You’d think by now we’d know exactly how long a day is on Saturn, but Cassini’s still working on it. On a terrestrial planet, you can time when a landmark cycles back around, but Saturn’s covered in clouds, so pinning down the day has been tricky. That’s why Cassini has been looking closely at Saturn’s magnetic field. Any tilt to the magnetic field would make the daily wobble of the planet’s deep interior observable, thus revealing the true length of day, but the probe discovered that Saturn’s field has almost zero tilt, so its day length remains elusive. When the Voyager 2 spacecraft flew by in 1981 it measured 10 hours 39 minutes; Cassini gave us 10 hours 47 minutes.
This surprising observation is just one of several early insights from the final phase of Cassini’s mission, known as the Grand Finale. Cassini is now in the 20th of 22 weekly orbits that pass through the narrow gap between Saturn and its rings. The spacecraft began its finale on April 26 and will continue its dives until Sept. 15, when it will make a mission-ending plunge into Saturn’s atmosphere.
“Cassini is performing beautifully in the final leg of its long journey,” said Cassini Project Manager Earl Maize at NASA’s Jet Propulsion Laboratory, Pasadena, California. “Its observations continue to surprise and delight as we squeeze out every last bit of science that we can get.”
In addition to its investigation of the planet’s interior, Cassini has now obtained the first-ever samples of the planet’s atmosphere and main rings. The spacecraft’s cosmic dust analyzer (CDA) instrument has collected many exceedingly small-size ring particles while flying through the planet-ring gap, while its ion and neutral mass spectrometer (INMS) has sniffed the outermost atmosphere, called the exosphere.
During the spacecraft’s final five orbits, as well as it final plunge, the INMS instrument will obtain samples deeper down in the atmosphere. Cassini will skim through the outer atmosphere during these passes to tell us what gases are there.
Cassini’s imaging cameras have been hard at work, returning some of the highest-resolution views of the rings and planet they have ever obtained including fresh, close-up views of Saturn’s C ring. This innermost of the three bright rings visible in small telescopes features mysterious bright bands called plateaus and surprisingly different textures in neighboring sections of the ring.
The plateaus appear to have a streaky texture, whereas adjacent regions appear clumpy or have no obvious structure at all. Ring scientists believe the new level of detail may shed light on why the plateaus are there, and what is different about the particles in them. On Aug. 17, Cassini examined the middle strands of the C-ring which seems to contain materials other than ice.
More recently, the Neptune happened to be in the probe’s field of view, so the mission team took the opportunity to snap a photo of the planet and its largest moon, Triton.
Launched in 1997, Cassini has orbited Saturn since arriving in 2004 for an up-close study of the planet, its rings and moons, and its vast magnetosphere. Cassini has made numerous dramatic discoveries, including a global ocean with indications of hydrothermal activity within the moon Enceladus, and liquid methane seas on another moon, Titan.
Cassini’s mission is ending because the craft is running out of fuel. Rather than let it continue orbiting the Saturn system and accidentally striking and contaminating Titan or Enceladus, each of which could potentially harbor life, the probe will be destroyed as it plunges into Saturn’s atmosphere.
“We cannot risk an inadvertent contact with that pristine body (Enceladus),” said Earl Maize, an engineer at NASA’s Jet Propulsion Laboratory (JPL) who manages the Cassini mission.
The probe’s final orbit will take it into Saturn’s atmosphere on Sept. 15 at 5:44 a.m. CDT (10:44 a.m. UT) traveling between 75,000 and 78,000 miles per hour (121,000- 126,000 kph). Shortly after, Cassini will break apart and burn up in Saturn’s atmosphere, appearing like a brilliant fireball across Saturnian skies.