NASA astronauts on Saturday performed an emergency spacewalk to try to stop ammonia from leaking from the International Space Station's power system, NASA television showed.

During the spacewalk, expected to last six and a half hours, flight engineers Tom Marshburn and Chris Cassidy will inspect and possibly repair the ammonia leak that affected the U.S. segment of the orbiting laboratory on Thursday.

The level of carbon dioxide in the atmosphere has surpassed 400 parts per million for the first time in human history, U.S. monitors announced Friday, sparking new calls for action to scale back greenhouse gases.

Climate scientists say the threshold is largely symbolic and has been expected for some time, but warn that it serves as an important message that people need to reverse the damage caused to the environment by the heavy use of fossil fuels.

The London Zoo is urgently seeking a female mate for the last-known males of a critically endangered fish species.

Zoo officials say the Mangarahara cichlid is thought to be extinct in the wild and that two of the last known individuals — both male — are in the zoo's aquarium. A third is in the Berlin zoo.

The act of exploring helps shape the brain and adventuring is what makes each individual different, according to a study out Thursday by researchers in Germany.

The findings published in the U.S. journal Science may offer new paths to treating psychiatric diseases, scientists said.

The International Space Station suffered a small leak of ammonia used to cool its power system on Thursday, but there was no danger to the crew, NASA said.

The U.S. space agency said crew had spotted small white flakes floating away from an area outside the craft before reporting the incident to Mission Control in Houston.

Sky-gazers were treated to an annular solar eclipse in remote areas of Australia on Friday, with the Moon crossing in front of the Sun to leave a "ring of fire" around its silhouette.

The eclipse, which occurs when the Moon passes between the Earth and the Sun but is too close to the Earth to completely cover the Sun, was seen in full across northern Australia, while Sydney saw a partial eclipse.

When Michael Gore stands, it's a triumph of science and engineering. Eleven years ago, Gore was paralyzed from the waist down in a workplace accident, yet he rises from his wheelchair and walks across the room with help from a lightweight wearable robot.

The technology has many nicknames. Besides "wearable robot," the inventions also are called "electronic legs" or "powered exoskeletons." This version, called Indego, is among several competing products being used and tested in U.S. rehab hospitals that hold promise not only for people such as Gore with spinal injuries, but also those recovering from strokes or afflicted with multiple sclerosis and cerebral palsy.

Beijing urged the European Union on Thursday to avoid engaging in protectionism, after Brussels proposed an anti-dumping levy on imports of Chinese solar panels.

The EU's executive arm, the European Commission, is considering a heavy tariff of 47 percent, an EU source told Agence France Presse, as European solar panel makers complain they are being undercut by cheap Chinese imports.

Scientists unveiled a new method on Wednesday for extracting metallic iron from its ore while curbing Earth-warming carbon dioxide emissions.

This is achieved with electrolysis and has the added benefit of releasing oxygen as a byproduct, a team from the Massachusetts Institute of Technology reported in the journal Nature.

For three quarters of an hour a giant swarm of locusts streams across the sky above southwest Madagascar.

Along National Route Seven, normally an artery for tourists enjoying breathtaking views of the island's vast open spaces, a 15 kilometre long (nine mile) swarm clouds the sky.