Illustration of Crab, Titan's Shadow and Chandra
In very rare instances, an object such as a planet or satellite of a planet will move in front of, or transit, an extended cosmic X-ray source, such as the Crab Nebula. Under these circumstances, an X-ray shadow of the planet or satellite can be imaged by Chandra. This happened in January of 2003 when Titan passed directly in front of the Crab Nebula. By using Chandra to capture an image of the X-ray shadow cast by Titan, astronomers were able to make the first X-ray measurement of the extent of its atmosphere.
More: A Comparison with Medical X-rays
(Illustration: NASA/CXC/M.Weiss)

Chandra's X-ray Shadow of Titan
A rare celestial event was captured by Chandra as Titan -- Saturn's largest moon and the only moon in the Solar System with a thick atmosphere -- crossed in front of the X-ray bright Crab Nebula. The X-ray shadow cast by Titan allowed astronomers to make the first X-ray measurement of the extent of its atmosphere.
Scale: Image is 19 arcsec per side
(Credit: NASA/CXC/Penn State/K.Mori et al.)

Titan's Path Across the Crab Nebula
This graphic shows how closely to the Crab Nebula's center Titan passed during the transit of January 5, 2003. Although Titan passes within a few degrees of the Crab every 30 years, it rarely passes directly in front of it. This may have been the first transit of the Crab Nebula by Titan since the nebula was formed by a supernova that was observed to occur in the year 1054. The next similar conjunction will take place in the year 2267, so this was truly a once in a millennium event.
More Information on the Crab Nebula
Scale: Image is 2.24 arcmin per side
(Credit: Titan's Path: NASA/CXC/Penn State/K.Mori et al.; Crab Nebula: NASA/CXC/ASU/J.Hester et al.)

Still Images of Titan's Transit of the Crab Nebula
On January 5, 2003, Titan crossed in front of the Crab Nebula, blocking some of X-rays emitted by the Crab. These still illustrations show how the event might look from the point of view of an observer watching the Chandra ACIS detector during the transit. This observer would not, of course, see the optical image of Titan, which is shown for reference. Although Titan passes within a few degrees of the Crab Nebula every 30 years, it rarely passes directly in front of it. This may have been the first transit of the Crab Nebula by Titan since the nebula was formed by a supernova that was observed to occur in the year 1054. The next similar conjunction will take place in the year 2267, so this was truly a once in a millennium event.
(Illustrations: NASA/CXC/A.Hobart;
Crab Nebula: X-ray: NASA/CXC/ASU/J.Hester et al.;
Optical: NASA/HST/ASU/J.Hester et al.; Radio: NRAO/AUI/NSF;
Titan's Shadow: NASA/CXC/Penn State/K.Mori et al.)
View Animation of Titan's Transit of the Crab Nebula

Chandra X-ray Image with Scale Bar
Scale bar = 60 arcsec
(Credit: NASA/CXC/Penn State/K.Mori et al.)