Cassini’s Last Picture Show

This September, one of NASA’s greatest success stories will come to an end.  After 13 years orbiting Saturn and sending a probe to make the most distant landing in history, the Cassini mission will end with a controlled descent into Saturn.  Thirty-five years in the making, Cassini was hatched during one of the darkest hours in NASA’s planetary program.  The mission will stand as a centerpiece of a golden era of planetary exploration that has included a Jupiter & Venus orbiter, asteroid missions, and several Mars orbiters and rovers.

In 1981, the Reagan administration proposed shutting down NASA’s planetary program.  For awhile, it appeared Voyager 2 would be terminated before its flybys of Uranus and Neptune.  The nation was in its worst economic crisis, up to that point, since the Great Depression as unemployment soared past 10%.  Scientists expressed deep alarm over planned spending cuts across the board.  It was in this environment a joint working group between the National Academy of Sciences and the European Science Foundation proposed the Saturn orbiter/Titan probe mission.  The intention was to follow-up the recent Voyagers 1 & 2 flybys of Saturn with a more in-depth research program.

By the mid ’80’s, the economy began to recover, and thanks to aggressive lobbying efforts by NASA, the planetary program (including Voyager 2) survived.  In 1989, Congress approved funding for the mission to go ahead.  During the early 90’s, on the heels of another economic recession, the Bush and Clinton administrations mandated NASA to cut the cost of the mission.  Many of the improvements made on Cassini during this phase were also implemented on NASA’s subsequent “cheaper, faster, better” planetary missions.  Finally, in 1997, Cassini-Huygens was ready to launch and begin its seven-year journey to Saturn.

NASA named the orbiter after Giovanni Domenico Cassini, an Italian astronomer who became director of the Paris Observatory in 1671.  Among his many discoveries was the division of Saturn’s rings.  The gap would separate what would be called the A Ring and B Ring.  Since then, several gaps have been discovered with major divisions designated as rings C through G.

A 1676 sketch by Cassini showing a gap in Saturn’s rings. Credit: Royal Society.

The Huygens lander, intended to land on Saturn’s moon Titan, was named after Christiaan Huygens.  In 1655, Huygens discovered Titan which earned him the honor over three centuries years later.

Cassini (right) and Huygens (left) at JPL in August, 1997, two months before launch. Credit: Gregory Pijanowski

Cassini was launched on October 15, 1997.  Its trajectory towards Saturn was not a straight shot but looped around the inner Solar System to complete two flybys of Venus, one of Earth, then finally, one of Jupiter in 2000 to put it on course towards Saturn.

Cassini trajectory to Saturn. Credit: JPL/NASA

The flyby maneuver uses a planet’s gravity to slingshot the probe to the required velocity to reach its target.  Not unlike Marty McFly in Back to the Future on his skateboard hitching on to a car to increase his speed.  This reduces the amount of fuel needed at launch which reduces weight.  In space exploration, weight means cost – about $10,000 a pound to lift payload from Earth.

Image of Jupiter taken by Cassini during flyby on December 29, 2000. Credit: NASA/JPL/Space Science Institute.

Cassini finally reached Saturn on June 30, 2004, twenty-two years after the original proposal.  On Christmas Day, Huygens departed from Cassini and landed on Titan on January 14, 2005.  Titan, the second largest moon in the Solar System, was a mystery as its hazy atmosphere shrouded the surface.  Besides Earth, Titan is the only body in the Solar System to have a nitrogen rich atmosphere.

Cloud shrouded Titan. Credit: JPL/NASA

What did we find out about Titan?  Methane plays the same role on the surface that water does on Earth.  Methane melts at -295.6 F, which is close to the surface temperature recorded by Huygens.  Before the landing, astronomers speculated Titan had large methane oceans.  In fact, Huygens was designed to float if that was the case.  Huygens descended on land, but the orbiting Cassini has detected methane lakes near the poles.

Methane lakes by polar regions of Titan. Credit: JPL/NASA.

While there is not an ocean on the surface of Titan, Huygens was able to detect a vast underground ocean via radio waves some 35 to 50 miles beneath the surface.

Surface of Titan. Credit: ESA/NASA/JPL/University of Arizona; processed by Andrey Pivovarov

And Cassini itself?  The discoveries made over the past 13 years are far too many to detail in a blog post, but here are a few of the highlights.

Enceladus

The Saturn moon Enceladus. Its plumes emanate from the tiger stripe formations. Photo: Cassini Imaging Team, SSI, JPL, ESA, NASA

The sixth largest moon of Saturn, its icy surface reflects almost 100% of light making it very bright.  In 2005, Cassini discovered icy plumes emanating in the Southern Hemisphere ejecting material thousands of miles into space.  Not unlike Old Faithful, but more powerful and a not understood heat source.  In 2015, Cassini flew through the plumes and detected, besides water vapor and ice, hydrocarbons such as methane and formaldehyde.  Cassini has also verified the presence of an ocean 26 to 31 km (16 to 19 miles) in depth.  To compare, the deepest point in the Pacific is 11 km.

Icy plumes of Enceladus. Photo: NASA/JPL/Space Science Institute

The Rings

Cassini has provided an in-depth look at perhaps the most famous feature in astronomy.  Among the many firsts was a view of the vertical structure of the rings.  In the image below, structures arise from the B ring to cast shadows much as buildings would do on Earth.  The structures causing these shadows are about two miles high.

Credit: NASA/JPL/Space Science Institute

Saturn’s Poles

Cassini gave us a look at the poles that earlier flyby missions could not.  When Cassini first arrived at Saturn, it was summer in the Southern Hemisphere so the South Pole was in daylight while the North Pole was dark.   What was found at the South Pole was a hurricane type storm two-thirds the size of Earth.  The dynamics of this storm are not completely understood as it is locked at the pole and there is no ocean to feed it energy.

Eye of the Saturn South Pole storm at different wavelengths. Credit: NASA/JPL/Space Science Institute/University of Arizona.

By 2009, it was spring in the Northern Hemisphere (A Saturn year equals 29 Earth years) and finally, the North Pole was in daylight.  Cassini discovered a hexagonal shaped jet stream formation.  Inside the hexagon is a hurricane type storm.  Just like the South Pole, this feature is not completely understood.

Saturn North Pole. Credit: JPL/NASA

Now in its 20th year in space, like an old car, Cassini is near the end of its useful life.  As its fuel supply gets lower and lower, there is the possibility that Cassini could be lost to ground control.  As the mission has discovered environments on Titan and Enceladus that could sustain microbial life, NASA wants to avoid the possibility of a crash on those two moons contaminating them with Earth microbes from Cassini.  Cassini will be in a ring-grazing orbital mode in the final phase of its mission until the end.  As Cassini is maneuvered closer and closer to the rings, it will give us with a look at the rings its namesake could only dream about.  Cassini will crash into Saturn on September 15, 2017.

Image of Saturn’s rings as Cassini enters ring grazing mode. Credit: NASA/JPL-Caltech/Space Science Institute.

The total cost of the Cassini mission has been $3.26 Billion or $163 million per year in space.  That’s a tad more than the $158 million the Buffalo Bills spent on their player payroll in 2016.

For a project conceived during a time when NASA’s planetary program was in danger of being terminated, Cassini has left a remarkable legacy 35 years in the making and spanning six presidential administrations.  Perhaps the most important lesson Cassini provides is no matter how dark times seem, keep pushing for your dream.

*Image atop post taken April 25, 2016 as Saturn’s Northern Hemisphere approaches summer exposing entire polar region to sunlight.  Credit:  NASA/JPL-Caltech/Space Science Institute.

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