After a 20 year mission, NASA’s spacecraft Cassini will meet its demise this week by plunging into Saturn’s atmosphere and burning up.
With propellant running low, NASA scientists are concerned that the probe might accidentally crash into one of Saturn’s nearby moons, which could contaminate it with Earthling bacteria stuck to the spacecraft. Instead, the spacecraft will be safely “disposed” in Saturn’s atmosphere.
Dr Nick Rattenbury, University of Auckland, comments:
“The Cassini/Huygens mission generated extraordinary information about Saturn and some of its remarkable moons. Cassini sent back exquisite images of Saturn, its swirling chaotic atmosphere and the beautifully complex ring system. Those images helped us understand that world and the forces that govern its extensive atmosphere and its orbiting disk of ring material. Apart from the planet itself, the mission made sensational discoveries on the moons of Titan and Enceladus.
“We’ve long known that Titan has an extensive atmosphere, thick and opaque, hiding that moon’s surface from us here on Earth. The Cassini mission revealed surface lakes and rivers on Titan’s surface, the first clear evidence of a liquid present on the surface of a world other than our own Earth. The Huygens probe descended into the thick atmosphere of Titan, recording and transmitting data as it descended and even after landing on the surface. Our new knowledge of these alien surface lakes and Titan’s thick atmosphere adds vital fuel to the debate on how and where life could catch a hold on alien worlds.
“Enceladus, another moon of Saturn, has plumes of material jetting out into space. The Cassini mission captured high-resolution images of these remarkable geyser-like jets, which contain water. To those of us interested in finding alien worlds on which life could emerge, water is considered as — almost certainly — the most likely medium in which life could emerge. However, Cassini wasn’t finished there, collecting evidence that in addition to water, there was a source of energy — methane — that could be used by life.
“Cassini can rightly take its place among the pantheon of other spacecraft explorers and after its final fiery demise, leave a lasting legacy of wonder and discovery.”
Professor Richard Easther, University of Auckland, (excerpted from this blog post):
“This week on September 15, the Cassini spacecraft will meet an emphatic demise. Launched 20 years ago, Cassini arrived at Saturn in 2004 and took up orbit around the giant planet, making hundreds of loops through its retinue of moons and skimming the iconic rings. It’s about to run out of the propellant needed for manoeuvring; theoretically, it might circle Saturn forever, but we could no longer steer it.
“Cassini’s spectacular finale is, in part, a tribute to its success. All spacecraft carry stowaways: bacterial spores which can, remarkably, remain viable in the vacuum of space. And one of Cassini’s many discoveries was that three of Saturn’s moons appear to have oceans of water beneath their solid crusts. Leaving the spacecraft to wander unguided around the Saturnian system would risk a collision with one of these moons. So, rather than let its microbial hitchhikers disembark onto a pristine world, Cassini will not ‘go gentle into the good night’ as its propellant runs low, but has been steered towards a fiery demise.
“NASA has an undoubted ability to sell a story, but the hype is not misplaced: Saturn has a key place in the evolving human understanding of the cosmos. The most distant planet easily visible to the naked eye, Saturn once marked the apparent edge of our solar system. Its rings are visible through even the smallest of telescopes, and seeing them this way still takes your breath away. Cassini has shown us Saturn with its rings and its moons up close and personal, with astonishing clarity and precision. The spacecraft and the team of scientists responsible for it have written themselves into the history books.”
The Australian Science Media Centre gathered expert commentary, including videos, on the Cassini mission and ran an online briefing, please feel free to use these comments in your reporting.
Charley Lineweaver is an Associate Professor in the Research School of Earth Sciences, at the Australian National University College of Science
“The single most impressive highlight of Cassini’s 20 year odyssey was the landing of the Huygens probe on the surface of Titan, 12 years ago.
“It showed us a moon with an atmosphere 50 per cent thicker than Earth’s atmosphere, above a landscape sculpted by methane rain, methane rivers and methane lakes — a methanological cycle at -180°C, comparable to the hydrological cycle at +15°C of our home planet. Can life be based on liquid methane rather than water?
“Your imminent death will not be in vain.
“You will burn up and become part of the planet that you have taught us so much about.”
Professor Trevor Ireland is from the Research School of Earth Sciences at The Australian National University
“Saturn’s rings have been a source of fascination for over 400 years. Galileo first saw them but then a couple years later they had disappeared going into an edge-on configuration. Maxwell the mathematician noted that the rings could not be solid and must be an assemblage of objects orbiting Saturn. A likely source of this material is a collision between moons or disruption by Saturn itself.
