NASA

The great robot tour of Comet 67P/Churyumov-Gerasimenko has been extended, and it may end next year with the Rosetta spacecraft taking up a permanent residence alongside the Philae lander on the surface of the icy space rock.

Meanwhile, NASA's Dawn spacecraft is descending closer to the dwarf planet Ceres in the asteroid belt, but NASA plans to end that mission by putting the spacecraft into an indefinite orbit around Ceres.

Why the different approaches to ending two equally fascinating landmark missions?

First, let's review the two missions and their targets. The European Space Agency launched Rosetta over a decade ago and the craft arrived at Comet 67P last year, deploying the Philae lander to the surface in November. Philae landed in the shade of a large, rocky outcropping on the surface of the comet, and when its battery ran out after a few days, the probe went silent. This month it finally began receiving enough solar power to call home.

On Tuesday, the ESA announced that it would extend Rosetta's mission -- which was initially funded through December of 2015 -- for an additional nine months, allowing it to continue scientific observations of the comet as it makes its long journey back to deep space following its pass around the sun. At that point, Rosetta will be low on propellant and receiving a dwindling amount of solar power.

"As (Rosetta is) riding along next to the comet, the most logical way to end the mission is to set Rosetta down on the surface," Patrick Martin, Rosetta Mission Manager, said in a statement. "But there is still a lot to do to confirm that this end-of-mission scenario is possible. We'll first have to see what the status of the spacecraft is."

Dawn's end-of-mission plan is very different. Launched in 2007, the NASA craft's mission is to explore two small worlds, Vesta and Ceres. Dawn spent a year circling Vesta in 2011 and 2012 and will spend at least the rest of this year orbiting Ceres. Already, Ceres has yielded mysterious bright spots, a weird pyramid mountain, huge hidden ocean and other curiosities, even as the craft continues to descend for an even closer look.

On Tuesday, as ESA outlined its end of mission plans for Rosetta, NASA reiterated on Twitter that it has no plans to bring Dawn anywhere near touching the surface of Ceres.

1. Mission planners don't want me to come in contact with the surface, even decades after the mission ends. — NASA's Dawn Mission (@NASA_Dawn) June 23, 2015

Instead, Dawn will be "orbiting Ceres indefinitely," meaning the dead craft will likely become a semi-permanent moon of Ceres, at least unless it gets destroyed by a stray asteroid.

So why not simply lay Dawn to rest on Ceres in much the same way Rosetta may come to its final resting spot on its comet? Is this approach reflective of differing European and American approaches to cosmic intimacy?

No, actually it's about something called "planetary protection," which is basically the real-world, pre-alien contact version of the "Prime Directive" from "Star Trek." While the fictional concept was about prohibiting interference in the development of alien civilizations, planetary protection is about trying not to screw up other worlds just as we're getting to know them, especially worlds like Ceres that may harbor some of the building blocks needed to support life.

@cosmos4u Yes, planetary protection. #Ceres may have chemistry that's related to the development of life. — NASA's Dawn Mission (@NASA_Dawn) June 23, 2015

This is particularly relevant in the case of Dawn, which isn't equipped with the right kind of propulsion system to make a soft landing on Ceres, so it might end up making more of a crash landing, turning Ceres into a distant wrecking yard.

Why is it fine from a planetary protection standpoint to leave space junk on a comet but not Ceres? Well, that's probably a point that some people would debate, but remember that a comet isn't a planet and it travels in a long, elliptical orbit that takes it through varied and torturous extreme temperature and radiation zones that would seem to make it utterly uninhabitable.

Then again, perhaps there are some bizarre, comet-riding, ultra-hearty life forms clinging to 67P and just hoping that the weird new satellite that started circling home last year won't fall from space and crush its entire family.

But probably not.