Goodbye Cassini

It’s been 19 years and 8 months since the launch of the two-part unmanned robotic spacecraft Cassini-Huygens that was sent to know more about Saturn, the sixth planet in our solar system. Cassini is the fourth space probe to Saturn, but the first-ever spacecraft to achieve orbit of the planet. Cassini, which has served as an encyclopaedia on Saturn, is all set to be plunged into Saturn on 15^th September 2017 as its mission draws to an end.

As the clock ticks down, let’s go back and retrace the long path Cassini has travelled and the milestones it has achieved in all these years.

Cassini: From an Astronomer to a Spacecraft:

A team of scientists from 16 European countries and the United States put their heart and soul into the mission and worked on the development of the spacecraft right from the 1980s. The design of the spacecraft included a Saturn orbiter and a lander for one of the Saturn’s moons, Titan. The mission was commonly called Saturn Orbiter Titan Probe (SOTP) in the initial stages. Components from different countries were assembled and managed by NASA’s Jet Propulsion Laboratory in the United States, with costs touching about US$3.26 billion for the mission.

ASI/NASA Cassini orbiter was named after the Italian-French astronomer Giovanni Domenico Cassini who discovered Saturn’s ring divisions and four of its satellites; and the ESA-developed Huygens was named after the Dutch astronomer, mathematician and physicist Christiaan Huygens who discovered Titan.

Cassini’s Journey to Saturn

Cassini started off with its launch on October 15, 1997 from the Cape Canaveral Air Force Station’s Space Launch Complex 40, using a U.S. Air Force Titan IVB/Centaur rocket. And what was this launcher made of? Well, a two-stage Titan IV booster rocket, two strap-on solid rocket motors, the Centaur upper stage and a payload enclosure or fairing. The two-part spacecraft was designed in such a way that when the spacecraft reaches Titan, it would detach itself and fly down towards the surface of Titan with the help of a parachute. Some truly magnificent engineering at work here!

Related: Is it really raining diamonds on Saturn & Uranus?

A technique called a ‘gravity assist’ (or ‘fly-bys’) was exploited to hurl this spacecraft to Saturn, as no launcher is capable of sending a 5600 kilograms spacecraft directly to Saturn. A gravity assist involves flying close to planets (or any other significantly massive celestial body). The planet pulls the spacecraft as it flies past the planet, but the spacecraft also pulls the planet. Thus, paving a way for an exchange of energy. Cassini made two fly-bys of Venus – the first on 26^th April 1998, and again on 24^th June 1999. This was followed by one of Earth on 18^th August of the same year. These three fly-bys provided enough orbital momentum for the spacecraft to travel to the outer Solar System. And Jupiter acted as the final energy booster for Cassini on 30^th December 2000 to project it to Saturn.

The spacecraft arrived at Saturn in July 2004. About 6 months after orbital insertion, Huygens was released from Cassini on 25^th December 2004. Huygens then landed on the mysterious surface of Titan on 14^th January 2005 – the first time ever we’d landed a craft on a moon of another planet!

Limelight on its Findings

From the moment Cassini reached Saturn, it’s enjoyed almost celebrity status, sending us valuable information that has helped scientists understand Saturn in fascinating detail. It takes 68 to 84 minutes for radio signals to travel from Earth to the spacecraft, and vice versa, providing no chance for ground controllers to give real-time instructions. If any unexpected event were to occur, and even if the transmission was immediate, it would still take two hours before the satellite instructions arrived at NASA.

The very first high-resolution image from Cassini caused quite a stir. It left scientists puzzled – there were no visible spokes in Saturn’s rings.  The next set of images showed clouds moving at high velocities around the planet – clearly visible in the equatorial and southern regions. Long-term observations of these cloud dynamics cast light on storms in Saturn’s atmosphere.

Two of Saturn’s moons previously discovered by Voyager 1 were again seen in the pictures taken by Cassini.  Prometheus and Pandora were no ordinary moons and they were called the shepherd rings due the gravitational effects they have on the “F” ring.

During Cassini’s first fly-by of Titan, Cassini passed over the south pole of the moon.  The pictures taken by the spacecraft confirmed that the darker and brighter albedo features on the surface do represent different materials. But the icy regions seem to be darker than the areas where other (possibly organic) matter is mixed in with the ice which was quite opposite to the expectation. Two new moons “Methone” (S/2004 S 1) and “Pallene” (S/2004 S 2) were discovered and they orbit between Mimas and Enceladus.

Perhaps most interesting findings by Cassini were the observations of volcanoes on Enceladus, or active venting. What’s more interesting is the fact that these vents create the “E” ring, so we have a ring created by material vented off Enceladus. This is very unusual as there are not many active objects in the Solar System.

Also Read: Can We Colonize Titan?

A closer study of Titan surface’s proved to be fascinating. It has open oceans or seas of hydrocarbons, with the additional possibility of an open ocean underneath the crust, just like that of the sixth moon of Jupiter, Europa. Another image, which seems to capture volcanic eruptions, has opened the door for new scenarios as in the outer solar system – such as – ice is a rock.

Except Io, all the moons in the outer Solar System have icy crusts. Now, if there is any possible volcanic eruption on Titan, there has to be an eruption of magma, and when ice is a rock, that eruption is water! So, we have cauldrons of life-giving water spewing out into space!

Mission Extension

In May 2008, Cassini completed its primary mission, surpassing our most optimistic expectations. Not surprisingly, it was given an extension, with the mission renamed to the Cassini Equinox Mission, to observe Saturn during its equinox crossing.

In 2010, NASA announced another extension of the mission, calling it the Cassini Solstice mission until May 2017, a few months past Saturn’s summer solstice. This schedule includes 155 orbits, with 54 fly-bys of Titan, 11 of Enceladus, 2 of Rhea and 3 of Dione. In one of its Titan fly-bys, Cassini will dip below the moon’s ionosphere – another first.

Related: Wanderers – Humanity’s Future in Space

Cassini’s been a rockstar spacecraft, setting record after record in space exploration, achieving dozens of firsts to its name. Almost all of our data on Saturn can be traced back to Cassini, which has been delivering for almost 2 decades. When its second extension ends in May 2017, the spacecraft will plunge into Saturn’s atmosphere, in order to prevent contaminating Titan and other worlds with Earthly debris that may endanger the potential life they harbour.

Cassini is not just a spacecraft, but also a proud example of how far we’ve come as a species exploring the universe. It is the culmination of decades of manhours, and a marvel of engineering brilliance. Here’s to the orbiter that  but let’s make it memorable and salute this spacecraft for the insights it provided all these years.