The European Space Agency launched its CryoSat-2 satellite today (April 8, 2010), thus completing the third phase of its Earth Explorer Series. The satellite will give us our best picture yet of the effects of global warming on Earth’s ice, and will allow us to predict its consequences with unprecedented accuracy. The information gained from this project will help the nations of the world understand this phenomenon and deal with its consequences in the years ahead.
This is an exciting time. As we have seen in previous articles, there are a lot of fascinating projects going on right now, with robot probes exploring places in the solar system and beyond. Places that used to be just names in textbooks are now real places to us, and we are learning more about them all the time.
However, it should never be forgotten that the exploration of the mysterious third planet on which we live can be just as interesting, and of even more vital importance. To help us understand more, there is now a legion of satellites orbiting Earth to study various aspects of its environment. Many of these are assessing the consequences of environmental tampering by humans, and finding that those consequences are proceeding at a faster pace than was originally thought.
In assessing the consequences of global warming, one of the indicators that is available to us is the Earth’s ice. We already know that ice in the polar regions is shrinking to levels unknown in recorded history. Ships can now sail through places that were completely blocked by ice only a few years ago, but this is only anecdotal evidence, and does not tell us how fast the problem is moving or how far it has already gone. Before 2000, it was thought that the interiors of the major ice sheets were largely stable. This was based on data gained by satellite altimetry, but the capability of those satellites to measure change at the margins of ice caps, where most of the change was occurring, was limited by their design. By 2006, new information was emerging which was causing scientists to doubt the stability of the ice caps. In that year, analysis of radar readings of the Pine Island Glacier in Western Antarctica showed a definite thinning of the ice layer. Data from other satellites, such as ESA’s Envisat, mapped the Earth’s ice and found that annual average Arctic sea ice extent had shrunk by 2.8% per decade since 1978.
But even with that information, we still don’t have the full picture. To get that, it’s necessary to measure the thickness of the ice. That’s the only way to get an accurate estimate of the amount of ice that is left, and how quickly it is retreating.
To address this need, CryoSat-2 was designed as part of Earth Explorer, a larger project which addresses key questions regarding the natural processes of our planet, and in some cases, how they are changing in response to the stresses placed on them by human activity.
CryoSat was originally intended to be the first of these probes, not the last. In one of those terrible disappointments that occasionally happen in space exploration, the first version of CryoSat was lost due to launch failure in October 2005. The ESA immediately started planning for a relaunch. Cryosat-2 took four years of preparation, and during that time, the other phases of the Earth Explorer Series proceeded as planned. While CryoSat was being rebuilt, the GOCE gravity mission and the SMOS water mission were launched successfully, and when CryoSat-2 went up today, it completed the series that it was supposed to start.
Cryosat-2 carries technology to measure changes in two types of ice: the huge sheets covering Greenland and Antarctica, and floating ice in the oceans. The satellite will travel in a highly inclined polar orbit which will reach 88 degrees latitude north and south, to get maximum coverage of the poles. Its main instrument is the Synthetic Aperture Interferometer Radar Altimeter (SIRAL). Radar altimeters used in the past have been designed for use over ocean or land, but SIRAL is the first one to be specifically made for use on ice.
SIRAL has a higher resolution than earlier radar altimeters. Whereas conventional radar altimeters send out radar pulses that are, on average, 500 microseconds apart, SIRAL send its pulses at intervals of only 50 microseconds. The onboard data processor can separate the echo into strips which are about 250 m. wide. Since the interval between bursts is arranged so that the satellite moves forward by 250 m. each time, the strips laid down by successive bursts can be superimposed on each other and averaged to reduce noise. This is known as the SAR (Synthetic Aperture Radar) mode.
In order to get accurate readings, it is necessary to pinpoint the position of the satellite with great precision. To accomplish this, CryoSat-2 uses the DORIS (Doppler Orbit and Radio Positioning Integration by Satellite) system. The satellite’s radio receiver measures the Doppler shift of signals from a network of more than 50 radio transmitters around the globe. The true accuracy of the reading can only be obtained by processing on the ground, but the raw data provides a usable estimate, good to about half a meter. The DORIS network has been in use for more than ten years, and has been used for various satellites, including ESA’s Envisat.
This locates the satellite itself, but in addition to this, it is necessary to determine the precise orientation of the baseline of the two antennas that receive the signal. To obtain this baseline, CryoSat-2 uses the sailor’s old friends, the stars in the sky. There are three startrackers on the antenna support structure, and each of them takes five pictures per second. The onboard computer compares the images to a catalogue of star positions. The orientation of the baseline of the receiving antennas is determined using this positioning data.
Unlike many other Earth-orbiting satellites, CryoSat-2 does not have a sun-synchronous orbit- in other words, it does not orbit in such a way as to receive sunlight all the time. Instead, the satellite’s path is such that there will be times when Earth is between it and the sun. This presented some design challenges, as the satellite will regularly be subjected to extreme temperature shifts. Because of this, it was necessary to insulate the precise instrumentation to maintain optimal operating temperature.
Except for a few valves in the propulsion system, CryoSat-2 has no moving parts at all. Even the solar panels, rather than being deployable as in most other satellites, are rigidly fixed to the body of the craft, forming the “roof” of the structure. The lack of moving parts allowed a significant cost savings in the satellite’s manufacture.
Knowledge is our best weapon in the fight against the long-term consequences of global warming. It is a sad truth that we gained the ability to destroy our world before we learned how to fully assess the damage, so the process went on for years before we realized it. Now we are in the uncomfortable position of having to measure just how far the process has progressed, and what, if anything, we can do about it. CryoSat-2 is a new tool for that measurement, and from it, we will gain a new understanding of our planet and its processes. If we could see the history books of tomorrow, how would they speak of this project, and the other Earth-sensing projects that are going on now? Will they see this as the beginning of recovery, or just a futile assessment of our hopelessness? Perhaps these measures will only provide us with a precise gauge of the inevitable end, or perhaps not. Much environmental damage has already been done, and will not be corrected soon- but the knowledge we gain from CryoSat-2 may help us to lessen the impact, at least.
And it will give us a lot of interesting things to ponder, too. From it, we may pry a few more secrets from this world on which we have always lived, but which we still do not know very well. In that sense, this is another fascinating, alien planet to explore, because the things we are learning now are totally new. It’s all part of our growing knowledge of the mysterious third planet.
Sources:
“Successful Launch for ESA’s CryoSat-2 Ice Satellite” at website of the European Space Agency: esa.int/esaCP/SEMH5ZZNK7G_index_0.html
“ESA CryoSat: an Earth Explorer” at website of the European Space Agency: esa.int/SPECIALS/Cryosat/SEMHSTOJH4G_0.html
“ESA CryoSat: an Icy Mission” at website of the European Space Agency: esa.int/SPECIALS/Cryosat/SEMFJ4908BE_0.html
“ESA CryoSat: Earth’s Changing Ice” at website of the European Space Agency: http://www.esa.int/SPECIALS/Cryosat/SEMQK4908BE_0.html
“ESA CryoSat: Platform” at website of the European Space Agency: esa.int/SPECIALS/Cryosat/SEMFN4908BE_0.html
“ESA CryoSat: the Instruments” at website of the European Space Agency: esa.int/SPECIALS/Cryosat/SEMRQ4908BE_0.html