On Saturday, NASA’s Opportunity rover celebrated a monumental anniversary all by itself on the surface of the Red Planet: surviving 5,000 Martian days. It’s an incredibly significant milestone for the little-wheeled robot, given the fact that it was only expected to last just 90 days. Opportunity is just one of two rovers currently functioning on the Red Planet, along with NASA’s Curiosity rover.

Opportunity launched on top of a Delta II on July 7th, 2003, along with a twin rover called Spirit. The pair then landed on Mars three weeks apart in January 2004, both going to opposite sides of the planet. The day of Opportunity’s landing, January 25th, was considered Sol 1 — or the first Martian day for the rover. The term “sol” is used to describe one “solar day” on Mars, which is how long it takes for the planet to rotate once around its axis. It’s nearly the same as an Earth day, but it runs about 40 minutes longer.

NASA didn’t think Opportunity would survive its first winter on Mars

NASA only planned 90 sols for both Opportunity and Spirit because the mission teams didn’t think the vehicles would survive their first winter on Mars — a season that lasts nearly twice as long as winter on Earth. The two rovers landed in Mars’ southern hemisphere, which doesn’t get a lot of sunlight during the planet’s winter, so NASA didn’t think they’d get enough power to function throughout the season. Yet NASA figured out a way to tilt the rovers northward, pointing their panels at the Sun, which appears in the northern sky during the winter. This allowed both rovers to well-exceed their intended timelines.

Spirit was eventually lost when it got stuck in a sand trap and couldn’t tilt northward during its fourth winter in 2009. Yet Opportunity is still kicking, currently enduring its eighth winter on Mars. It has driven more than 28 miles from where it originally landed and is responsible for making some big discoveries about the Red Planet, such as evidence that ancient Mars likely had ground and surface water. Now the rover is exploring a shallow channel called “Perseverance Valley,” helping scientists to figure out the geological processes that shaped the region.

So happy anniversary, Opportunity! Here’s hoping you make it through this winter and add even more sols to the mission.