2 min read

Halfway Home

Cassini engineers at the Jet Propulsion Laboratory
On June 28 in a testing lab at the Jet Propulsion Laboratory, Cassini engineers led by Julie Webster (right) were looking for any imperfections in the final commands to be transmitted July 7 to the spacecraft. Cassini will then autonomously operate under those instructions from July 10 through end of mission on Sept. 15.

Time is running out for Cassini.

In April, the Cassini spacecraft began the mission’s Grand Finale — a series of 22 dives through the 1,500-mile (2,000-kilometer) gap between Saturn and its rings, where no spacecraft had explored before. On June 29, the spacecraft completed its eleventh dive through the gap, putting it halfway through the final phase of its nearly 20-year journey in space.

So far, the science observations for the Grand Finale are going as planned, and the spacecraft — 19 years and eight months after launch — is in good shape. “It’s doing exactly what it’s supposed to do,” said Julie Webster, Cassini’s manager of spacecraft operations. “The spacecraft is executing every command that’s asked of it.”

Cassini needs only to stay out of trouble for the next two and a half months to reach the end of its mission, on Sept. 15, when the spacecraft will triumphantly plunge into Saturn’s atmosphere. On that day, Cassini will transmit data about its surroundings until the spacecraft can no longer fight the atmosphere with its attitude control thrusters. Before that day arrives, however, Cassini requires one more set of instructions.

Because communication with the spacecraft is not instantaneous, Cassini can’t be controlled in real time. Therefore, the spacecraft must operate autonomously using instructions transmitted from Earth. On the same day as Cassini’s eleventh dive through the gap, the mission’s engineers and scientists finalized the spacecraft’s last set of instructions.

Cassini mission
In the mission control room at JPL, a screen displays the days, hours, minutes, and seconds remaining until the Cassini spacecraft’s mission-ending plunge into Saturn’s atmosphere. The photo is from June 28 — a real-time countdown clock can be found in the bottom right of the mission homepage.

Each of Cassini’s sets of instructions are called a sequence — a series of computer commands for Cassini’s science instruments, communication system, attitude control thrusters and other subsystems to follow. A sequence is like a musical score, which can be separated into different sets of sheet music for each of a spacecraft’s subsystems to read and perform from. When all of Cassini’s systems perform in concert, they produce a symphony — not of music, but of engineering achievement and scientific discovery.

Cassini’s final sequence, called Sequence 101, controls the spacecraft from July 10 through Cassini’s last day, Sept. 15. The honor of transmitting, or “uplinking,” Cassini’s final sequence falls on NASA’s Deep Space Network station in Canberra, Australia, where the 70-meter diameter radio antenna will send the commands to the spacecraft on July 7.

After their final sequence approval meeting, the spacecraft operations team at NASA’s Jet Propulsion Laboratory celebrated with pizza.