After 20 years in space, a vintage probe called Cassini is entering its last waltz with Saturn with a series of rhythmical manoeuvres to explore one of the most dazzling planetary features in the cosmos - Saturn’s vast icy rings.
The NASA spacecraft has been swooping back and forth at 123,000 kilometres an hour through the narrow gap between Saturn’s rings and the planet’s cloudy surface before making a suicide plunge into Saturn’s atmosphere on September 15.
Mission managers programmed Cassini’s destruction to avoid contaminating any of Saturn’s moons that might be hospitable to life.
In its last days, the ageing spacecraft is bringing Saturn’s signature rings into sharp focus, detail that could help settle debates over their age, mass and origin.
“The ring scientists are having a field day,” said NASA deputy project scientist Scott Edgington. “When the navigators said we could come so close that we could dive through this gap, you can imagine how the scientists’ eyes lit up.”
It is the grand finale of a $3.27 billion mission that has involved 5,000 scientists in 17 countries. And it signals the end of an era of interplanetary exploration in which big multipurpose probes cruised the solar system as humanity’s eyes and ears, such as the Magellan mission to Mercury and Venus, the Galileo probe to Jupiter, and the 40-year-old Voyager spacecraft now entering interstellar space.
The Cassini probe is one of the heaviest and most complex interplanetary spacecraft ever built. Its components included the European Space Agency’s Huygens probe, which landed on one of Saturn’s moons in 2005. It has a dozen sets of sensors.
“Today there isn’t a US launch vehicle that could lift Cassini,” said Julie Webster, a spacecraft operations team manager at the Jet Propulsion Laboratory in Pasadena, California.
Launched in 1997 on a Titan IVB/Centaur rocket, Cassini took seven years to reach Saturn. It has spent the past 13 years exploring the planet, revealing secrets that bedevilled astronomers for centuries.
Among its findings, the probe found evidence that three major moons - Titan, Europa and Enceladus - might support the chemistry of life. It spotted seven previously unknown moons.
And in thousands of close-up images, the probe is detailing how Saturn’s rings, studded with thousands of tiny moonsets coalescing out of grit, likely mirror the dynamics of the giant, rotating cloud of gas and dust from which the sun and all the planets in the solar system formed 4.6 billion years ago.
“We have actually been able to see the rings evolving and changing,” said astrophysicist Robin Canup at the Southwest Research Institute in Boulder, Colo., who studies the origins of planets.
All of the large planets in the solar system have rings, but none as extensive or puzzling as those encircling Saturn.
As documented by Cassini’s sensors, the eight main bands of rings that surround Saturn are as flat as the broad brim of a stylish hat. They are composed mostly of unusually pure ice particles, with sprinkles of pink dust and rocks that range in size from a marble to a house.
Overall the rings extend about 280,000 kilometres out from Saturn, but for all their breadth, they are on average only 30 feet thick. As Cassini discovered, the rings have kinks, spokes and ripples. They wobble.
“They flap up and down,” said planetary scientist Jeff Cuzzi at the NASA Ames Research Centre in Moffett Field, Calif., who uses Cassini data to study the rings. “Imagine a flag flapping in the breeze.”
Using Cassini data, scientists discovered that the rings trap debris coming from the outer limits of the solar system. Some of it is so large that the rings vibrate from the impact for decades, like a bell struck by a cosmic clapper. Scientists identified one set of ring ripples that appeared to date from a major impact in 1983.
“It left an imprint in the rings that we can still see today,” said Matthew Hedman, a University of Idaho physicist who studies planetary rings. “These structures are echoes of events.”
Two of the most fundamental facts about Saturn’s rings - their mass and age - are a mystery that Cassini may help solve in its final hours.
“There are people who argue that the rings should be as old as the solar system,” said Dr. Hedman. “There are others who argue that they can’t be any older than the dinosaurs.”
Researchers hope that gravity measurements during Cassini’s final orbits will pin down the true mass of the rings, which in turn would clarify their age. The more massive the rings, the older they likely are.
By NASA’s calculation, Cassini has travelled more than 5 billion miles, and it is showing signs of its own age. Its bearings are almost out of lubricant; its reaction controls are wearing out; and the power supply for one sensor has failed.
