This is a sequence of images obtained at the
telescope at the
Kitt Peak National Observatory,
by Paul Smith
and Gillian Rosenstein, on the night of Thursday March 12th 1998 when the
asteroid was about 150 million miles from Earth (and somewhat over
190 million miles from the Sun).
As you step through the animation, you may notice two separate features. The second frame, just after the asteroid passes the central bright star, has slightly different brightnesses for some of the stars. Because the frames have all been normalized, this means the relative brightnesses change (for example, the two stars about a quarter of the way up in the middle seem to get brighter). This is because the second frame in the sequence was taken through an I (far red) filter, while the other three used an R (red) filter. The change in relative brightness of some stars in the second frame is therefore an indication of their colours.
Better colour information obtained by imaging the asteroid through different filters may enable the size of the rock to be determined more accurately. The problem is that we cannot actually `see' details on the asteroid at this distance, and so the only way to estimate its size is by its brightness. To do this, we assume that it reflects about as much light as similar objects in the solar system. If 1997 XF11 is actually a much whiter, brighter colour, then it will be smaller for the same brightness: conversely, if it's made of especially dark rock the suggested one mile diameter could be a serious underestimate.
Secondly, although the first three exposures lasted for five minutes each,
the last one lasted for ten minutes, during which the asteroid travelled
twice as far. Thus, the slight elongation along the path visible in
frames 1 to 3 is doubled, and the asteroid shows significant streaking.
The individual frames from the sequence are available at higher resolution: frame 1, frame 2, frame 3, frame 4. All images are oriented in the conventional astronomical sense, with N up and E to the left, and are about 100 arc seconds square.
A different but similar view was earlier obtained at the ARC 3.5m telescope. This asteroid was discovered by Jim Scotti with the University of Arizona SpaceWatch project, and was thought for a while to be passing dangerously close to the Earth in the year 2028. We now know that it won't even come within the Moon's orbit. The collision idea very rapidly led to a great deal of uninformed conspiracy theory speculation, such as this one about a coverup (remember to treat this as amusing and not serious). The reprieve has also generated some interesting non-technical journalism. A full and sensible technical analysis is available from NASA's Jet Propulsion Laboratory.
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Last updated: 18 March 1998