On Sept. 15 at about 10pm, NASA’s Cassini spacecraft is to plunge deep into the hostile atmosphere of Saturn on an historic but suicidal course. It is the grand finale of a 20-year mission which has revolutionized our understanding of the solar system and sent home more than 250,000 stunning images of Saturn and its moons.
Cassini’s instruments will be running to the last, capturing every possible byte of data from its closest encounter with the ringed planet before it ultimately evaporates.
About 1.2 billion kilometers away, in a valley just outside Canberra, Glen Nagle and his colleagues will be listening intently to what he calls the “whispers” from deep space.
Photo: AFP / NASA /JPL-CALTECH
“I’m going to be here for 24 hours and I won’t be sleeping,” he said.
Nagle works at the Canberra Deep Space Communication Complex, also known as Tidbinbilla Tracking Station, home to four antennae which help track and command the many spacecraft in our solar system.
Run by the Commonwealth Scientific and Industrial Research Organisation, Australia’s national science agency, but funded by NASA, Tidbinbilla is one of just three stations in NASA’s Deep Space Network (the others are in California and Madrid) and it is here that Cassini’s final radio signals are to be received and relayed to a global audience.
“We’re going to be responsible for capturing Cassini’s last breath of data,” Nagle said.
“It’ll be a bittersweet moment. NASA can’t do it without us because the other stations are completely facing in the wrong direction. Saturn will be in our skies, our field of view. It’s literally the way the planets have aligned,” he added.
Opened in 1965, Tidbinbilla is a serene station enveloped by national parks. It is a place where the low hum of the moving antennae and the occasional paging announcements are the only sounds that punctuate the silence.
The dishes look surprisingly small from a distance, dwarfed by nature itself, but up close their scale is imposing. The largest is 70m in diameter and 109m across its curvature — “you could throw a football field into it,” Nagle said — and weighs about 4,000 tonnes.
They are almost millimeter-perfect parabolic surfaces.
Each dish acts as both a gigantic ear and a gigantic loudspeaker, telling spacecraft how to behave, ensuring their health and collecting their data. The dishes operate night and day, whether or not the skies are clear to the naked eye.
“At the present time we, Earth, have about 30 missions in the solar system, so about 40 individual spacecraft,” Nagle said. “We communicate with them using radio waves — the invisible part of the electromagnetic spectrum.”
“We just talk to the missions that have headed out across the solar system,” Nagle said.
The furthest of them, Voyager 1, is so far from Earth that it seems a minor miracle its signal can be heard at all.
“Right now Voyager 1 is roughly 20.7 billion kilometers away and moving further away by about 1.4 million kilometers every day,” he said. “That’s about four and a quarter times further away than Pluto. So it’s way out there. It takes over 30 hours to get a signal there and back.”
“To give you some idea of what that signal is like now: Voyager transmits at around 19 watts, about half the power it’s taking to run the light bulb in your fridge. So imagine already trying to see half your fridge light from four and quarter times as far away as Pluto — you’re not going to see it,” Nagle said. “And it gets even smaller because as that signal travels across that 20 billion kilometers of space it spreads out, it becomes thinner and more diffuse.”
Australia’s involvement in space exploration is six decades old and even though Nagle thinks “Australia doesn’t see itself as a space-faring nation” it has played a critical role in some of the most inspiring moments in the history of humankind.
“The dish out the front is the one from Honeysuckle Creek that received and relayed to the whole world the first pictures of Neil Armstrong walking on the moon in 1969,” Nagle said.
In its lifetime Cassini and its accompanying probe, Huygens, have revealed many of the secrets of the Saturnian system: How the particles that make up Saturn’s rings range in size from smaller than a grain of sand to as large as mountains; how Titan, one of the moons, has prebiotic chemistry as well as rain, rivers, lakes and seas; how icy plumes of water are spraying upwards from “tiger stripe” fractures on Enceladus, an otherwise frozen moon.
It has also witnessed giant hurricanes at both of Saturn’s poles and captured the first complete view of the north polar hexagon — not bad for a one megapixel camera. The finale should reveal yet more about the interior of the planet as the craft measures its gravity and magnetic field.
The decision to hurl Cassini into Saturn’s deadly, gaseous atmosphere next month has been made through necessity and responsibility. The craft has run out of fuel and contains a nuclear battery; NASA’s scientists fear it might contaminate one of the surrounding moons should it crash into them.
“We have to dispose of the spacecraft safely,” deputy project scientist Scott Edginton said, who is based in California. “Because Titan and Enceladus have been shown to be places where there are conditions for habitability, conditions that we think are appropriate for life.”
“So our navigators came up with this series of grand finale orbits, flying through the gap between the planet and the rings and eventually ending in Saturn’s atmosphere,” Edginton said.
“When the scientists saw that plan they were like, ‘Wow, this is unexplored territory, we’re going to learn so many new things.’ So starting April this year we entered into the grand finale orbits. It’s hard to believe we’re almost done,” he said.
Future missions to Saturn and its moons may yet reveal some answers, but for Cassini the deadly denouement is imminent.
“Cassini’s going to end its life as a shooting star in the atmosphere of a giant ringed world,” Nagle said. “There’s no more poetic way for a spacecraft to finish what has been a magnificent mission.”
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