“The spacecraft is an amazing machine and it’s operating almost flawlessly,” said Cassini program manager Earl Maize. Even so, “the warning light is on,” he said. “We are essentially out of gas.”
The NASA spacecraft has been swooping back and forth at 123,000 kilometres an hour through the narrow gap between Saturn’s rings and the planet’s cloudy surface before making a suicide plunge into Saturn’s atmosphere on September 15.
Mission managers programmed Cassini’s destruction to avoid contaminating any of Saturn’s moons that might be hospitable to life.
In its last days, the ageing spacecraft is bringing Saturn’s signature rings into sharp focus, detail that could help settle debates over their age, mass and origin.
“The ring scientists are having a field day,” said NASA deputy project scientist Scott Edgington. “When the navigators said we could come so close that we could dive through this gap, you can imagine how the scientists’ eyes lit up.”
It is the grand finale of a $3.27 billion mission that has involved 5,000 scientists in 17 countries. And it signals the end of an era of interplanetary exploration in which big multipurpose probes cruised the solar system as humanity’s eyes and ears, such as the Magellan mission to Mercury and Venus, the Galileo probe to Jupiter, and the 40-year-old Voyager spacecraft now entering interstellar space.
The Cassini probe is one of the heaviest and most complex interplanetary spacecraft ever built. Its components included the European Space Agency’s Huygens probe, which landed on one of Saturn’s moons in 2005. It has a dozen sets of sensors.
“Today there isn’t a US launch vehicle that could lift Cassini,” said Julie Webster, a spacecraft operations team manager at the Jet Propulsion Laboratory in Pasadena, California.
Launched in 1997 on a Titan IVB/Centaur rocket, Cassini took seven years to reach Saturn. It has spent the past 13 years exploring the planet, revealing secrets that bedevilled astronomers for centuries.
Among its findings, the probe found evidence that three major moons - Titan, Europa and Enceladus - might support the chemistry of life. It spotted seven previously unknown moons.
And in thousands of close-up images, the probe is detailing how Saturn’s rings, studded with thousands of tiny moonsets coalescing out of grit, likely mirror the dynamics of the giant, rotating cloud of gas and dust from which the sun and all the planets in the solar system formed 4.6 billion years ago.
“We have actually been able to see the rings evolving and changing,” said astrophysicist Robin Canup at the Southwest Research Institute in Boulder, Colo., who studies the origins of planets.
All of the large planets in the solar system have rings, but none as extensive or puzzling as those encircling Saturn.
As documented by Cassini’s sensors, the eight main bands of rings that surround Saturn are as flat as the broad brim of a stylish hat. They are composed mostly of unusually pure ice particles, with sprinkles of pink dust and rocks that range in size from a marble to a house.
Overall the rings extend about 280,000 kilometres out from Saturn, but for all their breadth, they are on average only 30 feet thick. As Cassini discovered, the rings have kinks, spokes and ripples. They wobble.
“They flap up and down,” said planetary scientist Jeff Cuzzi at the NASA Ames Research Centre in Moffett Field, Calif., who uses Cassini data to study the rings. “Imagine a flag flapping in the breeze.”
Using Cassini data, scientists discovered that the rings trap debris coming from the outer limits of the solar system. Some of it is so large that the rings vibrate from the impact for decades, like a bell struck by a cosmic clapper. Scientists identified one set of ring ripples that appeared to date from a major impact in 1983.
“It left an imprint in the rings that we can still see today,” said Matthew Hedman, a University of Idaho physicist who studies planetary rings. “These structures are echoes of events.”
Two of the most fundamental facts about Saturn’s rings - their mass and age - are a mystery that Cassini may help solve in its final hours.
“There are people who argue that the rings should be as old as the solar system,” said Dr. Hedman. “There are others who argue that they can’t be any older than the dinosaurs.”
Researchers hope that gravity measurements during Cassini’s final orbits will pin down the true mass of the rings, which in turn would clarify their age. The more massive the rings, the older they likely are.
By NASA’s calculation, Cassini has travelled more than 5 billion miles, and it is showing signs of its own age. Its bearings are almost out of lubricant; its reaction controls are wearing out; and the power supply for one sensor has failed.
“The spacecraft is an amazing machine and it’s operating almost flawlessly,” said Cassini program manager Earl Maize. Even so, “the warning light is on,” he said. “We are essentially out of gas.”