A top secret U.S. robot space plane landed back on Earth on Friday after a 22-month orbit, officials said, although the craft's mission remains shrouded in mystery.

The unmanned X-37B, which looks like a miniature space shuttle, glided into the Vandenberg Air Force Base in California after having launched on December 11, 2012, on a mission that military officers say is still strictly secret.

A month after world leaders pledged to curb the threat of climate change, their words will be put to the test when talks for a new global pact resume in Bonn next week.

Negotiators will gather from Monday, tasked with ironing out differences over how to save Earth from potentially catastrophic climate damage.

Argentina launched its first domestically built communications satellite Thursday.

The ARSAT-1 satellite is the first to be constructed with local technology in Latin America. It was built by a crew of about 500 scientists over seven years at a cost of $250 million. The satellite was launched from a base in French Guiana and is to orbit 22,000 miles (36,000 kilometers) above Earth.

A pair of American astronauts stepped outside the International Space Station Wednesday for a spacewalk to do repairs and upkeep at the orbiting outpost, NASA said.

Reid Wiseman and Barry "Butch" Wilmore finished their spacewalk after six hours and 34 minutes, right on schedule, the U.S. space agency said.

A fast-moving comet is about to fly by Mars for a one-in-a-million-year encounter with the Red Planet, photographed and documented by a flurry of spacecraft, NASA said.

The comet, known as Siding Spring (C/2013 A1), has a core about a mile (1.6 kilometers) wide in diameter, but is only as solid as a pile of talcum powder.

Australia's Environment Minister Greg Hunt has pledged to end the extinction of native mammal species by 2020, with a focus on culprits such as feral cats.

Hunt said Australia had the worst rate of mammal loss in the world and the nation's "greatest failure" in environmental policy was protecting threatened species.

A forerunner of today's crocodiles that lurked in coastal waters and estuaries some 150 million years ago measured 9.26 meters (some 30 feet) from snout to tail, scientists reported Wednesday.

Paleontologists from half a dozen European countries took a fresh look at the classification of so-called crocodylomorphs that go by the name of Machimosaurus.

Nigeria's chaotic megacity Lagos on Wednesday sought to impose a one-day ban on the use of the car horn, hoping to raise awareness about damaging noise pollution and improve quality of life.

The occasional honk still rang out on the crowded streets of the country's financial capital yet drivers, public transport users and pedestrians said they noticed that things were a bit quieter.

Space enthusiasts planning a move to Mars may have to wait to relocate: conditions on the Red Planet are such that humans would likely begin dying within 68 days, a new study says.

Oxygen levels would start to deplete after about two months and scientists said new technologies are required before humans can permanently settle on Mars, according to the study by researchers at the Massachusetts Institute of Technology (MIT).

Japan's utilities say they are being swamped by green power because of rules forcing them to buy up every last watt produced from renewable sources, as new generating companies seek to cash in on premium prices.

Power firms say the grid does not have enough capacity to cope with the rocketing levels of electricity that is coming from a growing number of solar power facilities, possibly risking blackouts.