NASA made its final attempt to uplink a command to the Spirit rover May 25. That afternoon, a group of us who work with the rovers at Cornell gathered at a bar to mourn. We drank sangria outside, soaking up the solar energy that Spirit needed so desperately, and toasted the rover's seven years on Mars.

Although Spirit hadn't driven in over a year, and we all knew that this day would come someday, this official "last call" was hard news to receive. One former team member said that she broke down and cried as she read the e-mail sent by project manager John Callas explaining to the team that this was the end for Spirit.

I had also held back tears when I read that e-mail. I started wondering why we responded so emotionally — why the official end of this rover could make grown scientists cry. Certainly part of it has to do with the community of the team. In planning meetings, the team tends to refer to the rovers as "we" as opposed to "she" or "it" — "We'll do the Pancam imaging, then we'll drive west," for example — and the end of a mission means the end of our shared experience.

But there's more to it than that. For one, it meant I'll never get more data for my thesis chapter on the Troy soil exposure. But there is something else about Spirit that separates this rover from every other spacecraft, something that tugs at our hearts. I think I finally figured it out: It's cute.

I'm serious. The Spirit and Opportunity rovers brought something brand new to space exploration: cuteness. The rovers' solar panels vaguely resemble wings, the camera masts look like long necks, and the Pancam instruments — two on the top of each rover's mast — seem to be the "eyes" of space-faring creatures.

Why is cuteness so important? Because humans tend to respond emotionally to cute things. It's hardwired into us, as it's related to our instincts to protect our young.

It's not necessarily "human" characteristics that trigger this biological response in us; it's the "juvenile" characteristics — such as big eyes, flat faces and large foreheads. These are the characteristics that make us go "awww" when we see puppies and that compel us to watch videos of kittens on YouTube.

I think this is why kids respond so strongly when I give presentations about the Mars Exploration Rover mission at schools and museums. No matter what age, they're always enthralled by the animation of the rovers, and they really seem to identify with these spacecraft. With kids under the age of 10 or so, I always get a question along the lines of, "Do the rovers get lonely?" Once I even got, "Do the rovers miss their mommies?"

And each time, while I explain to the children that the rovers are machines that have no real emotions, some deep part of me aches to travel to Mars and give lonely Spirit a hug.

Spirit's cuteness is, I believe, a big part of the public's response to the Mars Exploration Rover mission in general. It's why someone wrote LiveJournal entries for Spirit and Opportunity, giving them the personalities of vain teenage girls. (My favorite Spirit entry: "ARGH, can't i just explore mars in my own way? i don't need this constant ordering around! when do i get to make my own choices, anyway? it's my life, after all.")

I'm sure that Spirit and Opportunity were not engineered to be cute -- it was a bonus to their engineering design. But maybe it's something that should be kept in mind for future spacecraft. I'll be interested to see how the public response to Mars Science Laboratory (aka Curiosity), arriving at Mars next summer, compares to MER. It will be harder to anthropomorphize that beastly rover, with its off-center Cyclops eye (the ChemCam laser) and boxy frame. Even with its cutesy name, Curiosity isn't cute -- and it never gets the same "aww" response when I show pictures of it to kids.

Will the end of Curiosity's mission pull at our heartstrings the same way the end of Spirit's mission has? Hopefully we won't find out for many years. But for now: Goodbye, Spirit. We'll miss you, cutie.

Melissa Rice is a Cornell graduate student (and Cornell Chronicle writer intern) using Mars Exploration Rover Pancam data to study the surface composition and history of water at Gusev Crater. She also works on the rover's operations team as a pancam payload downlink lead. This article is an abridgement of the original published on The Planetary Society website at http://planetary.org/blog/article/00003065/.