|
|
The Planetary Society Blog
By Emily Lakdawalla
The Winds of Change and Annoyance
Sep. 28, 2006 | 17:11 PDT | Sep. 29 00:11 UTC
by David Seal
Titan's atmosphere poses a similar dilemma for us as the dust hazards I talked about on Tuesday in that it's one of our prime scientific targets, it was poorly understood before our arrival, and it poses a potential hazard to the mission. In this case, however, we're not really worried about losing the spacecraft, but if we do fly a bit too low, the atmospheric drag could cause us to tumble out of control. The spacecraft would enter "safe mode", kill the active sequence of measurements, and regain control shortly after the encounter, but take no further observations until we verified the spacecraft is healthy from the ground (which usually takes a couple of days, since we tend to be careful about these sort of things).
It's worth reviewing that Titan is unique in that it's the only satellite in the Solar System with an appreciable atmosphere. It resembles the prebiotic Earth with organic molecules and is mostly nitrogen, so a large and diverse community of scientists are very interested in studying it in line with NASA's life origins vision. If Titan weren't orbiting Saturn (and "cleared the neighborhood" of its heliocentric orbit, as is now required), it would be a perfectly respectable planet at 5150 km in diameter (not counting atmosphere), which is larger than Mercury. Titan's atmosphere is surprisingly denser than Earth's at the surface by about 50% (it's got a *lot* of atmosphere). Cassini can only get about as close as 950 to 1000 km to the surface without losing attitude control - compare that to the altitude of the International Space Station, which floats happily above the Earth at a mere 390 km! Titan Compared with other Solar System BodiesTitan (lower right) illustrated along with other comparably-sized bodies in the Solar System to scale.Credit: NASA / JPL / Caltech |
Atmospheric drag does slow down the spacecraft slightly; for a close flyby, the maximum force on the spacecraft provides a paltry twenty millionths of a gee, so it really has little influence on the trajectory. However, drag also places a torque on the orbiter which has to be compensated for by the thrusters. This is caused by an offset between the center of pressure - the balance point of the visible area - and the center of mass for all of the attitudes we want to fly. The center of mass is determined by the structure of the spacecraft and the propellant in the tanks, which are in the lower half of the spacecraft body as seen in the attached image, one possible attitude facing the wind direction. The center of pressure is higher and to the left due to the high-gain antenna and the magnetometer boom, so the atmosphere will push on the center of pressure and try to rotate the spacecraft around the center of mass. Cassini Spacecraft Seen From one Possible AngleThe Cassini spacecraft seen from one possible angle by the atmospheric wind during a Titan flyby. Approximate locations for the center of pressure (for this attitude) and center of mass are shown. Since the two are offset, Titan's atmosphere will induce a torque on the spacecraft since the drag will be centered on the center of pressure, attempting to rotate the spacecraft around the center of mass.Credit: NASA / JPL Digital Image Animation Laboratory / Caltech |
And again, as with the dust modeling, the big problem was gathering up what observations we had - which were not always consistent - and assessing the hazards so we could fly low enough to sample the atmosphere well, but not so low that we risked tumbling. Befor arrival, all the models we had suggested that an altitude of 950 km was the right number - safe, but reasonably deep in the upper atmosphere.
