2004 CZ1 (fast moving small Apollo makes close
approach)
The Spacewatch 0.9-m telescope at Steward Observatory, Kitt Peak
picked up 2004 CZ1 as a mag +20 object two weeks before it
was predicted to pass by Earth at about 6 lunar distances.
It was caught in the image above two days before closest approach.
Moving at 38" per minute the 18th magnitude object was imaged by taking a
succession of short (4 second) exposures to keep the image of the minor
planet virtually circular.
20 exposures were then combined into two stacked frames to strengthen
the weak image recorded in each short exposure. All the 'odd' numbered
images (i.e. 1,3,5,7...) were stacked as one frame and all the 'even'
images (i.e. 2,4,6,8...) stacked as a second frame. In doing this the
start times and therefore the position of the object in the two frames are
separated by just 35 seconds of time, which translates to the object
moving 22" of arc in the animation above. The next night it was moving at
52"/minute and was recorded with a set of 2 second exposures, though not
as obvious to see in the resultant stacks.
2004 CZ1 was also noted as being a fast rotator - it could
be seen varying in brightness by 1-1.5 magnitudes within a few minutes,
varying from a fairly obvious 17th mag to not being recorded at all in
frames reaching about mag +18.5, and going though several maxima and
minima within 20 minutes.
2004 CZ1 was last recorded at Tenagra and Camarillo
observatories during the first few hours of 24th Feb 2004, about 12 hours
before closest approach, moving at 56"/minute.
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