**6. EOM statistics**

*Power usage:* **Figure 9** shows the entire power history telemetered by the spacecraft during its 20-year mission, including the very last data point sent just before EOM. The overall RTG power decay shows an exponential behavior starting from 882.1 W on the Day 1, to 600.3 W on the last day of the mission (indicating a total power decay of 32%). The data plot indicates nominal RTG performance, with some peculiarities: during the first 3 years of the mission, the power output decayed by 70 W at an accelerated rate due to the dopant precipitation in the SiGe thermocouple, which reduces the available current carriers.

*Cassini's SSRs used as a radiation detector:* As discussed above, Cassini's SSRs were susceptible to high SBE and DBE occurrences due to environmental effects. These elevated bit error counts often occurred in the presence of high dust and radiation. In this way, the SSRs were inadvertently turned into uncalibrated and unofficial radiation detectors. **Figure 10** shows the effects of radiation on the SSRs during

**145**

**7. Saturn science**

**Figure 10.**

**Figure 9.**

*Robotic Autonomous Spacecraft Missions: Cassini Mission-To-Saturn Example*

*Cassini recorded power usage over its 20 year mission (credit: Grandidier et al. [17]).*

several ring flybys, relative to the SBE error count. The spacecraft's flybys between the D-Ring and Saturn's atmosphere are shown as the "proximal orbit" region.

Many incredible discoveries were uncovered by the Cassini-Huygens mission during its 20 years of flight; a few of those fascinating encounters are mentioned here. **Figure 11** depicts six of the more than 60 of Saturn's known moons, which range in size from a few hundred meters to larger than planet Mercury. The top row of this figure, from the left (not to scale), shows the tiny odd looking moon Pan, Mimas (which looks like the "Death Star" space station from movie Star Wars), and Hyperion, which resembles a sponge. On the bottom row are Iapetus, Titan (the largest of Saturn's moons), and Enceladus, which contains "tiger stripe" fissures with erupting plumes, implying an underground reservoir of water that is suspected to be around 10 km deep (i.e., an underground ocean). **Figure 12** depicts an artist's impression of the hydrothermal activity taking place on this south polar

These results were consistent with MIMI's radiation model.

*SSR SBE counts from late 2015 through EOM (courtesy of S. Adamiak).*

*DOI: http://dx.doi.org/10.5772/intechopen.82161*

*Robotic Autonomous Spacecraft Missions: Cassini Mission-To-Saturn Example DOI: http://dx.doi.org/10.5772/intechopen.82161*

#### **Figure 9.**

*Aerospace Engineering*

the remainder of the mission.

**5.4 The grand finale**

mission [16].

**6. EOM statistics**

(millions to one), that no fix was implemented.

by the spacecraft, but switches over to the DST's VCO).

couple, which reduces the available current carriers.

however, the quality of radio science observations was reduced.

*Fix:* The chances of a cosmic ray hit on an uplinked command are so unlikely

*Loss of the USO:* At the beginning of the DSN track on December 23, 2011, no downlink signal was received from Cassini. The suspected cause was bad predicts used at the DSN station. New predicts were built and two different DSN antennas were used to acquire the spacecraft's signal to no avail, ruling out the bad predicts as the cause of the anomaly. Attention then turned to the USO as the source of the problem. Cassini's signal was acquired after RTLT (when the USO is no longer used

*Fix:* A test was devised to determine if the DST's downlink path or the USO was the cause of the loss-of-signal problem. It was determined that the USO had failed. The Auxiliary Oscillator was used for the remainder of the mission, which yielded a "rattier" signal. Spacecraft operations were not affected by the loss of the USO;

The Cassini mission ended with 20 orbits of the F-Ring, followed by a 22 orbit ballistic trajectory through the D-Ring, and a highly successful final plunge into Saturn. Unique science data was captured during this final flight phase, and no significant anomalies occurred, ending the highly successful, nearly 20-year

*Power usage:* **Figure 9** shows the entire power history telemetered by the spacecraft during its 20-year mission, including the very last data point sent just before EOM. The overall RTG power decay shows an exponential behavior starting from 882.1 W on the Day 1, to 600.3 W on the last day of the mission (indicating a total power decay of 32%). The data plot indicates nominal RTG performance, with some peculiarities: during the first 3 years of the mission, the power output decayed by 70 W at an accelerated rate due to the dopant precipitation in the SiGe thermo-

*Cassini's SSRs used as a radiation detector:* As discussed above, Cassini's SSRs were susceptible to high SBE and DBE occurrences due to environmental effects. These elevated bit error counts often occurred in the presence of high dust and radiation. In this way, the SSRs were inadvertently turned into uncalibrated and unofficial radiation detectors. **Figure 10** shows the effects of radiation on the SSRs during

*CAPS instrument failure:* In April 2011, Cassini's power bus suffered unexpected swings. The imbalance remained in place until June of that year, when another shift occurred. Engineers suspected the high-rail short to be within the CAPS instrument and 3 days later the instrument was turned off. The bus returned to near preanomaly values, and the CAPS instrument was left off while an investigation was conducted into the cause of the short condition, and whether CAPS could be turned back on. The conclusion was that it was safe to turn the CAPS instrument back on. Two days later the short condition reappeared, causing the bus voltage to shift again. The CAPS instrument was left on and the shifted values remained until June 2012, until a series of voltage swings occurred over a 24-h period. The condition culminated until CAPS was autonomously shut off by the SSPS switch, by an overcurrent draw from the instrument. A second investigation was undertaken after this CAPS anomaly, leading to a decision to leave the CAPS instrument off for

**144**

*Cassini recorded power usage over its 20 year mission (credit: Grandidier et al. [17]).*

#### **Figure 10.**

several ring flybys, relative to the SBE error count. The spacecraft's flybys between the D-Ring and Saturn's atmosphere are shown as the "proximal orbit" region. These results were consistent with MIMI's radiation model.
