There are 17000 artificial objects currently being tracked in Earth orbit by the
space surveillance network of the United States, some smaller than 10 centimeters
in size. The orbital data are made public as 'Two-Line Elements' (TLEs). Astronomers
search for natural Near Earth Objects (NEOs): asteroids that could pose an impact
threat to our planet. In recent years these searches have become more comprehensive,
with the most successful current effort being the Catalina Sky Survey (CSS) which
monitors the northern sky from Arizona.
When INTEGRAL changed its orbit in January/February 2015 not everywhere were its
TLE parameters updated. At the end of November 2015, Jonathan McDowell
(Harvard-Smithsonian Center for Astrophysics) tweeted that asteroid-observing
astronomers had been tracking an unidentified object, WJ2B5AC, with a 3831 min
period. It had been found by the Catalina Sky Survey, while looking for asteroids.
It was checked against known satellites using TLEs, but got no match. And thus it
became a candidate space rock (see Jonathan's full story at
"How INTEGRAL was 'lost' and became a space rock".
Shown are the Catalina Sky Survey discovery images of WJ2B5AC, which turned up
in May 2015 in the course of their usual NEO survey, from the 0.7-m f/1.8 Schmidt
telescope. This telescope is located on Mt. Bigelow in the Catalina Mountains just
north of Tucson, Arizona, and had a field-of-view of 2.85 degrees x 2.85 degrees and
a magnitude limit of V~19.7 mag. When detected, WJ2B5AC was passing through the
northern skies at around 21 degrees per day. The 4 images are each 30 second exposures,
and the trailing is clearly seen over the course of the integration.
In June 2015 it was noted, however, that the orbit looked a lot like that of a
satellite with COSPAR identification 2002-048A (or NORAD identification 27540) and C,
i.e., the INTEGRAL spacecraft and booster. And, so, subsequently, it was realized that
the TLEs were in fact not up-to-date, and that they were still calculated using the old
orbit. As of December 2015 reliable TLEs for INTEGRAL were again used. Case closed - but
the episode reminds us that the deep Earth orbit where INTEGRAL resides is still a Wild
West frontier for space tracking.
Image credits: Eric Christensen / Catalina Sky Survey / The University of Arizona
Text credits: Jonathan McDowell / Harvard-Smithsonian Center for Astrophysics
