Insider’s Cassini: ‘Going Low at T-70’

Todd J. Barber, Cassini lead propulsion engineer

As summer begins in Earth’s northern hemisphere, our thoughts turn to barbeques, long evenings, and maybe even dipping our toes in the pool. The Cassini spacecraft will celebrate the northern hemisphere’s summer solstice with a bold “toe-dip” of its own this Sunday evening (PDT), plunging deeper into Titan’s atmosphere than it ever has before. I recently interviewed Julie Webster, the Spacecraft Operations Team Manager (and my immediate boss) about this unique event as Cassini’s Equinox Mission nears completion. Believe it or not, the first work for “going low at T-70” began nearly 3.5 years ago. We knew that Titan closest approaches below 940 to 970 kilometers (584 to 603 miles) in altitude had some non-zero risk of spacecraft tumbling, depending on the spacecraft pointing (or attitude). But the question familiar to every limbo fan (“How low can you go?”) was investigated extensively. The team decided to look at an 880 kilometers (547 miles) altitude flyby: not too much of a change, but enough to make our magnetometer scientists salivate, since their instrument's sensitivity for understanding the Titan subsurface structure increases as the inverse fourth power of distance to the center of Titan. Interestingly enough, at the minimum torque attitude (which was still excellent for science, by the way), there was margin against tumbling at 880 kilometers! However, the margins were tighter for aero-heating thermal limits on the spacecraft (specifically, the stellar reference units) — in fact, the thermal limits were the ultimate driver in selecting a closest approach altitude no lower than 880 kilometers.

I think I was most astounded to hear about the amount of work that went into assessing this flyby. Julie conservatively estimated the team put in more than 5,000 hours of work in the analysis of spacecraft health and safety for this record-low altitude plunge! We also had external help to validate our results, including the aerodynamic expertise of NASA Langley engineers in Virginia. Three candidate attitudes during the flyby were investigated — the JPL optimal attitude (our assessment of the minimum torque pointing), the NASA Langley optimal attitude (their assessment of minimum torque pointing), and the safing attitude (pointing the high-gain antenna to Earth — the condition we would encounter in case we entered safing just before the flyby). The first two attitudes compared favorably (as expected), but through an unbelievable stroke of good fortune, the safing attitude was within roughly just one degree of the other attitudes as well! This not only gives us confidence that the spacecraft will remain healthy even if safing occurs, it also allows us to obtain real-time telemetry during the plunge through Titan’s atmosphere, since the safing attitude is pointing the high-gain antenna to Earth! For this reason, our baseline attitude will be high gain antenna pointed to Earth and thus we’ll relish having about thirty minutes of real-time data during the flyby. In fact, Julie herself will be blogging about the outcome of the T-70 flyby when it’s all over, based on observations of real-time data during closest approach.

Other preparations at JPL for this scientifically crucial flyby included hundreds of simulations of different “error monitor” tripping, the ability to recover from loss of spacecraft pointing control, and many simulations in our ground testbeds. We also held a meeting to walk through what would happen in the extremely unlikely event the spacecraft were to tumble during T-70. As Julie reminded me, “At JPL, we have a very strong urban myth that if you simulate an anomaly thoroughly then (1) it won’t happen or (2) if it does, we’ll be ready.” This event is the last “big-ticket” item in the Equinox Mission — in fact, it was put near the end because it’s a bit more sporting than our typical Titan flyby. I rest easy when I hear Julie say “we’ve done everything we know how to do for safety.” Despite our prospects for seven more years in the Cassini Solstice Mission, Julie said “This is the time to respond to science pushes” (which T-70 definitely embodies). I asked for her final thoughts, and she said she was anxious to get T70 behind her, not because she’s worried, but because it represents a large step in engineering capability and it gives us more confidence for future Titan flybys at higher altitudes. Thank you, boss, for putting my mind at ease. Now we can sit back and enjoy a front-row seat to this scientifically historic swoop through Titan’s atmosphere. Enjoy your plunge, Cassini! May the only waves you generate be waves of interest and scientific surprise.