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.
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.
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.
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.