This week marks the end of an important era in space exploration. On Friday, NASA's Cassini spacecraft will take a death dive into Saturn, where it has been in orbit for 13 years.
Cassini was more than a spacecraft that provided us with beautiful photos of Saturn and its intricate system of moons and rings. It changed the way planetary scientists approached the search for life beyond Earth.
The search is really a search for life as we know it, since we only have one sample: Earth. And that means our vision is narrow. Until recently, we didn't even know what types of moons existed beyond that of Mars.
- Stunning photos taken by Cassini
- Enceladus's north pole in stunning detail
- NASA pilots spacecraft between Saturn and its rings
"When we launched Voyager in the late '70s, we knew very little — nearly nothing — about the outer solar system in great detail," Larry Soderblom, an interdisciplinary scientist on the Cassini mission, told CBC News. "And we expected that the moons of the giant planets would be lifeless, dead, geologically inactive, boring, dull, cratered bodies."
With images from the Voyager missions, however, scientists suddenly saw the moons of the outer planets as intriguing and worthy of further study. That in turn, led to the birth of Cassini.
Exploring new worlds
When Cassini arrived, it found small, rich worlds orbiting Saturn. Enceladus and Titan are two of the most geologically intriguing, and both hold potential in the search for life.
Shortly after its arrival in 2004, Cassini detected geysers of water vapour streaming into space from Enceladus at nearly 400 metres per second.
What made this discovery particularly exciting was that, before Cassini, astronomers believed Enceladus — with a radius of 252 kilometres, slightly less than the distance between Regina and Saskatoon — was too small to hold any potential heat that would be generated by the push and pull of Saturn.
Later, the spacecraft flew through the plumes and discovered organic material, carbon dioxide and carbon monoxide.
Then, in 2015, a study suggested that the moon's slight wobble could only be accounted for if there was an ocean of water beneath its icy surface, likely the source of the water plumes.
With water, and likely hydrothermal vents — areas where life thrives in our own oceans — could Enceladus harbour life?
The possibility of microbial life gained even more support when NASA announced in April that a potential food source was found in those vents, in the form of molecular hydrogen, which microbes on Earth use.
Life beyond Earth — on a moon thought only decades earlier to be desolate — became a real possibility.
Then there was Titan. While Voyager 1 imaged the moon, it couldn't penetrate its smoggy atmosphere. But new instruments on Cassini, as well as a European Space Agency lander named Huygens, could.
'Cassini, in some ways, represents the best of humanity.' - Ray Jayawardhana, York University
Huygens detached from Cassini in January 2005, descending to Titan's surface. It provided us with the first images of the moon. And scientists were floored by what they found.
"The most surprising thing to me is how Earth-like Titan appears, although it's clearly an alien chemistry and an alien environment," Soderblom said.
Titan had lakes and weather, just like Earth. However, instead of water, it was lakes and rivers of methane and hydrocarbons.
"As we descended … lo and behold, it looked like we were landing on the Earth. We saw drainages, river valleys cutting through mountains, what looked like coastlines," Soderblom said. And while the plains were dry, it looked like something had flowed and scoured the surface. "Sure enough, liquid methane … is basically the water equivalent of Earth's hydrosphere."
And that opens up the possibility that an entirely different type of life could exist on its surface.
Life outside our solar system
Cassini turned planetary science on its head.
"From a broad perspective, it's really drawn our attention to ocean worlds in our solar system and beyond, as potential habitats for life," Ray Jayawardhana, dean of science and a professor of physics and astronomy at York University in Toronto, told CBC News.
And that has wider repercussions.
"Now we realize when we go to the next solar systems around exoplanets, around stars … we're not going to be surprised at the diversity, variety and activity we're going to find when we get there one day far in the future," Soderblom said.
But Cassini was more than just a mission to explore new worlds.
"Cassini, in some ways, represents the best of humanity. It's really a testament to our endless curiosity, our collective passion to continue exploring the world and the solar system we live in," Jayawardhana said.
To Soderblom, a seasoned spacecraft mission specialist, the end of a mission and the death of a spacecraft isn't anything new. Still, Cassini has had a special place in his heart. While he's sad to see Cassini go, he's happy with the results.
"It's going to be a shock when we incinerate our own creation," Soderblom said. "But the other thing is the people, the data and the excitement and the knowledge that's important. And that goes on."