"Near-Miss" Space Object remains mysterious

Here is a story I read in our local paper today (Sacramento Bee-
Sacramento,CA): UFO SLIPS BY EARTH,REMAINS A
PUZZLE By David L. Chandler Boston Globe BOSTON -
The unidentified object that hurtled past Earth before dawn on
Thursday remains a mystery, astronomers said Friday, and it
apparently slipped by without them getting a good look at it.
A week ago astronomers had decided that the tiny object, which
passed slightly farther away from Earth than the moon's distance
of 240, 000 miles, was probably an asteroid - a chunk of
interplanetary rock probably less than 30 feet across. But
observations on Monday by European astronomers working in Chile
showed dramatic variations in brightness, which suggest an
irregularly shaped, tumbling object with great variations in its
surface reflectivity, much more so than expected for a natural
object like an asteroid, said astronomer Brian Marsden.
Marsden said Friday that the variation in brightness "means we
are dealing with some very peculiar object. I would say man-made. I
think it would be hard to explain this kind of variation with a
natural object, even a small one."
But Marsden and others are still puzzling over what kind of
man-made object it is.
Marsden said two weeks ago that it was probably the leftover
Centaur rocket used in 1974 to launch a solar satellite called Helios.
But more precise calculations of the object's orbit, based on the
latest observations, show that, traced backward, it never quite meets
Earth as it should if it had been launched from Earth.
There is a possible explanation for that discrepancy, Marsden
said. Centaur rockets sometimes contain leftover hydrogen fuel after
they reach orbit, and this fuel could continue to leak out, perhaps
for years. This could propel the rocket in unpredictable directions.
During its closest approach to Earth on Thursday, the object was
too far south to be visible from anywhere in the Northern Hemisphere.
Astronomers at the European Southern Observatory in Chile tried to get
pictures of it shortly after its closest approach in order to get more
precise data on its exact path and to make further measurements of its
brightness varations.
But as of Friday, they were unable to find any sign of the object
in their pictures.
Because of the difficulty of the observation and the faintness of
the object, astronomers Richard West and Oliver Hainaut at the
observatory in the Chilean Andes continued to examine their images
carefully Friday.
These observations are essential to guarantee the success of a
planned radar observation of the object when it returns to visibility
in the Northern Hemisphere next week. A successful radar observation
"would clinch it" as to whether the object is natural or artificial,
Marsden said.
Because of the uncertain movements of a rocket leaking leftover
fuel, however, it may never be possible to identify it as a specific
rocket.
If it is artificial, but not a known rocket, that might explain
the astronomers' inability to locate it in their pictures, Marsden
joked.
"Maybe that's why they couldn't be found last night," he said.
"Maybe they landed."