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Sky & Telescope News Bulletin - 06/13/97

SKY & TELESCOPE NEWS BULLETIN
JUNE 13, 1997

EARTH's COMPANION ASTEROID

Two interesting asteroids are in the news this week. One of them is
unnamed but designated 3753. Discovered in 1986, this 5-km-wide body
has an inclined and eccentric path that crosses the orbits of both
Earth and Venus. Simulations by Paul Wiegert (York University) and two
colleagues show that 3753 circles the Sun in what's termed a horseshoe
orbit with respect to Earth. It works like this: the asteroid circles
the Sun in just a bit under 1 year. Over time this slight mismatch
allows it to overtake Earth, every 385 years to be exact. But as it
comes near the Earth's gravity adds a little energy, expanding its
orbit. Suddenly circling the Sun in just *over* 1 year, the asteroid
never quite catches Earth and moves away. Then, 385 years later, Earth
catches up, this time *robs* 3753 of a little energy, and drops it
into a smaller, faster orbit.

The asteroid thus appears to be a companion of sorts, under Earth's
control. The only other known example of such horseshoe orbits involve
Janus and Epimetheus, two small satellites of Saturn located just
beyond the ring system. Sometime within the next 100 million years,
however, asteroid 3753 will escape its captivity, either crashing onto
one of the inner planets or being flung away onto a totally new path.
Details appear in the journal NATURE for June 13th.

DOUBLED DIONYSUS?

The second newsy asteroid is 3671 Dionysus, an Earth-crosser in a
3.25-year orbit. When astronomers Stefano Mottola and Gerhard Hahn
(German Aerospace Research, Berlin) observed its rotational light
curve earlier this month, they noticed that every 1.15 days the
asteroid's brightness would suddenly drop by 0.08 magnitude. But the
asteroid itself is known to rotate every 2.7 hours. So they've
concluded that Dionysus probably has a satellite, and we're looking
right along its orbital plane. If they're right, every 28 hours this
little moon either passes in front of or behind the main asteroid.
Mottola and Hahn point out that Dionysus will come within about 17
million km of Earth in early July, when it will brighten to 15th
magnitude. Astronomers around the world are trying to learn more in
the coming weeks, as the asteroid won't be near us again for 13 years.


[snip]

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