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.

Opportunity was one of two six-wheeled rovers that NASA launched to Mars in 2003.

The rover blasted off from Kennedy Space Center at 11:18 p.m. on June 7, 2003.

Opportunity's twin, Spirit, left Earth on June 10 of the same year. Scientists wanted the rovers to help them figure out whether Mars might have once been a place where life could exist.

It took the two rovers more than six months to fly the roughly 283 million miles to Mars.

Opportunity arrived on Mars on January 25, 2004, and was drop-bounced onto the ground inside of a kind of heavy-duty bubble wrap.

The rover's heat shield protected the machine as it whizzed through the Martian atmosphere, helping Opportunity land safely.

The rover then disembarked and drove off in search of adventure.

Opportunity hasn't really been in a hurry on Mars: it's logged an average of roughly two miles of exploration per year.

The rover travels up to about 130 feet each Martian day, according to NASA.

Engineers at NASA's Jet Propulsion Lab in California tried to simulate some of the tough driving conditions that the rovers would face on Mars before they launched.

The Spirit rover spent just shy of seven years exploring the red planet. It got caught in some soft soil and lost contact with NASA in 2010.

The instruments on the Opportunity rover include cameras, spectrometers that study chemicals and minerals, a rock grinder, and a brush for sweeping dust off rocks.

NASA calls Opportunity a "a golf-cart-size robotic field geologist."

The rover's robotic arm, seen here in its stow position, is designed to dispatch four different kinds of tools that the machine uses to inspect its surroundings. But not all of those tools function anymore — at least two instruments have stopped working, and the robotic arm is now creaky and old.

Some of the first things Opportunity spotted on Mars were these small, round balls that NASA nicknamed "blueberries."

Opportunity caught a glimpse of these hematite-rich spherules on the Martian surface near the Fram Crater in April 2004.

Their round shape gave scientists good evidence there was probably water on Mars at some point.

Much of Opportunity's time on Mars has been spent examining rock layers in craters. The rover has been exploring the area around the Endeavor Crater, which is 14 miles wide and at least 3.6 billion years old, since 2011.

The 10-million-year-old Orion crater sits on the western rim of the Endeavor Crater and is just 90 feet wide. Opportunity passed by Orion on April 26, 2017 and captured this panorama of pictures using three kinds of light filters to capture near-infrared, green, and violet light. The image has been enhanced so that we can see differences in the surface material on Mars more clearly.

Scientists think the rocks pictured here hint that Mars might've harbored life long ago because there are signs that water moved through rocky cracks.

The rover explored Mars' Endurance crater from May to December 2004.

The rover's front hazard avoidance camera, which took this shot, looks out for danger ahead.

Engineers back on Earth rehearsed moves the rover might make into the Endurance crater, hoping to avoid disaster.

Opportunity examined the crater's steep, rocky walls for clues about the planet's past and discovered that water flowed on the planet on more than one occasion.

Scientists at NASA also found minerals like jarosite and hematite there, which crop up in wet environments.

The rover hovered at the edge of Erebus Crater in November and December 2005. In this image, we see a bird's eye view of Opportunity projected from the rover's panoramic cameras. Opportunity is the bright spot in the middle — its solar panels glint in the sun.

The "pancam" is actually two digital cameras that work like 360-degree viewing eyes. The device is mounted on poles that can raise the eyes up over 4 fee, which is how the rover captured this view.

In this image, the rover is surrounded by rocks and sand dunes. The Martian soil is not actually blue — the false-color image serves to emphasize some features of the land that the rover is checking out.

Opportunity saddled up to the Victoria crater, which is five times wider than the Endurance crater, on September 26, 2006. In this image, the rover appears as a tiny speck on the rim.

This picture was not taken by the rover — it's a shot from the High Resolution Imaging Science Experiment (HiRISE) that's been orbiting Mars and taking super high-resolution photos of the planet since 2006.

In the Victoria Crater, Opportunity spied this promontory called Cape Verde on October 20, 2007.

Once inside the half-mile-wide crater, Opportunity spent nearly a year examining ancient rock layers before climbing out in August 2008.

Opportunity has become something of a Mars rock expert after repeatedly drilling into the planet's surface to analyze chemicals inside.

"The reddish color around the holes is from iron-rich dust produced during the grinding operation," NASA said.

Between the dust storms and cold, dark winters, Mars is a tough place to be a solar-powered machine. By 2014, Opportunity's solar panels looked much browner and dustier than they did when the rover landed.

But Opportunity kept chugging along.

In 2015, NASA announced that the rover had traveled a marathon's worth of miles on Mars: 26.2 miles over 11 years and two months.

That's the farthest any Earthly machine has ever traveled on another world.

"This mission isn't about setting distance records, of course; it's about making scientific discoveries on Mars and inspiring future explorers to achieve even more," Opportunity mission leader Steve Squyres, a planetary scientist at Cornell University, said of the feat in 2015. "Still, running a marathon on Mars feels pretty cool."

In total, Opportunity has logged more than 5,100 Martian days, or "sols," on the red planet.

That's pretty impressive for a machine that was expected to last just 90 sols.

The Opportunity rover now faces its toughest physical challenge to date. A global dust storm has rendered its solar panels useless, and the rover has gone to sleep.

NASA last made contact with the rover on June 10, 2018. The space agency says it has been listening every day since then for signs that Opportunity might wake up again.

If dust continues to coat the solar panels that power up the rover, Opportunity may not be able to wake in time to save its electronic circuits from freezing forever in the bitter Martian cold.

"This is the worst storm Opportunity has ever seen," Squyres told the Planetary Society. "We're doing what we can, crossing our fingers, and hoping for the best."