“Cassini has shown that the rings are incredibly thin, less than a kilometre thick, and shepherded by a number of small moons. They are a mixture of mud and ice. A key question to be established by Cassini in its final orbits is how massive are the rings, and how old are the rings?
“In passing inside the rings, Cassini can determine the gravitational perturbation associated with the mass of the rings. The take on it is that the higher the mass in the rings, the older the rings are likely to be. Are they as old as Saturn itself? Highly unlikely.
“The shepherding moons are pushing and pulling the material in the rings, causing friction and likely evaporation. The rings must be relatively young, but that still means they could be hundreds of millions of years old. Cassini has provided a wealth of information on the Saturnian system. From Enceladus to Titan it has been absolutely thrilling. Farewell Cassini.”
Dr Alice Gorman Senior Lecturer in the College of the Arts, Humanities and Social Sciences at Flinders University and an internationally recognised leader in the emerging field of space archaeology
“There’s a long history of spacecraft doing science while in their death throes.
“Galileo, the only spacecraft to orbit Jupiter, was sent to its death in Jupiter’s atmosphere in 2003 to avoid contaminating the moons with bacteria. It transmitted data right up until the last minute, when the heat and pressure created by plunging into the atmosphere at 48 km/s tore it apart.
“In 2012, NASA deliberately crashed the Ebb and Flow spacecraft into a mountain at the Moon’s north pole to make sure they didn’t accidentally land on lunar heritage, destroying sites like Apollo 11 where humans first set foot on the Moon. Their demise was incorporated into an experiment to calculate fuel needs for future lunar missions.
“ESA’s Rosetta spacecraft was crashed into Comet 67P Churyumov-Gerasimenko in 2016, measuring gas and dust, and sending back images until it was about 20 m above the surface.
“Without Cassini, we’ll have no eyes or senses close to Saturn any more. We’ve become used to seeing the close-up images of one of the solar system’s most beautiful features. Cassini might be a distant robot, but for many people, it’s like saying goodbye to an old friend after seven years of captivating interplanetary postcards.”
Fred Watson AM is an astronomer at the Australian Astronomical Observatory
“If there was ever a space project that totally exceeded the expectations of the scientific world, it has been the Cassini mission to Saturn.
“Launched in 1997, Cassini reached its target after a seven-year journey, and immediately began re-writing the textbooks.
“Now, 13 years later, we have a remarkable catalogue of amazing discoveries. Storms and a hexagonal jet-stream in the planet’s atmosphere, ripples and spokes in its rings, global oceans and oily seas on its moons. And they are just the start – the best may be yet to come.
“With the spacecraft’s fuel running out, mission controllers have thrown caution to the winds in an audacious series of orbits that have threaded Cassini between the rings and the planet before the final plunge into Saturn’s atmosphere on 15 September.
“The last photos and data sets are likely to be spectacular.”
Dr Brad Tucker is a Research Fellow and Outreach Manager at Mt. Stromlo Observatory at the Australian National University
“Cassini has been a part of our lives for over two decades. Cassini has changed our understanding of Saturn and its moons – from the composition of the rings to the potential for life that exists on Enceladus and Titan. It is only fitting that because of what Cassini has shown, that we do not want to contaminate potential habitable system with Earth-born material.
“At the same time, we will be able to see as deep as possible into Saturn. We live in a time where we must carefully think of what we do, and be responsible stewards of our Solar System. It is an exciting time to understand our Solar System and Cassini has set us up for future projects in the quest for knowledge and life.”
Dr Alan Duffy is a Research Fellow at Swinburne University of Technology in Melbourne
“As long as I’ve been an astronomer Cassini has been orbiting Saturn sending back breathtaking images. I’ll shed a tear at the end of this spacecraft as it has been one of the greatest scientific explorers of all time. Cassini revealed Saturn to be beautiful but also full of surprises, with an Earth-sized six sided storm at the Pole still shocking to me.
“The greatest surprise Cassini uncovered was in the moons rather than the planet itself. The giant Titan with thick orange smog hiding its surface from view revealed rain, rivers and seas but of liquid methane not water. The frozen moon Enceladus has salt water geysers which erupt, sending all the ingredients life needs into space from vast oceans beneath the ice-crust.
“Cassini’s greatest discovery, of a world more capable of sustaining life than almost any other besides Earth, means that we have to destroy it at the end of its mission than risk it contaminating Enceladus with stowaway Earth-born bacteria.
“One last great effort for science at the end of a glorious mission.”