GALILEO'S JOURNEY

The reasons for Galileo's strange journey are at least partly political. Launched with a Centaur rocket, it could have gone to Jupiter directly. But in 1989 NASA, Congress and many other institutions were still in the grip of the Challenger disaster and would not permit the powerful Centaur rocket to be used.

So scientists thought up a scheme whereby Galileo would only have to reach our neighbor Venus, where it was accelerated by the Venusian gravitational field, and then again by the terrestrial field in its flyby in 1990, and after an excursion into the asteroid belt (not shown in the figure), it returned to the Earth's field a second time today, as I am writing this (12/8/92), to be accelerated yet again by the terrestrial field, whence it will fly through the asteroid belt to Jupiter, to arrive on December 7, 1995. The entire process is somewhat reminiscent of a child on a swing which is given only a small initial push, and it then maneuvers itself into higher and higher swings with each cycle.

Not a peep out of the Union of "Concerned" "Scientists," nor the sages who went to court claiming that the earth "could" be showered with plutonium. The earth has already been showered with plutonium in the 1950s bomb tests, 3.5 tons of it, and no harm is known to have come to anybody, for reasons that I explained last time. But why should Galileo's small quantity of plutonium get detached from the satellite and, driven by unknown forces, head back for the earth?

In any case, my point here is not so much the Galileo mission itself as the problem of providing on-board electricity at a level of the order of 500 W for no less than 6 years.

To get an idea of the problem, consider an automobile battery, which at the moment of cranking (starting) applies a power of something over 1 kW. But for how long? If the engine does not start, you try again and again. After a net time of a couple of minutes the battery is dead. So to provide 500 W for 6 years you would need somewhere around 1.5 million car batteries, whose weight would (see chart in the editorial) put it far beyond the Karl-Harpo limit. Solar? Something like 5 square meters of cells would be needed, and if they did not unfold, the entire mission would fail. (Galileo's high-gain antenna did indeed fall to unfold, but the fault was not fatal.)

An alternative that is small fight and highly reliable is a thermocouple generator. When the joint of two different metals is heated (the two metal bars usually forming a V, touching at the bottom of the V along the two areas where the bars have been cut obliquely), a voltage appears at the top end of the V. This is known as the thermoelectric effect.

So all we need is a source of heat that will last for 6 years, and that is provided by a radioisotope. Heat is the vibration of the molecular grid of a material and one of the ways to get it to vibrate is to bump into it with nuclear particles. These, in turn, are provided by any radioisotope, for radioactivity consists in shooting out particles as the nucleus of one of its atoms disintegrates.

Another way of looking at this is to remember that the particles (including gamma photons) shot out of the nucleus have energy, and that energy has to appear somewhere. It appears as heat in the material that absorbs the particles. As a matter of fact, radioactivity is what keeps the interior (and by conduction, to some extent also the exterior) of our earth warm. So every time you see the ecofreaks' bumper sticker "Love your mother" with a satellite picture of the Earth, remember it means "Love radioactivity." Without it the Earth would be as barren as the Moon.

The area below the corresponding Karl-Harpo Time is marked "radioisotopes" in the editorial box for this type of generator. It can be used for on-board power generation, but not for propulsion. For a 6-year mission, plutonium is a natural choice, for 1) it emits alpha particles (helium nuclei), which are heavy and therefore have high energy, 2) its halflife of 24,900 years will keep the heat energy going. At the beginning of the mission, the source had 570 W, which is expected to decrease to 486 W after 6 years as the plutonium decays.

There is no way to overstep the Karl-Harpo curve; but there are plenty of ways to inform oneself what is going on in astronautics, who wants to wreck it, and why.