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Originally Posted by Fram
Jerry, I don't want to quote your whole post, but why is Cassini so in trouble by Titan and not in trouble at all by Saturn? Wouldn't the miscalculated effect of gravity be much much greater with regards to Saturn? Why does only Titan have such an effect in your mind?
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Yes, but with some qualifications. Remember the basic idea is that the Strong Equivalance Principle is false: We cannot use Newtonian estimates of gravity to predict the mass of anything outside of a lunar-earth orbit. The difference is small, but by the time we arrive at the orbit of Saturn, the predicted mass, using Newtonian physics, is off by a factor of more than two.
As you know, to the first order, orbits are not a function of the mass of the orbiter, so using Newton's mechanics to plunk Cassini into an orbit does not test this concept. But changing the orbit, moving closer to Saturn, or moving close to one of the moons reveals perturbations that should be interpreted as greater mass in the system, not bugs on the windscreen or the other usual suspects.
Posters on this board have been dogging me for months now, pointing out these perturbations should exist, and all I have been able to do is kind of half heartedly suggest that NASA is not reporting these minor deviations from nominal trajectories. Now look what we find out: The IR mirrors are vibrating, the thrusters are thrusting and these potentially mission compromising events are not even making it into the significant event log.
(Although you can read, about troops of girl scouts visiting the mission control center.)
Could this be one of the reasons NASA is having so many mission failures? Could the servo problem with Cassini's mirrors have something in common with Dart's navigational problem? I don't know. Does anybody?
The weakness in detail in reporting the navigational trials of Cassini demonstrates that
NASA still does not get it: Missions fail when everyone puts on a smilely face and pretends everything is according to Hoyle. Chunks of foam and ice were falling off the Shuttle fuel tank on every flight, some of them seriously damaging flight hardware. Did the wingmen even know it? The public? The astronauts?
If a software problem is causing Cassini to panic and fire thrusters, shouldn't the Dart engineers have known that as soon as it happened? If the Descent, entry and landing phase of
Spirit and Opportunity produced data streams that can't be modeled, shouldn't ESA and Japanese engineers planning missions to Mars know that? Shouldn't you and I know that?