When the Curiosity rover studies soil on Mars, it does it with a little shimmy. Its robotic arm collects a pinch of soil and drops it into the sample-analysis unit in the robot’s belly. The unit vibrates at different frequencies, shaking the powdery sample so it settles down into small cups. There, the unit heats up the soil, causing the grains to release fumes that scientists can study for hints of organic compounds.

In August 2013, NASA decided to use the sample-analysis unit’s vibrations for something a little different. To celebrate the mission’s first successful year on Mars, engineers programmed the unit to vibrate to a musical tune. From inside a Martian crater, millions of miles away from home, Curiosity sang “Happy Birthday” to itself.

The news of Curiosity’s mini-celebration of perhaps the loneliest birthday in the galaxy prompted a deluge of empathy in comment sections around the Internet. “WHEN HUMANS LAND ON MARS, WE BETTER DAMN GIVE THAT ROVER A HUG!!!﻿” one user wrote. The thought of a space robot serenading itself all by its lonesome certainly tugs at the heartstrings.

Curiosity marked its fifth year on Mars last week. (NASA counts the anniversaries in Earth years, which are shorter than Martian years.) But the rover didn’t sing. “The reports of my singing are greatly exaggerated,” the rover’s Twitter account reported, presumably referring to news coverage about its fifth birthday. “I only hummed ‘Happy Birthday’ to myself once, back in 2013.”

Huh. Why, over five years, did Curiosity get only one birthday party?

To find out, I emailed Florence Tan, the deputy chief technologist at NASA’s Science Mission Directorate. Tan was the electrical lead engineer for Curiosity’s sample-analysis unit, known as SAM.

“The answer to your question will sound rather cold and unfeeling,” her email began.

Oh, no.

“In a nutshell, there is no scientific gain from the rover playing music or singing ‘Happy Birthday’ on Mars,” Tan said. In the battle between song and science, science always wins.

Tan acknowledged that this may be a difficult truth for some. “We earthlings,” as she put it, tend to anthropomorphize robots. Many studies have shown that people can have feelings of attachment and protectiveness toward robots as they would for humans. The more “alive” a robot appears, the more likely people are to react to it in ways usually reserved for living beings. In various scenarios, people have felt empathy for Roombas, dinosaur toys, and bomb-disposal robots. All three are nothing more than boxes of wires, circuits, and sensors, but they exhibit enough autonomy, enough signs of “aliveness,” to trigger emotional responses. Scientists and astronomy fans are currently slogging through a weeks-long public mourning period for the Cassini spacecraft, which will end its mission next month. One planetary scientist recently told me she feels like she should be sipping vodka the day Cassini burns up in Saturn’s atmosphere, in a somber tribute to a brave pioneer.

So it’s no surprise that the idea of a space robot “celebrating” its birthday—the very thing that makes something alive—makes people feel all the feels, even if they know it doesn’t make sense. As one user wrote back in 2013, “It’s literally an inanimate object why am I still crying.”

For Tan, Curiosity isn’t a cute robot, it’s a $2.5 billion national asset. Tan said she and other Curiosity team members are mindful of the purpose of the mission, which is to study Mars. The process of generating and transmitting commands to Curiosity takes weeks of preparation and involves reviews and walk-throughs with various groups. “It’s not just, ‘Oh, I’m ready to send a command, just send an email to somebody,’” Tan said in a phone interview. The rover’s activities are scheduled down to the minute, and SAM requires power to operate. Curiosity runs on a nuclear battery that turns heat into electricity, and it will eventually die.

“I’m sorry. I’m a cold-hearted engineer.”

Tan and Tom Nolan, her husband and fellow Curiosity engineer, came up with the idea of turning SAM’s vibrations into music in 2007 as they worked on the hardware. Nolan quickly built a program and got SAM to “sing” “Twinkle Twinkle Little Star.” In August 2013, as Curiosity’s first anniversary approached, Tan asked Paul Mahaffy, the lead scientist on SAM, if they could get Curiosity to sing “Happy Birthday.” The team tested the commands on an identical version of SAM that resides at NASA’s Goddard Space Flight Center, and then transmitted them to the real thing. The tune, which you can listen to here, sounds almost like the humming noises of the animated robot, WALL-E.

If someone had been standing next to Curiosity when it sang “Happy Birthday,” they could probably hear it through their spacesuit, Tan said. But it would be very, very faint. Sound waves need air particles to carry them to our ears, and the lower the atmospheric pressure, the slower they travel. The atmospheric pressure on Mars is 1/100th that of Earth’s, so SAM will sound quieter than its twin back on Earth. We can’t know for sure how Curiosity’s various noises—the crunch of its wheels on terrain, the whirring of the camera atop its long neck—sound on Mars. Unlike NASA’s next class of Mars rovers, Curiosity doesn’t have a microphone.

So, how does Tan feel about Curiosity? Does she see it, like others do, as something other than a machine, as something a little alive?

“No, I’m sorry. I’m a cold-hearted engineer,” she said, laughing. She suspects she knows it too well. “I feel a different thing. I feel a sense of ownership. I don’t feel that it’s alive, but I feel in awe of its capacity and capability.”

But she totally gets why others might experience something more.

“When I watch WALL-E, I definitely feel the same feeling that everybody feels, so I understand,” Tan said. “When WALL-E was all alone ... I watched that movie and I shed a tear.”