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Let’s Celebrate Cassini

Let’s Celebrate Cassini


Have you ever skipped to the back of a story because you simply could not wait another minute to find out how it all ends? Planetary astronomers, in contrast, must be very patient people. Some of them have worked on a project for years, even decades before they ever begin to collect any information. The good news for curious and impatient people is that this is a particularly good time to learn new details about the planets.

One really dramatic piece of news is that a tiny spacecraft called Voyager 1 is about to leave our solar system. Currently it is moving into a completely unexplored region called interstellar (between the stars) space. Apparently our solar system has an edge and beyond that is interstellar space. It so happens that our sun continuously emits a stream of electrically charged particles (called the solar wind), and this exerts an effect on a huge area of space.

Eventually however, the solar wind meets resistance from charged particles and gas atoms coming from other bodies in interstellar space. Scientists imagine that there is an invisible boundary zone where particles streaming from different directions cause a turbulent effect. They call this the region of termination shock. As Voyagers 1 and 2 have proceeded outward well beyond the farthest planets, scientists have been on the lookout for this boundary to our solar system. In August of 2002, 25 years after the launch of Voyager 1, when it was already 12.6 billion km away from us, there was still no sign of the termination shock.

Finally in December 2004, Voyager 1 recorded sudden increases in the strength of the magnetic field and in the temperature of the few gas atoms. Now, five months later, the elevated magnetic field and temperature continue. The spacecraft is now 14 billion km away and going strong. Scientists consider that Voyager 1 has now passed through the termination shock. Everyone is eager for information about interstellar space. No doubt there will be surprises. There always are in space related research.

During the past 30 years, we have enjoyed a feast for the eyes and the mind, as various probes have beamed back dramatic images and data from the planets. The Voyagers have been particularly successful. Each has only 80 kilobytes of computer memory and only a 23-watt transmitter, yet they continue to make history. Both spacecraft astonished the world with demonstrations of delicate detail in Saturn’s ring system including braided rings and spokes.

We can of course celebrate these discoveries of the past, but the exciting news is that right now is the time to pay attention to space exploration. The 3.3 billion dollar Cassini mission to Saturn represents the largest and most expensive interplanetary mission perhaps ever. It is expected to complete its mission by 2008. Until that time, this is our chance to enjoy superb information from space. Some scientists have been working on this project for 24 years. By the end of this study, some workers will have devoted almost their whole careers to this study. Now that is real patience!

This probe was first proposed in 1982. Actual development, a cooperative effort by NASA, the European Space Agency and the Italian Space Agency (scientists from 18 nations in all), began in 1989. Launch of this exceptionally heavy spacecraft took place in October 1997. Finally in June 2004, the craft entered orbit around Saturn. This probe is named after astronomer Giovanni Domenico Cassini (1625-1712) who discovered four of Saturn’s moons.

One well known science commentator, John Nobel Wilford, remarked about the results of the Voyager probes, that we see revealed in space, as elsewhere “nature’s unending capacity for surprise, diversity and breathtaking beauty.” (Edmonton Journal August 21, 2002 A13). His remarks apply just as well to today’s results from the Cassini probe.

One of the earliest studies which Cassini has begun, is of the rings of the giant planet. This first radio occultation of Saturn’s rings was performed on May 3, 2005. When the probe beams radio signals through the rings towards Earth, particles in the rings intercept some of those signals. As a result, the radio beam received on earth is weaker depending upon the number and size of particles which it hits in the ring. Unlike the Voyagers, this probe beams signals in three radio frequencies rather than two. With the combined data, scientists can distinguish particles from 1 mm in diameter up to objects larger than many metres. Altogether 20 radio occultations are planned, mostly in the near future.

The observations made by these probes have not been exactly what astronomers might have predicted or even have hoped for. Based on their belief in an age for Saturn of billions of years, most scientists would have preferred observations from the rings which could be interpreted in terms of very long ages. This however was not to be. The data from Cassini continue the trend, first noted with the Voyagers, of ring characteristics which demonstrate diversity and instability.

Diversity, in this case, is a term applied to observations so different from each other that they require separate explanations as to their causes. The rings can be said to demonstrate diversity since there is no general force or process which can explain all their characteristics. For example, astronomers definitely would have preferred to see objects arranged in a regular order in the rings, for example spread out according to size and weight. This condition of regular distribution of objects is not however what the Voyagers found, nor is it what Cassini has observed. Thus one general explanation for these observations will not work.

The data so far from Cassini have indicated that all ring bands appear to include a broad range of particle sizes, everything from boulders to powder. However the distribution of particle sizes varies greatly among the rings. Again there is no logical progression. The other problem with these rings, from the point of view of long age interpretations, is their apparently unstable nature. This means that according to the laws of physics, they must be continuously in process of change. The very existence of these rings with particles dispersed thickly enough to be observed, is surely an unstable phenomenon. The smaller the particles and the more dense their distribution, the more quickly they can be expected to disappear as a result of pulverizing collisions. The narrower each ring is too, the more quickly it can be expected to spread out into an invisible sheet.

The Voyagers revealed that the rings were not broad and featureless as was formerly believed, but rather many were narrow and highly varied in appearance with some sinewy strands apparently braided together. Also at least 2 rings were slightly off centre, like wobbly wheels. The braids and wobbles would be even more unstable than the other narrow rings.

Cassini further extends this trend of diversity and instability. Apparently the large B ring, which was not previously studied, is remarkably different from the A and C rings, its neighbours on either side. It consists of smaller bands which vary greatly in their content. A thick 5000 km wide core contains several bands with ring material nearly four times as dense as that of ring A and nearly twenty times as dense as ring C. Indeed “the dramatically varying structure of ring B is in sharp contrast to the relatively flat structure of ring A or the gentle, wavy structure of ring C, where many dense, narrow and sharp-edged ringlets permeate its outer part.” (NASA)

The NASA press release of May 23, 2005 also declares that “The origin of Saturn’s rings is a mystery.” When no general process can be identified but only many explanations specially proposed for each particular situation, then we may well become suspicious of natural explanations. Are the rings merely the result of mindless chance, or do they (and all nature) demonstrate the conscious choices of God who loves diversity and beauty?

Moxie’s challenge is to look for further examples of unstable phenomena and unexpected diversity in the Cassini reports. Draw your own conclusions.

July 2005

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