May 20, 2003

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May 20, 2003: Crowded, bustling, full of life: coral reefs are the big cities of the sea. One-fourth of all known marine species live in reefs, including at least 4000 varieties of fish and thousands of kinds of plants and microorganisms. Reefs are critically important to our planet's ecosystem.

They're also endangered. Pollution, global warming, cyanide fishing and coral mining are among the threats. Even tourism can be a problem. "We're loving them to death," says Cindy Evans, an earth scientist at the Johnson Space Center (JSC). Resorts are built near reefs to attract snorkelers and divers. Construction and human traffic, however, are bad news for these delicate marine communities.

Today's picture from the International Space Station shows more than a dozen underwater reefs in the Red Sea. Six percent of all the world's reefs are found in the Red Sea, and while these particular reefs look well, 60% of the reefs in the area are thought to be in peril.

The space station is a good place to study reefs. Four hundred km above Earth, the crew can take pictures with both a wide field of view (capturing entire reef communities) and high resolution. "It's a wonderful combination," says Evans. This Red Sea image, for example, spans 85 km from side to side and reveals details as small as 100 m. Other photographs of coral reefs from the space station can discern details as small as 5.5 m.

And that's not all:

When ordinary Earth-observing satellites pass over a spot, it's usually at the same local time. The sun is in the same part of the sky ... flyby after flyby. Because their sensors look straight down the view never changes.

The ISS is different. Each time the space station passes over some location, like the Red Sea, the sun is in a different place in the sky. With different lighting, the camera can capture different features. "Furthermore," adds JSC earth scientist Julie Robinson, "astronauts are able to look out different windows of the station, and they can look out each window at a variety of angles. This allows humans in orbit to observe features in ways that satellites cannot."

A remarkable feature of today's image is the bright spot in the lower right--a reflection of the sun overhead. "We call this 'sunglint,'" says Evans. Snaking away from that spot are silvery tendrils where sunlight is glinting from oily surface films. The oils come from shipping traffic and the natural biological activity of marine life. Wave motions and flows concentrate the films into telltale patterns. "By tracing their shape, we can identify eddies, flow features between the islands, and the overall water dynamics."

Flowing water helps shape and sustain reefs, so this is important information.

"For most Earth satellites, sunglint off the surface of the ocean is considered a nuisance," adds Robinson. "It interferes with observations of underwater features. However, glint can be used to interpret the physical properties on the sea surface, and even infer wind speed and direction in a local area."

Sunglint a nuisance? It all depends on your point of view....

This image is a mosaic of two digital photographs taken from the International Space Station on April 11, 2003: ISS006-E-45935 and ISS006-E-45936. The mosaic was provided by the Earth Sciences and Image Analysis Laboratory at Johnson Space Center. Additional images taken by astronauts and cosmonauts can be viewed at the NASA-JSC Gateway to Astronaut Photography of Earth.