Editorials

Farewell, Cassini

The Cassini-Huygens space probe ended its 20-year mission last Friday with a fiery dive into the atmosphere of Saturn, ensuring its destruction after 13 years studying the massive planet, its rings and its moons. Data collected from the mission have changed the way scientists now think about Saturn and our part of space: There is open speculation about the possibility of life within our solar system. Space scientists are already thinking about ways to follow up on Cassini’s incredible discoveries. There is no guarantee that there will be a next mission to Saturn, however. If ever there was a reason for multinational collaboration and cooperation, space exploration is it.

Cassini, a collaboration between the U.S., European and Italian space agencies, was launched Oct. 15, 1997. It took nearly seven years for the probe to travel the 1.4 billion km to Saturn, where it settled into a planetary orbit.

A year after its arrival, on Dec. 25, 2004, the Huygens module separated from Cassini and descended to Titan, the largest of Saturn’s moons, where it successfully landed about three weeks later. Huygens was the first spacecraft to successfully land in the outer solar system and the first landing on a moon that is not our own. The probe sent data for nearly two hours before shutting down. While data collection and transmission was short, the content was astounding. The surface of Titan is pocked with valleys, rivers, lakes, and seas that distribute liquid methane across the moon. In one of the most striking discoveries, it appears as though there is an ocean beneath Titan’s crust. The probe confirmed the composition of the hydrocarbon lakes and discovered indications of other organic molecules.

Scientists have been astounded by similar discoveries on Enceladus, another of Saturn’s moons. On one of its early flybys of the moon, Cassini detected geysers of gas, ice and dust shooting out of cracks near its south pole. The plumes contained carbon-bearing molecules, salts and other chemical indicators of hot water beneath Enceladus’ surface.

With these findings, Cassini transformed our understanding of the solar system. The previous belief that only Earth could sustain life in our planetary neighborhood was wrong. Four moons in our solar system — Enceladus, Titan, Dione (another of Saturn’s moons) and Europa, one of Jupiter’s moons — appear to have liquids on them and thus may be capable of supporting the building blocks of life. Explained Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate, “Cassini’s discovery of ocean worlds at Titan and Enceladus changed everything, shaking our views to the core about surprising places to search for potential life beyond Earth.”

Those are only the most striking findings uncovered during 13 years of orbits and exploration. Cassini discovered six new moons and propeller-shaped gaps in Saturn’s rings that appear to have been made by a new class of objects: “moonlets,” about 1 km in diameter but larger than the material that make up the rings. The data suggests that much of the material for the planet’s E-ring — a diffuse ring outside the main rings — comes from those geysers on Enceladus. And Cassini found features that look like propellers that can be thousands of kilometers long. They appear to have been produced by the gravitational influence of those moonlets.

In all, Cassini took more than 453,000 images during its mission, and scientists will be studying them and additional data for years to come.

Even Cassini’s end came in the service of science. The probe conducted 22 “Grand Finale” dives between the planet and its rings. As its fuel dwindled mission controllers decided to destroy the probe to ensure that it did not crash on any of the moons and contaminate them; although Cassini was decontaminated before it took off, some microbes might have survived. And so Cassini was steered into Saturn’s atmosphere where it would burn up, yet its instruments transmitted data till they were destroyed.

It will take at least a year for that data to be analyzed. In the meantime, space administrators are debating future missions: Do they follow up on Cassini’s discoveries or head off in new directions to discover what else we know about our solar system is wrong? Europa, Jupiter’s moon, is one of the targets of NASA scientists, but Cassini veterans want to return to Enceladus and Titan. Each mission carries an $800 million price tag and while that is not an exorbitant amount given the scale of national budgets, it is a daunting sum nevertheless. Here the case for multinational efforts is plain. Japan should be ready to join in this and other endeavors.