Say Hello to Phoebe


I am mostly a biology person, and PT is mostly a biology blog, but I think we can all take a moment to have a gasp at the results of the first moon flyby of the Cassini-Huygens mission to Saturn.

Quite an improvement over the last few days, eh? Check the continually-updated latest images from Cassini page at JPL, where you can view the images at almost whatever resolution you desire. The Cassini team will probably be adding images rapidly from now on.

For the past several months, the Cassini-Huygens probe has been racing towards Saturn, after a seven-year journey that included four gravity-assist manuevers in the inner solar system to pick up speed. The Cassini website has been posting pictures about once a week, mostly of Saturn getting bigger…and bigger…and bigger in the zoom cameras. However, the first stop on the Deluxe Grand Tour of Saturn was Saturn’s largest outer moon, Phoebe. Cassini flew past Phoebe late last week and the images have been coming in this weekend.

The Cassini probe is the size of a bus, has a mass of 6000 kilograms*, and is packed with high-tech cameras and instruments far superior to those carried on the Voyager probes. It appears that all sensors worked perfectly on the Phoebe flyby, as well as the antenna that transmits the data back to Earth. This is very important – the big problem with the Galileo space probe to Jupiter was that its main antenna wouldn’t open, meaning that all data had to be trickled back through a low-bandwidth backup antenna. The Galileo probe still did an excellent job, but Cassini will be able to send us gigabytes of data a day.

Phoebe is interesting because it has a “retrograde” orbit around Saturn, meaning that it orbits in the opposite direction to Saturn’s inner moons (Saturn has a few outer moons smaller than Phoebe that also have retrograde orbits – they may be bits of Phoebe blasted off by impacts). Phoebe’s orbit is also in the plane of the solar system, rather than the plane of Saturn’s rings and inner moons. These facts, along with Phoebe’s dark surface, indicate that Phoebe may be a bit of the outer solar system that wandered in and was captured by Saturn’s gravity. Therefore, today’s images might be the first good look that we have ever gotten of a primitive, unprocessed piece of the outer solar system.

A few things that I find interesting about the photos thus far: Phoebe is clearly so small that it barely has enough gravity to keep a spherical shape, and impacts are able to blast large pieces off of it. I suspect that the steepness of the craters is also a result of the low gravity. It is also clear that some craters are fresher than others, and one fresh crater appears to reveal some layering of ice (brighter material) and darker material (perhaps organic compounds?).

But, this is only the first stop on the tour. Cassini is NASA’s last “Cadillac” probe, where billions of dollars were spent to pack everything onto one probe. After problems with previous Cadillac missions, NASA switched to a “faster, better, cheaper” philosophy. This may be appropriate for Mars missions, where the travel time is only 6 months, but for the outer planets, big probes with lots of sensors are really the only way to go. And finally, it appears that we have a deluxe probe that is functioning perfectly.

What we have to look forward to: 30 more (known) moons, 70-some orbits of Saturn over the next 4 years, and the December dropping of the Huygens probe into Titan’s mysterious organic-haze atmosphere. But the next big event is on July 1: a flight through Saturn’s rings (between the F- and G-rings) and an ultra-close approach to Saturn:

I think I know what I’ll be doing over the July 4 weekend.

* Mark Perakh pointed out that my original description, “weight of 6,000 kilograms” was only correct while Cassini was on the Earth’s surface. In space its weight is ~0. Its mass remains 6,000 kg. You can’t get away with anything here in the Thumb


Boy, I really hope NASA’s right about the amount of empty space between the F and G rings. I know that space is really large and really, really empty, but still, it seems that there might have been just a little safer way to enter into orbit. They’re going to feel like idiots if they spent seven years getting there, just to smack into something inbetween those rings.

*knocks on wood*

It all is fine except for Cassini’s weight. First, it cannot weigh 6,000 kilogramm because a kilogramm is a unit of mass, not of weight. Second, the weight depends on the location. So, I believe you meant to say that Cassini’s MASS is 6,000 kg. If that is indeed so, then when on the surface of Earth its weight would by 6,000 times 9.8 = 58,800 Newton or about 13,200 pounds (a pound is a unit of weight like Newton - it is the weight of one slug of mass on the surface of earth). On the Moon it would be about six times less than that, and during most of its flight its weight was practically zero (“weightlessness”) while its mass was practically constant - 58,800 Newton(because its speed was well below the speed of light). Cheers.

Mark’s got me dead to rights, there. Now you can see why I’m not a physicist…

[correction will be made shortly]

Back in the early ’60s I spent a couple of years on a Polaris launch crew at Cape Canaveral before the test center was renamed for JFK. We used to watch night launches of the big (well, they were big then) experimental liquid fueled rockets being developed to throw nukes at the Soviet Union. (Yes, Mark P., I’m an old cold warrior!) They’d rise a a few miles, accelerating with their splendid flame of exhaust lighting the sky, and then the range safety officer would see something wrong in the telemetry and push the destruct button and a multi-million dollar bloom of an explosion would fill the night sky. While we knew the manned space program was progressing – the original 7 astronauts were making the transition from Mercury to Gemini in those days – it’s still hard for me to concenive of putting that massive an object into orbit around Saturn and confidently expecting it to function for years. Shoot, one of the principal investigators was quoted as saying she expected it might last a decade up there, cranking out data. That is totally amazing for one who saw those early days of night destructs and 18 hours on hold waiting for the range to clear and numerous sea dumps of out-of-control ballistic missiles.


Mark Perakh Wrote:

a pound is a unit of weight like Newton - it is the weight of one slug of mass on the surface of earth

The term “pound” appears to be ambiguous.

According to both the current US and British official standards, the pound is a unit of mass, not weight. See, for example, the Statutory Instrument No 1804 of the British units of measures regulations 1995, or page 5 of the US National Institute of Standards and Technology’s general table of units and measurements.

When I was in high school in the early 60s (in Australia), one of the systems of measurement we were taught was the British foot-pound-second system, in which the foot was the unit of length, the pound the unit of mass and the second the unit of time. The unit of force in this sytem is the poundal, the amount of force which would produce an acceleration of 1 ft/sec² when applied to a mass of 1 pound.

We were required to use the term “pound-weight” rather than “pound” to refer to amount of force which would produce the standard acceleration of gravity (approx. 32 ft/sec²) when applied to a mass of one pound. If we ever committed the egregious blunder of referring to the weight of an object as so many pounds, rather than that same number of pounds-weight, the more pedantic of our teachers would feel compelled to give us a rap over the knuckles. Judging from various pages now available on the web (including the British Statutory Instrument No 1804 mentioned above), the term “pound-force” rather than “pound-weight” is now used to refer to this unit.

Until Prof. Perakh’s post induced me to do a little research, I had never realised that the term “pound” by itself was also a genuinely respectable way of referring to this unit of force.

Mark Perakh Wrote:

a pound is a unit of weight like Newton - it is the weight of one slug of mass on the surface of earth

While checking the definitions of the terms “pound” and “slug” for my previous comment I failed to notice that Prof. Perakh’s statement is incorrect, regardless of any ambiguity in the use of the term “pound”.

A slug is defined to be the mass which will be accelerated at 1 ft/sec² when subject to a force of 1 pound-force. Its weight at the Earth’s surface is approximately 32 pounds, not 1 pound.

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This page contains a single entry by Nick Matzke published on June 13, 2004 5:15 PM.

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