The Last Signal From the Cassini Spacecraft

The contribution of Sapienza and the scientific results of a space journey that lasted 20 years: from the exploration of Saturn to the discovery of seas and lakes made of liquid hydrocarbons on the Titan and Enceladus satellites

On Sept. 15, at 2 pm Central European Summer Time, the large NASA antennas located in Canberra, Australia, received the last signal from the Cassini space probe, sent from the upper layers of Saturn’s atmosphere one hour and 23 minutes earlier. A few second later the probe was torn apart by the powerful aerodynamic forces generated by its large velocity (30 km/s – at this speed the Rome-Milan distance would be covered in just 20 seconds). Luciano Iess, a professor of aerospace engineering at Sapienza University who participated in the Cassini project since its start, flew to Pasadena, California, to give the “goodby kiss” to the probe.

In the last phases of the mission, which started on April 26, Cassiny flew by Saturn 22 times at a distance of just 3000 km, shedding light for the first time on the interior structure of the gas giant and reveal the age of its rings. The investigators from Sapienza presented just yesterday the last, revolutionary, results from the last data obtained from the spacecraft.

Cassini started its journey on October 15, 1997, with a spectacular launch from NASA’s Kennedy Space Flight Center in Cape Canaveral. The mission, led by NASA with important contributions from the the Italian Space Agency (ASI) and the European Space Agency (ESA), targeted the Saturnian system, with its many moons and rings. After a seven year cruise phase, Cassini arrived at Saturn on July 1, 2004, starting a complex sequence of orbits which took the spacecraft close to many moons and allowed close observations of the rings and the planet itself. The discoveries went well beyond expectations.

Sapienza University gave a major contribution to this undertaking with two important onboard instruments (the radat and the frequency translator, built by Thales Alenia Space Italy) and by achieving some of the most important scientific results of the mission. A first goal was achieved already in 2002, when the spacecraft  was cruising between Jupiter and Saturn, with a precise measurement of the delay in the propagation of radio signals caused by solar gravity. These data provided the most accurate experimental confirmation of Einstein’s theory of general relativity to date.

During the Saturnian tour, researchers from Sapienza revealed the hydrocarbon seas of Titan (the second largest moon of the solar system) and discovered a rocky core of about 2000 km radius made up by low density hydrated silicates. The measurement of large tides showed that a global, liquid water ocean must be present at depth, under the thick, external, ice shell. The bathymetry of the seas, enabled by the radar, indicated a depth as large as 170 m.

In 2014 Cassini measurements revealed a large amound of liquid water in the southern polar region of Enceladus (about 240 times larger than the Lake Garda), below the cold (-190° C) icy crust of the small moon (252 km radius). As the water reservoir is in contact with the inner silicate core, complex chemical reactions may occur, such as those necessary for life.

The members of the Cassini science team and their research group are based at the Department of Mechanical and Aerospace Engineering, under the lead of Prof. Luciano Iess, and at the Department of Information Engineering, Electronics and Telecommunications with Prof. Roberto Seu.

“In more than ten years in the Saturnian system, Cassini visited all its main moons and provided an extraordinary sequence of discoveries, leaving an immense heritage to the scientific community” says Luciano Iess.

Wednesday, 13 September 2017

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