NASA has confirmed that the Cassini spacecraft successfully completed its final flyby of the moon Titan and is now on course for its fatal encounter with Saturn on Friday. After 13 years orbiting the giant ringed planet and exploring its moons, the unmanned, nuclear-powered probe will be incinerated at 7:55 am EDT (4:55 am PDT) on September 15 in a planned maneuver that will see it plunge into Saturn's upper atmosphere.
Friday's self-destruct dive is the culmination of Cassini's "Grand Finale," as NASA calls the series of close orbits around Saturn that the probe has executed since April. The reason for this seemingly callous disposal of the robotic explorer at the end of its 20-year mission is due to the spacecraft running out of the propellants needed to control its trajectory. In order to protect potentially habitable moons orbiting Saturn from being contaminated with terrestrial microbes, NASA has ordered Cassini to destroy itself.
In an outline of Friday's event, NASA says that Cassini will enter the Saturnian atmosphere at 7:54 am EDT (4:54 am {DT). In fact, this will be the time that the radio signals reach mission control from Saturn – a trip that takes 83 minutes at the speed of light. However, signal reception is generally used to mark mission times to avoid confusion. The space agency says that Cassini's final transmissions will be picked up NASA's Deep Space Network complex in Canberra, Australia.
As Cassini approaches Saturn over the next two days, it will take its final images of the planet and its moons. It will then make a final dump of its entire memory in a download to mission control before reconfiguring itself to send all its science data and telemetry live.
Unfortunately, Cassini's cameras will shut down on Thursday, so there will be no live images of its final moments. However, eight of Cassini's 12 science experiments will continue to run until the last second, gathering never before seen data about Saturn's atmosphere.
According to NASA, when Cassini hits the atmosphere at about local noon 10 degrees north of the Saturnian equator, it will be traveling at about 70,000 mph (113,000 km/h) and accelerating. As the air becomes thicker and drag increases, the probe will use its thrusters to remain stable and keep its main antenna pointed at Earth, increasing the thrust from 10 percent to 100 percent.
After the thrusters reach maximum, Cassini will no longer be able to control its attitude and will begin to tumble. When the antenna shifts a fraction of a degree, contact with Earth will be lost and the probe will start to break up about 930 miles (1,500 km) above Saturn's cloud tops. It will then burn up like a meteor in about two minutes while still hundreds of miles above the clouds.
"The spacecraft's final signal will be like an echo," says Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "It will radiate across the solar system for nearly an hour and a half after Cassini itself has gone. Even though we'll know that, at Saturn, Cassini has already met its fate, its mission isn't truly over for us on Earth as long as we're still receiving its signal."
For those who wish to share Cassini's last moments, mission control at JPL will stream the event live on NASA TV and website from 7:00 am to 8:30 am EDT (4:00 am to 5:30 am PDT) on September 15.
The animation below shows Cassini's last orbits.
Source: NASA
