A MYSTERIOUS SERIES

Any planet in the solar system must travel along an elliptic orbit with the Sun at one of the foci of the ellipse. (A circle is merely a special case of an ellipse, with the two foci merged into a single point the center, or an ellipse with equal axes. The other conic sections are not special cases of an ellipse.) This goes for each asteroid in the Asteroid Belt just as it does for the planets. With a few exceptions to which we will come in a moment the planets and the asteroids have orbits with very small eccentricities, i.e., they run along ellipses that are very close to circles.

It is interesting to sidetrack to the history of how the Asteroid Belt was discovered. in 1764 a German astronomer, J.D. Titius, found that the mean distance of the planets from the Sun followed a certain pattern, which fitted a simple formula: mean distance from the Sun to the nth planet (he thought) was rn=4 + 3x2^n units. Mercury's distance is 4 units (n=minus infinity) and the other distances are found by successively substituting n = 0,1,2,3,4....His formula got a strong boost 1781 when Uranus was discovered, which fit n = 6 quite closely, though much later, when Neptune and Pluto were discovered the agreement for n = 7 and n = 8 was less impressive, though still quite tolerable. But there was a gap in the series between n = 2 and 4, which stimulated a search for the "missing planet." What it found was not a single planet, but the Asteroid Belt.

Today me know that the formula is not as simple as Titus thought but the principle of the series remains: plotting the distances on semilogarithmic paper with n = 1, 2...on the horizontal axis, we find points not too far removed from a straight line (which corresponds to a constant raised to a linearly increasing power); even more amazingly, this holds for all four known planetary systems: solar, and those formed by the moons of Jupiter, Saturn and Uranus, respectively. End of side track.

Among the plants, only Mercury has a considerable eccentricity, i.e., runs along an ellipse with significantly unequal axes; the other orbits are quite close to circles. Most asteroids, too, have close to circular orbits; but there are five large asteroids with eccentricities so large that their orbits cross (in projection, anyway) the orbit of the Earth. They are Adonis, Apollo, Eros, Hermes, and Icarus. Many smaller asteroids of this type may be orbiting the Sun. Only three such orbits are shown in the figure quite symbolically by the three dashed ellipses.

In 1968, Icarus approached the Earth to within 4 million Miles, a stone's throw in astronomy, and in 1937 Hermes sailed by at a mere 400,000 miles.

Do I talk like a Green now? No: for instead of panicking and using the phenomenon to shut down industry, let me report how such a disaster might be averted.

[More: For the Titius series, take a good book on astronomy or the solar system, but look for the "Titius-Bode" or even the "Bode" series. But since Bode, also a German astronomer, had nothing to do with the series except to popularize it, I refuse to attach his name to it.]