On Sept. 30, 2016, the European Space Agency's Rosetta spacecraft purposely crashed into a comet.
The Rosetta spacecraft launched in 2004 and spent 10 years chasing down Comet 67P, a rubber-duck-shaped space rock that orbits the sun between Earth and Jupiter.
Rosetta spent the next two years tagging along with the comet as it traveled through the solar system. When it was time for the mission to end, Rosetta gently crashed into the comet.
Why it mattered
The European Space Agency's Rosetta mission to a comet was a first-of-its-kind mission.
Not only was it the first spacecraft ever to orbit a comet, but it was the first to follow its comet target as the icy object approached the sun, watching the comet evolve as it warmed up during its journey across the solar system.
ESA launched Rosetta in March 2004, with the probe taking a full decade to reach its target: Comet 67P/Churyumov-Gerasimenko, an icy remnant left over from the birth of the solar system 4.6 billion years ago.
The 1.3 billion-euro ($1.46 billion) Rosetta probe reached comet 67P in August 2014 and spent two years orbiting the comet, capturing stunning photos and studying its composition and changes over time.
Perhaps the most memorable moment during Rosetta's time at comet 67P was the landing of Philae, a tiny lander released by Rosetta that bounced several times before coming to rest at a strange angle on the comet's surface. The lander's batteries ran out after a few hours (since its solar arrays couldn't face the sun), but the landing itself was considered mostly a success.
Like a silent sentinel, Rosetta continued its observation of comet 67p, and even rediscovered Philae (which had disappeared from view after landing), just weeks before the orbiter crashed into comet 67P.
You may be wondering why, if Rosetta was still functioning, ESA opted to crash it into comet 67P.
The Rosetta probe followed comet 67P as it flew through the inner solar system and looped around the sun. By September 2016, comet 67P was headed back out to the outer solar system in the direction of Jupiter and Rosetta's solar panels would not be able to generate enough power to keep pace with the comet so far out.
It took 14 hours for Rosetta to make its slow-motion crash landing on comet 67P as mission scientists watched from a control center in Darmstadt, Germany. It hit the comet at leisurely walking pace of about 2 mph (3.2 kph).
The impact Rosetta's mission had on comet science, as well as our understanding of the the early composition of the solar system, is still being felt to this day.
"I can announce the full success of this historic descent," Patrick Martin, Rosetta mission manager, said at the time as he declared mission operations ended. "Farewell Rosetta, you've done the job. That was pure science at its best."
