Story highlights
- Cassini probe is nearing the end of its 20-year mission to explore Saturn
- Unmanned spacecraft will carry out a series of weekly dives through the planet's icy rings
- In September, Cassini will enter Saturn's atmosphere and burn up, like a meteor
(CNN)NASA's Cassini spacecraft has begun an unprecedented series of space dives that will see it plunge through the icy rings of Saturn, in the final phase of its 20-year mission.
For the first of 22 weekly plummets, the unmanned probe has altered its trajectory, allowing it to maneuver through the 1,500-mile-wide gap between the planet and its rings, at a top speed of more than 76,800 mph (120,000 kph).
Because the region has never been explored before, Cassini's controllers are using the craft's dish-shaped antenna as a shield, to protect it against particles of debris, while it voyages past the rings.
This means it will be out of contact with Earth through the dive, but the Cassini team says it expects to regain a signal from the probe by about 3:00 a.m. ET Thursday.
Unlocking puzzles of Saturn
During Cassini's final revolutions around Saturn, the probe will collect rich scientific data, giving scientists clues to help explain the evolution of giant planets and planetary systems.
Its controllers hope to use the information it collects to create detailed maps of Saturn's gravity and magnetic fields, allowing them to solve the mystery of how fast Saturn rotates.
The closing plunges of the mission could also reveal the make-up and origin of the rings, as well as providing valuable and detailed photographs.
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Fast approaching what NASA hails "the grand finale," Cassini is now set on a terminal collision course which will see it lose contact with Earth before burning up like a meteor on September 15 at 9:45 a.m. GMT (6:45 a.m. ET).
"The spacecraft is now on a ballistic path, so that even if we were to forgo future small course adjustments using thrusters, we would still enter Saturn's atmosphere on September 15, no matter what," said Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory (JPL), in a statement.
Shortly before impact, Cassini will emit a final signal which will be received on Earth just over an hour later.
Why destroy Cassini?
Cassini's mission has twice been extended -- in 2008 and 2010 -- but the probe is now running low on rocket fuel.
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Deliberately allowing the craft to burn up in Saturn's atmosphere cuts the risk of it damaging one of Saturn's moons if scientists lost the ability to control it enough to prevent a collision.
This way, Cassini should not contaminate future scientific work.
Before Cassini's mission, little was known about the planet. Previous missions, including Pioneer 11 and Voyagers 1 and 2, all undertook flybys providing glimpses of Saturn and yielding ground-breaking discoveries.
But it was Cassini's first close-up survey of the planet and its system of rings and moons in 2004 that changed scientists' understanding of the planet and altered our approach to future planetary exploration.
Photos: Photos: Cassini's Saturn discoveries
Cassini's top 10 discoveries about Saturn – To celebrate the 10th anniversary of Cassini's Saturn orbit insertion, NASA has listed the top 10 things we wouldn't know if the spaceship hadn't traveled to the ringed planet. Also: Cassini images of Saturn and its moons.
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Photos: Photos: Cassini's Saturn discoveries
Cassini's top 10 discoveries about Saturn – 10. First complete view of the north polar hexagon and discovery of giant hurricanes at both of Saturn's poles. Saturn's polar regions have surprised scientists with the presence of a long-lived hexagonal-shaped jet stream in the north and hurricane-like storms at both poles. The driving forces of each remain a mystery.
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Photos: Photos: Cassini's Saturn discoveries
Cassini's top 10 discoveries about Saturn – 9. Mystery of the dual bright-dark surface of the moon Iapetus solved. The origin of Iapetus' two-faced surface has been a mystery for more than 300 years. The Cassini spacecraft solved the puzzle, showing that dark, reddish dust in Iapetus' orbital path is swept up and lands on the leading face of the moon.
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Photos: Photos: Cassini's Saturn discoveries
Cassini's top 10 discoveries about Saturn – 8. Study of prebiotic chemistry on Titan. Titan's atmosphere is the most chemically complex in the solar system. Here, bodies of liquid near Titan's north pole can be seen.
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Photos: Photos: Cassini's Saturn discoveries
Cassini's top 10 discoveries about Saturn – 7. Vertical structures in the rings imaged for the first time. Once about every 15 years, the sun shines on the edge of the ring plane and northern and southern sides of the rings receive little sunlight. Cassini measured the thick, long shadows from this rare event to determine the heights of structures within the rings.
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Photos: Photos: Cassini's Saturn discoveries
Cassini's top 10 discoveries about Saturn – 6. Radio-wave patterns shown not to be tied to Saturn's interior rotation as previously thought. Saturn emits radio waves known as Saturn Kilometric Radiation. A similar radio wave pattern was measured at Jupiter to deduce the length of that planet's day. However, it was discovered that the variation in radio waves controlled by the planet's rotation is different in the northern and southern hemispheres. The northern and southern rotational variations also appear to change with the Saturnian seasons and the hemispheres have actually swapped rates. Saturn's length of day is still not known.
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Photos: Photos: Cassini's Saturn discoveries
Cassini's top 10 discoveries about Saturn – 5. Studies of the great northern storm of 2010-2011. In 2010 Cassini got a front-row seat to a massive storm that disrupted Saturn's relatively tranquil atmosphere. The largest temperature increases recorded for any planet were measured. Molecules never before seen in Saturn's upper atmosphere were detected. The storm diminished shortly after its head collided with its tail, a little less than a year after it began. Saturn's rings are seen as the thin blue line in this image, due to the filters used to show methane absorption. The rings are outside the atmosphere, and therefore are not affected by methane absorption.
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Photos: Photos: Cassini's Saturn discoveries
Cassini's top 10 discoveries about Saturn – 4. Titan revealed as Earth-like world with rain, rivers, lakes and seas. Titan is the only known place in the solar system, other than Earth, that has stable liquid on its surface. Rather than water, its lakes are made of liquid ethane and methane.
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Photos: Photos: Cassini's Saturn discoveries
Cassini's top 10 discoveries about Saturn – 3. Saturn's rings revealed as active and dynamic. Cassini's mission allowed scientists to observe changes in Saturn's dynamic ring system, including what could be the birth of a moon. The disturbance, visible in the lower left of this image, is thought to be an icy body migrating out of the ring, believed to be part of the process required to form a moon.
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Photos: Photos: Cassini's Saturn discoveries
Cassini's top 10 discoveries about Saturn – 2. Icy plumes on Saturn's moon Enceladus. Here, the plumes can be seen back-lit by the sun. The discovery was such a surprise that the mission was completely reshaped to get a better look. Evidence of water-based ice in the plume excited scientists further, as life as we know it relies on water.
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Photos: Photos: Cassini's Saturn discoveries
Cassini's top 10 discoveries about Saturn – 1. The Huygens probe makes first landing on a moon in the outer solar system. The Huygens probe's historic landing in 2005 was the most distant to date. In addition to providing this photo, the probe found the moon to be similar to Earth before life evolved, with methane rain, erosion, drainage channels and dry lake beds.
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