Earlier this week I wrote an article about NASA’s Cassini mission to Saturn ... one of the most successful missions in the agency’s history.
This morning, that mission ends in spectacular fashion, as the spacecraft will be intentionally terminated with a dive into Saturn’s atmosphere.
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The mission’s final calculations predict that loss of contact with the Cassini spacecraft will take place today at 7:55 a.m. EDT. Cassini will enter Saturn's atmosphere approximately one minute earlier, at an altitude of about 1,190 miles above the planet's estimated cloud tops (the altitude where the air pressure is equivalent to sea level on Earth). During its dive into the atmosphere, the spacecraft's speed will be approximately 70,000 miles per hour. The final plunge will take place on the day side of Saturn, near local noon, with the spacecraft entering the atmosphere around 10 degrees north latitude.
When Cassini first begins to encounter Saturn's atmosphere, the spacecraft's altitude control thrusters will begin firing in short bursts to work against the thin gas and keep Cassini's saucer-shaped high-gain antenna pointed at Earth to relay the mission's precious final data. As the atmosphere thickens, the thrusters will be forced to ramp up their activity, going from 10 percent of their capacity to 100 percent in the span of about a minute. Once they are firing at full capacity, the thrusters can do no more to keep Cassini stably pointed, and the spacecraft will begin to tumble.
When the antenna points just a few fractions of a degree away from Earth, communications will be severed permanently. The predicted altitude for loss of signal is approximately 930 miles above Saturn's cloud tops. From that point, the spacecraft will begin to burn up like a meteor. Within about 30 seconds following loss of signal, the spacecraft will begin to come apart; within a couple of minutes, all remnants of the spacecraft are expected to be completely consumed in the atmosphere of Saturn.
Cassini's last transmission comes 86 minutes later
Due to the travel time for radio signals from Saturn, which changes as both Earth and the ringed planet travel around the Sun, events currently take place there 86 minutes before they are observed on Earth. This means that we will not receive Cassini’s last data transmission until 86 minutes after the fact.
"The spacecraft's final signal will be like an echo. It will radiate across the solar system for nearly an hour and a half after Cassini itself has gone," said Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "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."
Cassini's last transmissions will be received by antennas at NASA's Deep Space Network complex in Canberra, Australia.
Cassini is set to make groundbreaking scientific observations of Saturn, using eight of its twelve science instruments. All of the mission's magnetosphere and plasma science instruments, plus the spacecraft’s radio science system, and its infrared and ultraviolet spectrometers will collect data during the final plunge.
Atmosphere composition, structure
Chief among the observations being made as Cassini dives into Saturn are those of the Ion and Neutral Mass Spectrometer (INMS). The instrument will directly sample the composition and structure of the atmosphere, which cannot be done from orbit. The spacecraft will be oriented so that INMS is pointed in the direction of motion, to allow it the best possible access to oncoming atmospheric gases.
The University of Michigan’s Dr. Tamas Gombosi has worked with the Cassini science team since 1990. The mission’s end is hitting him hard: “Everyone, including me, were so young!”, said Dr. Gombosi. “ We had a lot of fun for 27 years, and we grew close to each other. We know each other’s families, children, and grandchildren. It is almost like a close-nit neighborhood. This end of mission feels like you are moving out of a nice neighborhood after 27 years. Sad.”
Read more about the Cassini mission, including spectacular photos taken by the spacecraft and more about the University of Michigan connection, here.
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