THE USM SOUTHWORTH PLANETARIUM
207-780-4249     www.usm.maine.edu/planet
70 Falmouth Street     Portland, Maine  04103
43.6667° N                   70.2667° W
Altitude:   10 feet below sea level
Founded January 1970
Julian date:  2458626.5
            "Even mushrooms inhabit their own universe."

THE DAILY ASTRONOMER
Thursday, May 23, 2019
Binary Asteroid Fly By


And now for the latest news from the inner solar system's energetic
shooting gallery:  a binary asteroid dubbed 1999 KW4 is due to fly close to
our home planet this weekend.  We would have loved to have written
"perilously close" because such phrasing sounds much more melodramatic.
However, this double space rock won't venture closer than 3.2 million miles
from terra firma: more than a dozen times the moon's average distance from
Earth. This mile-wide menace from the inky black will prove boringly
harmless on this go-around.    Yet, its passage serves as a wholly unneeded
reminder that our lush sphere is constantly racing through a sparse, but
still somewhat dangerous, debris field:  remnants of the solar system's
most violent youth.

[image: Asteroid_1994_KW4.jpg]
*1999KW4: * No, your screen is not out of focus.  Astronomers using the
Goldstone Radar Telescope captured these images of the binary asteroid that
will "fly by" Earth this weekend.  Note the curling "tail" in each panel:
 that was formed by the asteroid's moon as it raced around its parent body
in a swift, 16 hour orbit.

Discovered in 1999, hence its catchy sobriquet, this asteroid will make it
closest approach to Earth at 7:05 p.m. on May 25th. Even at this position,
it won't be visible to the unaided eye.  After all, it is only about a mile
wide:  not much surface from which to reflect light.  Even its "moon" won't
contribute significantly to its brightness.   Yes, this asteroid has its
own moon: an orbiting body.*   Although it is not the solar system's only
binary asteroid (see footnote), it is one of the few Earth crossing
binaries.   As such, it is also considered a "potentially hazardous
asteroid."   one that could pose a danger to Earth were it to veer too
close.  Fortunately for us, it is not expected to do so.      After the May
25th passage, it won't swoop by for another "near miss" until May 2036.
 While this next rendezvous will draw it closer to Earth, it will still
move harmlessly along without inflicting any damage on this battered planet.

Good news for those afflicted with asteroid anxiety:   after this passage,
no asteroid of comparable size is expected to come this close until 2027.
In that year, the asteroid 4953 will come within 12 lunar distances of
Earth:   hardly a matter to perturb our sleep.





*Binary asteroids are not as rare as one might think.   As of now,
astronomers have found 352 asteroids with 370 known satellites.  Yes, that
means that some asteroids, such as 3122 Florence, are trinary asteroids:
three in a system.