly from next month.
For all the applications derived from Envisat's 10 instruments this is no big deal, bar one - InSar [6Mb PDF].
To produce interferograms requires the orbit of the spacecraft to be very tightly controlled and unfortunately, that's heavy on fuel. But the Envisat operators have come up with a clever trick.
They can fix a point above the Earth in this more relaxed orbit where Envisat will always come back to, enabling radar images to be accurately overlaid and permitting InSar studies to continue albeit in a very narrow band centred on 38 degrees North. But as Dr Henri Laur, Envisat's mission manager, explained to me this week, 38 degrees North is an important line across the Earth:
"First we have to think to our continent, Europe, and here we have Italy and in particular central-south Italy. 38 degrees is exactly Etna, which is certainly one of the most studied volcanoes in the world; and Vesuvius is not far away. It also covers Greece and Turkey, which are the most tectonic places in Europe. Going to the east, we cover northern Iran, part of China and Japan. And then in North America, we can see the northern part of California and San Francisco.
"We will have an excursion of plus or minus four degrees over three years. So we will have an eight degree band which will be good for InSar."
For the future, InSar studies will get a big boost when Envisat's radar replacement mission, Sentinel-1 gets into orbit. And, in fact, there'll be two of them, Sentinel-1a and 1b, returning more data, more frequently than Envisat is capable of doing right now.
Envisat's UK-supplied radar instrument has made a major contribution to InSar science
As impressive as all this is, there's a bit of a sad story here for British space enthusiasts.
You see, the UK played a leading role in the development and construction of Envisat.
Sentinel-1a is expected to get into space in late 2012 or in early 2013
Its magnificent radar instrument, Asar, which delivers all these interferograms was produced in England. But the UK will not be producing Sentinel-1 because the government decided not to put the required funds into the project - though it has probably invested something like 750 million euros in the Envisat programme down the years.
So despite Britain having the heritage and the expertise, and despite UK engineers doing the early design work on Sentinel-1 - the job of building the new spacecraft has been handed to Italian and German industry.
Only the central radar electronics subsystem will come from the UK.
And it should be remembered that the Sentinels are a recurring series - as well as 1a and 1b, there will be a 1c, 1d, 1e, 1f, and so on.
Long-term, these contracts will be worth hundreds of millions of euros; and assuming there is no massive foul-up, all this work will continue to be given to Italy and Germany.