2001, Then and Now

Miscellaneous ramblings for February, 2001

by John Rummel

I recently found myself in the awkward position of having no good books to read (an odd feeling for me, an library addict), and had to dig into my library and dust off an old classic, but one that seemed due for a "timely" reread: Arthur Clarke's 2001 a Space Odyssey. Having read this work several times before, there were few surprises, but I did find deep satisfaction in reading the following passage:

Like a ball on a cosmic pool table, Discovery had bounced off the moving gravitational field of Jupiter, and had gained momentum from the impact. Without using any fuel, she had increased her speed by several thousand miles an hour.

Yet there was no violation of the laws of mechanics; Nature always balances her books, and Jupiter had lost exactly as much momentum as Discovery had gained. The planet had been slowed down - but as its mass was a sextillion times greater than the ship's, the change in its orbit was far to small to be detectable. The time had not yet come when Man could leave his mark upon the Solar System.

What struck me was the fact that NASA's Cassini spacecraft has just executed this exact maneuver, using Jupiter's gravitational field to propel it on to Saturn. But contrary to the book's perspective, this maneuver is not now considered a novelty. It's been used successfully many times now - by the Pioneer, Voyager and Galileo spacecraft, and even by Cassini prior to reaching Jupiter. Cassini's unusual flight plan involved two flybys of Venus and one of Earth - all designed to use the gravity of these planets to sling it on to higher speeds as it looped through the inner solar system for two years while gaining enough speed to reach the gas giants. (read more about Cassini's flight path to Saturn here.

Arthur Clarke is regarded by many science fiction aficionados as the dean of scientific fiction. With other science trained authors such as Isaac Asimov, he is credited with putting the science back in science fiction. Clarke was remarkably on target with many of his predictions of science advancements in the 20th century. The following is from one of his early papers:

" ... An 'artificial satellite' at the correct distance from the earth would ... remain stationary above the same spot and would be within optical range of nearly half the earth's surface. Three repeater stations, 120 degrees apart in the correct orbit, could give television and microwave coverage to the entire planet."

Clarke was describing a geostationary communications satellite, something so patently commonplace today we barely give it any thought. But Clarke penned that paragraph in 1945, over 12 years before the first successful launch of an artificial satellite (basically a ball of metal with a radio beacon) into low earth orbit.

While remarkably accurate in some predictions, Clarke, like any writer willing to take a chance on predicting the future, made a few inaccurate guesses. We still have no moon bases, nor any planned manned excursions to the outer planets. While we do have a space station, it doesn't have artificial gravity and a population of hundreds. And computers have come a long way, but instead of the villainous HAL, we have desktop machines capable of 1 billion calculations per second.

While science fiction writers, politicians, and psychics attempt to predict the future, we have the privilege of living it. I recall as a child thinking of the magical year 2001, so immortalized in fiction. I tried to imagine what life would be like in that distant future, when I would be nearly 40 years old. Things didn't turn out like many of the predictions, but we've survived and arrived. Now we can imagine the next step as life in the 21st century unfolds before us.

***

How does a gravity assist work?

Imagine a ball rolling down a hill. It gains speed rolling downhill, but then loses speed as it rolls up the next upslope. It's hard to see how speed can be permanently gained this way. But now imagine that the hill is being propelled forward as you roll down it. Now you're not only gaining speed due to the slope, but due to the motion of the hill as well. As you reach the bottom, you have gained more speed than you would from the gravity alone. With a moving hill and proper timing, one could utilize a "slingshot" effect to gain speed as a result. This is exactly the idea, only instead of a hill, you are approaching the moving gravitational field of a planet from behind. The spacecraft "falls" in the planet's gravitation field, and then goes "up" the other side, gaining speed in the process by "stealing" a tiny bit of the planet's momentum.

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