The Mars Opportunity rover, which has been exploring the red planet for the past 15 years, is in danger of succumbing to a fierce dust storm that will soon encircle the entire planet, NASA scientists said Wednesday. The dust storm is blocking the sunlight that the rover needs in order to charge its batteries, forcing it to go into an emergency mode to conserve energy.

Why it matters: The ongoing dust storm is no ordinary tempest, it's reached a severity that has not been seen before in the nearly two decades of detailed observations, NASA said. The Opportunity rover is one of several rovers that NASA operates on Mars. Its research helps us learn more about the red planet in order to pave the way for eventual human exploration there.

The timeline: The dust storm was first spotted by the Mars Reconnaissance Orbiter on May 30, according to John Callas, Opportunity project manager for NASA's Jet Propulsion Laboratory in California. The storm began limiting the amount of sunlight reaching the rover on June 3, with an atmospheric opacity level at a record 10.8 by June 9. This means that day had effectively turned into night on Mars, as sunlight was absorbed by the dust in the air.

"I made the decision to declare a spacecraft emergency," Callas said, referring to the period when opacity readings increased during the past few days. He said the spacecraft is likely in a "low power fault" situation, and that if the storm abates, the rover will attempt to charge itself and relay a message to the MRO or another spacecraft.

• "Were listening every day for possible signals from the rover," Callas said during a press call.

The details: Scientists expect the rover to stay warm enough to avoid even more severe problems, but it could have so little power left in the near future that its clock will shut down, which would involve a more complicated process to restart itself.

What's next: In just a few days from now, according to Rich Zurek, Mars program office chief scientist at JPL, the dust storm will meet the definition of a planet-encircling event. Such massive dust storms do not occur on Earth, but Mars has a thinner atmosphere and more dust on its surface.

The big question: Scientists that are part of the Opportunity team have a tight bond with the intrepid little rover. They're rooting for it to survive this storm, but no one knows for sure if it will. Should it succumb to the sudden darkness, it would end a mission that has outlasted its intended design lifetime, while leaving NASA with other rovers still left on the planet, including the Curiosity rover, and another, known as Insight, on the way.

• "Prior to the storm the vehicle was in remarkably good health," Callas said, noting one arthritic joint on its robotic arm, and a challenge with its steering mechanism.

The scientists working with Opportunity for all these years are suffering along with their scientific research vehicle. "This team has a very strong bond with the rover," Callas said. "It’s like you have a loved one with a coma in a hospital."

"We anthropomorphize this rover," he said. "This storm is threatening."