Icarus May Be Sphere Of Solid Iron

Press, Volume CVIII, Issue 31729, 12 July 1968, Page 15

Icarus May Be Sphere Of Solid Iron

(By

WALTER SULLIVAN)

NEW YORK, June 29. Icarus, the asteroid that flew past the earth on June 14, may be a ball of almost solid iron, according to a first look at data obtained in recent days.

Icarus never came closer than 3.9 million miles. It is probably less than a mile in diameter and can be seen only through powerful telescopes, showing up then as no more thrn a pinpoint of light.

Nevertheless, through a variety of sophisticated observational techniques, astronomers are trying to determine its nature. Every 19 years Icarus passes close enough to the earth to be seen. Only one of these passes had been observed before the recent one. That was in 1949, when it was discovered. From 71 photographs taken then it was concluded that Icarus is an irregular object in an extremely eccentric, or elongated, orbit round the sun. Orbit Close to Sun This brings it far closer to the sun than any other asteroid whose orbit is known. It was named for the mythical youth who perished when he flew too close to the sun. Its orbit is so much like that of a comet that some speculated it might be the remains of a comet head, its gaseous portion boiled off by

■ close encounters with the sun. What would remain, in | the words of Dr William Liller of Harvard, would be a i “gravel bank” with only a faint, comet-like plume. Observations by Dr Liller and Joseph Veverka have disclosed no such plume. Rather, they found Icarus a good deal bluer than most asteroids. This has been confirmed in observations by the University of Arizona. Dr Tom Gehrels said the manner in which Icarus reflects and polarises sunlight is “strikingly different” from the reflecting properties of the moon.

Rough surfaces tend to orient or polarise, the vibration modes of light waves reflected from them. These observations and the bluish colour of Icarus suggest that it may be made of iron, Dr Gehrels said. Some Fragmented

Most of the asteroids, some of them classed as minor planets, orbit the sun in an almost circular region between the orbits of Mars and Jupiter. Those that swing in close to the earth are thought to have been knocked out of their normal orbit by collisions.

Some are fragmented, providing chunks of rock or iron that fall to earth as meteorites. It has generally been thought that the iron meteorites are fragments from the

core of a minor planet shattered by collision. The roundness of Icarus has been inferred from measurements of its brightness, using sensitive photometers. If, for example, the moon were spinning, relative to a distant observer who was measuring its brightness, he would see a rhythmic fluctuation. When the seas on the earth-facing side of the moon were toward him it would be darker. When the mountainous far side was facing him, it would be brighter. Rate of Spin '

The rhythm of these fluctuations would tell him the spin rate of the moon. The extent of the brightness variations would Indicate whether the moon was a sphere or a jagged object, for in the latter case the brightness would be more variable. Both the Harvard and Arizona observations have shown Icarus to be spinning once every two or possibly four hours.

In explaining the latter possibility Dr Liller pointed out that if Icarus were shaped like a brick and spinning end over end, its brightness would reach a maximum twice during each spin. This would make it appear to be spinning twice as fast as was the case. However, Dr Gehrels has found the fluctuations so subtle that a roughly spherical shape is probable. At first he thought the spin axis might be pointed at the earth, minimising the brightness changes, but Icarus is now being viewed at a different angle and the variations continue to be moderate.—Copyright, 1968. "New York Times” News Service.