Andy Calloway loses a friend today.

Calloway is the missions operation manager—the "MOM"—for Messenger, an uncrewed spacecraft that has, for four years, orbited Mercury, 48 million miles away. He has essentially been the project hub. Each day he and his team interface with the engineers, scientists, management and, of course, the craft itself. He even takes a peek at how it's doing on weekends from his laptop at home.

Today, Messenger will make one last orbit around the planet, disappear behind the far side, and crash into its surface.

Messenger—an amalgam-acronym for Mercury Surface, Space Environment, Geo-chemistry and Ranging—has endured the most hostile space weather in the solar system. And, despite solar flares and coronal mass ejections, Calloway says, the craft has surpassed every challenge and met every request.

It has mapped new territory, discovered that Mercury has shrunk over its lifespan, found new and mysterious geological features, and sent back data that forced planetary scientists to reconsider how planets formed in the early solar system. "It's been such a reliable, durable little spacecraft—just doing everything we've ever asked of it," Calloway says.

Earth's crewed and robot space programs have a healthy rivalry—robots have journeyed farther from home than humans by several orders of magnitude. When humans relegate robots to a second-class status, it's unfair—not just to the people like Calloway who run the missions but to the robots themselves. These space probes are the products of human ingenuity and imagination. They are our intellectual offspring. We send them to the stars as our proxies, and some of them voyage on forever. Most perish in space—and they do it for us.

Andy Calloway. Ed Whitman

After launching in August 2004, Messenger's mission got off to a hesitant start. To conserve fuel, it slingshotted around Earth, Venus (twice) and Mercury (three times), altering its trajectory with each "gravity assist," before inserting itself into Mercury's orbit in March of 2011. Each flyby taught Messenger's humans how to direct the craft more efficiently. All the same, Calloway says, they knew entering Mercury's orbit was fraught with danger. A heat-shield protects Messenger's instruments (it's 300 degrees on the sunny side surface, and room temperature an inch or two behind on the back) but a tilt of a few degrees in the wrong direction would have fried the instruments.

It had been more than 30 years since Mariner 10 completed the first reconnaissance mission of the inner solar system. Before it went into permanent orbit around the sun, Mariner flew past Mercury, beaming home a rough map of about 40 percent of the planet. It took all those intervening decades for a group to propose a design it was confident could survive another mission to Mercury.

Calloway knew Messenger might be the last mission to Mercury for decades, even centuries. "We treated each Mercury flyby as if it was the last chance to see the planet," he says. During the first flyby in particular emotions ran high. His team had spent months preparing to collect as much data as possible during the 30-hour visit. A picture mosaic of the planet's surface was the most anticipated grab. "A bunch of us were sitting in the control room at midnight when the images started coming down," Calloway says. "It was amazing to think that we were the first humans in history to see this territory."

To date, Messenger has sent an astounding 275,000 images home. Still, the mission's kept Calloway and his team on their toes. "This mission has been non-stop with engineering challenges to overcome," he says. "It's the antithesis of boredom." This is due primarily to Mercury's closeness to the sun. Solar flares and coronal mass ejections have at times caused bit flips (switches between 0 and 1 or 1 and 0) in the spacecraft's code, causing instruments to malfunction.

Each time, the Messenger team—headquartered at Johns Hopkins University—has been able to get them online again. "It's just like trying to deal with a storm on Earth," Calloway says. "You know how in general to deal with a storm, but each one is different in how it affects you."

But now, finally, the end is coming, as Calloway and his team knew it would. The sun's gravity has nudged Messenger toward Mercury. Every time it got too close, the spacecraft performed a "boost up" maneuver, using precious fuel to rocket outward. Last month, the fuel ran out.

The team had a plan. They used the helium that pressurized the onboard fuel supply—like the last bit of air in a can of Reddi-wip—as propellant. It kept Messenger in orbit a month longer than the original mission had aimed for. "Even in the face of impending doom—impact on the 28th of March—we asked it to give us another month of operations," Calloway says. "It performed flawlessly." The team even got a pulse of low-altitude data they didn't expect.

Today, on the far side of Mercury, Messenger will descend through solar wind and the planet's attenuated atmosphere. It'll encounter temperatures as low as -280 degrees F. The sun-blasted surface will rise up; Messenger will gouge a crater 50 feet wide. Calloway will never hear from the craft again. "It will be surreal," he says.

Humanity's robot proxies will only get smarter. Today they're purpose-built computers studded with sensor packages. But as artificial intelligence gets more sophisticated, so will the robots. Humanity's children will spread throughout the solar system, whether or not human beings are with them, at the controls. With a few rare exceptions, none of those robot explorers will ever return.

Messenger will live on. Scientists and the public will be poring over data and images from the mission for years. And if the next proposed mission to Mercury—a joint project between the European and Japanese space agencies named BepiColumbo—takes flight, it may launch with one particular mission: to check up on Messenger's final resting place, a newly made crater in the backyard of our nearest star.