NASA's Mars Opportunity rover is celebrating its 15th birthday with a nap because of a giant dust storm. Look back at its unlikely journey.

NASA's Opportunity Mars rover was built to last just 90 Martian sols, or 92 Earth days. But the scrappy machine shocked engineers by lasting far longer than that. The rover is now celebrating 15 years since it first launched from Earth.

Opportunity set off for Mars in the dark of night on July 7, 2003. Engineers at NASA never expected the solar-powered machine to weather a Martian winter, but the golf-cart-sized rover has traveled more than 28 miles on the red planet since it landed there on January 25, 2004.

Today, the teenager is undergoing one of its toughest tests to date: a global dust storm is covering Mars, making it tough for the rover to capture much-needed solar power, so it has gone into safe mode. In other words, Opportunity is celebrating its record-breaking tenure on the red planet with a nap.

NASA plans to try to reconnect with the rover once the storm passes, but on July 3 the space agency reported that it was "still waiting for the dust to settle."

Here's a look back at what the Opportunity rover has accomplished so far on its unlikely journey on Mars.