Ocean colour seen - from the sky

The countdown has begun to the launch of a satellite that will monitor changing hues of the sea, giving clues to the planet's future.

Oceanographers and climatologists are not usually the ones watching with fingers crossed when a rocket takes off. But that is what will happen on 1 August with the launch, rescheduled from last week, of the world's first commercial remote sensing satellite. Called SeaStar, and built by Orbital Sciences Corporation (OSC) of Fairfax, Virginia, it will be dedicated to the seemingly esoteric study of ocean colour. It will be carried on the notoriously unreliable Pegasus rocket, which sea scientists hope fervently will this time fulfil its mission.

For most of us, the colour of the ocean is largely a reflection of the weather: its colour is generally that of the sky. Hence grey seas are associated with our dismal climate, while the brilliant blue hue of sunnier climes is associated with summer holidays in more exotic locations. But scientists are interested in the more subtle colour variations that can best be seen from space.

In the open ocean, the water colour is largely determined by the amount of microscopic plant life, or phytoplankton, living close to the surface. Near the coast, the picture becomes more complex, with sediment and river outflows also affecting the colour of the sea. It is these differences which SeaStar will map, using its state-of-the-art SeaWiFS (Sea-viewing Wide Field-of-view Sensor) instrument.

SeaWiFS is designed to study light reflected from the oceans at eight wavelengths in the visible and near-infrared spectrum. From a near-polar orbit, it will be able to scan each square kilometre of cloud-free ocean every 48 hours, searching for variations in the amounts of chlorophyll and phytoplankton.

The satellite is intended to operate continuously for five years, giving a unique opportunity to study the changing patterns of microscopic marine life on a global scale. "It will give us accurate information on the abundance of marine phytoplankton over the whole world on an almost daily basis - information that cannot be gathered any other way," says Dr Jim Aiken of Plymouth Marine Laboratory.

One key application of SeaWiFS data will to support the international Joint Global Ocean Flux Study, covering the role of the ocean in the carbon cycle. Data received from the satellite will tell us more about the ways in which the oceans absorb carbon dioxide, the most important greenhouse gas.

Just like land-based plants, marine phytoplankton use energy from sunlight to grow and multiply, taking in carbon dioxide dissolved in sea water. What they remove is replaced by carbon dioxide from the atmosphere. As the plankton die, some of their carbon is released back into the air, but some ends up in deep sea sediment where it is locked up and eventually forms carbonate rocks such as limestone. The oceans thus act as a major sink for carbon dioxide, and slow the rate of global warming. By monitoring phytoplankton patterns, SeaWiFS will help scientists to measure this effect and should allow them to predict more accurately future global climate change.

"We desperately need this sort of data if we are to understand the way in which biological patchiness influences the way carbon dioxide is removed from the atmosphere by plankton," says Peter Challenor, head of satellite remote sensing at Southampton Oceanography Centre's James Rennell Division.

Marine organisms also modify climate more subtly by releasing a gas called dimethyl sulphide (DMS). Some of this escapes into the atmosphere, where it is oxidised to form aerosol particles. These absorb and reflect solar energy back into space, and also increase the likelihood of cloud formation, by acting as nuclei around which water droplets can condense. An increase in global cloud cover could reflect more sunlight back into space and have a cooling effect on the climate. But anything that affects the balance of microbial ecology, ranging from wind speed to changes in sea temperature, can affect how much DMS is released into the atmosphere.

Apart from studies of the open ocean, SeaWIFS will also allow scientists to study changing conditions in coastal regions, including concentrations of pollution and sediment as well as marine organisms such as poisonous algal blooms - known as "red tides" - which can kill fish, ruin holidays and devastate the economies of coastal resorts.

Interpreting the water colour in such regions will be a major aspect of SeaWiFS research in the UK. "It is optically much more complex," says Dr Ian Robinson of Southampton Oceanography Centre. "Dissolved organics, suspended sediment and so on make it harder to interpret the data."

Dr Robinson's group intends to use the new data to study features such as the plume of sediment which extends all the way across the North Sea from the eroding cliffs on the English east coast to the shores of Holland, and the ways in which this feeds back into the biological cycle.

Using data from other ocean colour satellites, British scientists have been preparing for the SeaWiFS mission for several years. They have had to learn, for example, how to calibrate and validate the stream of data from space before feeding it into their oceanographic models.

Regular surface measurements have to be made for comparison with the satellite results, since satellites only skim the surface and cannot measure conditions at greater depths. UK oceanographers use a variety of sources for their calibration data. For example, scientists from Southampton Oceanography Centre have recently completed a SeaWiFS preparatory cruise in the Mediterranean. A data buoy operated by Plymouth Marine Laboratory will be used to measure sea colour each day as the satellite passes overhead. For more extensive coverage, the British Antarctic Survey's research ship RRS James Clark Ross takes ocean colour readings during its biannual 12,000-kilometre (7,500-mile) voyage to and from the frozen southern continent.

Since SeaStar is being operated on a commercial basis, the scientists will be second in the queue for data, with a two-week waiting period. Orbimage, an OSC subsidiary, is promoting SeaWiFS data for land remote sensing and as a means of guiding fishing fleets to the most productive areas: certain high-value fish, such as tuna, tend to concentrate where phytoplankton levels are high.

The success of this technique will be enhanced still further by SeaShark computer software, developed in the UK by the Terrastar consortium led by the Vega Group. This allows commercial customers to combine SeaStar's ocean colour data with ocean surface temperatures measured by US weather satellites.

The unusual hybrid mission is the outcome of a new approach introduced by the American space agency Nasa. Under a unique agreement signed in 1991, Nasa agreed to purchase five years of SeaStar data at a cost of $43m (pounds 26m) as part of its "Mission to Planet Earth" initiative. In return, the satellite development was left entirely to OSC.

But the arrangement was far from successful. The launch date slipped by four years due to technical problems, and Nasa has had to advance most of its cash allocation to ensure that development problems were overcome. "I can say when I think they're doing something stupid, but I can't direct them," says Mary Cleave, a Nasa project scientist . "It's a very ... interesting way to do business."

Scientists will hope that deployment of the satellite is more straightforward than its prolonged, tortuous development. SeaWIFS is being launched at a time when a gap has opened in ocean colour monitoring. Although it has been studied from space since the late Seventies, only one operational sensor is now in orbit, installed on board the Indian IRS-1 satellite.

This month, the Japanese Adeos satellite stopped operating, apparently because of a design fault. There was another ocean colour sensor in space. But that was on board the Mir space station