However, once we got there, we got a few surprises. The image here is an illustration of the first few data points we collected on the atmospheric density. What conclusions would you draw? The density we had assumed for safety at 950 km was around where the leftmost circle is on the plot. So not only was the atmosphere more dense - possibly, in spots - but there was some indication of an atmospheric bulge at the equator (which isn't unrealistic). And since we prefer not to gamble with a priceless national asset, we updated our models and raised a number of altitudes (mainly those near the equator) by 50 km or so - enough to lower the drag effects by a factor of two or more. Illustration of early atmospheric density data (scaled to one altitude) vs. spacecraft latitude at closest approach. Credit: D. Seal / NASA / JPL / Caltech |
Now that we've flown more Titan encounters low enough to enter the atmopshere - including Titan 18 last Saturday - our plot looks a lot different, like the second image here. The green line is roughly the curve fit we had some months ago, and now that we have these other data points, what conclusions would you draw now? I don't think there's an atmospheric bulge anymore, do you? It looks like a scatter plot at a fixed density, with some variations that can be modeled statistically. As our lead attitude control engineer Allan Lee says, "that's research". Illustration of updated atmospheric density data (scaled to one altitude) vs. spacecraft latitude at closest approach. The green curve is a fit to some of the early data, which now appears not to match the updated data set. Credit: D. Seal / NASA / JPL / Caltech |
So if we now fit a straight line through these points, our best guess at Titan 18 was that our attitude control thrusters would have fired up to about 49% of their capacity at closest approach (which was at an altitude of 960 km). And as Allan and the attitude control team reported on Wednesday morning, they saw a 42% duty cycle (refer to the next image). So that was good news. Now the question is, should we go back and lower those altitudes again? Ha - not so fast. See the spread of the points that still remains? Is it possible that Titan's atmosphere just varies with time, and could give us grief in the form of the higher data points in the future? Certainly. Is it worth it to create a whole bunch of work for all of the science and sequencing teams to change all of the encounter designs yet again to implement slightly lower altitudes? Definitely not - at least not at this time. Even at a duty cycle of 40% or so, the Ion and Neutral Mass Spectrometer - the primary instrument that conducts in-situ atmospheric sampling - is collecting plenty of gas to satisfy the scientists. So we don't have to lower the altitudes. Illustration of thruster profile during the Titan-18 flyby. The peak "duty cycle" or thrusting effort was 42% of capacity, which matched well with predictions. Credit: Allan Lee / JPL / NASA / Caltech |
So, again, that's research. That's engineering. Our Titan atmospheric modeling working group will be meeting after every low Titan flyby to look at the latest data. You do the best you can with what information you have, become comfortable with uncertainty, stay on top of things when you get new information, constantly reevaluate your strategies, and make changes with conviction when needed.
And don't screw it up.
Other Titan-18 Results
The Cassini RADAR team has posted some of their SAR lake images, and I should certainly include them here. It looks like they are seeing more lakes, as with Titan 16. The excitement in the air when I talk to the RADAR folks is tangible. Titan's "Kissing Lakes"This Cassini radar image shows two lakes 'kissing' each other on the surface of Saturn's moon Titan. The image from a flyby on Sept. 23, 2006, covers an area about 60 kilometers (37 miles) wide by 40 kilometers (25 miles) high. This pass was primarily dedicated to the ion and neutral mass spectrometer sensor, so the volume of radar data was small, but amazingly Earth-like lakes are seen. With Titan's colder temperatures and hydrocarbon-rich atmosphere, however, the lakes are likely to contain a combination of methane and ethane, not water. In this example, near 73 degrees north latitude, 46 degrees west longitude, two lakes are seen, each 20 to 25 kilometers (12 to 16 miles) across. They are joined by a relatively narrow channel. The lake on the right has lighter patches within it indicating that it may be slowly drying out as the northern summer approaches. Credit: NASA/JPL |
Shorefront property, anyone?This lake is part of a larger image taken by the Cassini radar instrument during a flyby of Saturn's moon Titan on Sept. 23, 2006. It shows clear shorelines that are reminiscent of terrestrial lakes. Centered near 74 degrees north, 65 degrees west longitude, this lake is roughly 20 kilometers by 25 kilometers (12 to 16 miles) across. It features several narrow or angular bays, including a broad peninsula that on Earth would be evidence that the surrounding terrain is higher and confines the liquid. Broader bays, such as the one seen at right, might result when the terrain is gentler, as for example on a beach. Credit: NASA / JPL / Caltech |
|
|
[ what is this? ]