Credit: ESA/XMM-Newton/C. Lisse, S. Cabot & the XMM ISO Team
A Halo from Beyond the Solar System
Comets are sometimes called "dirty snowballs" since that's what they
resemble - they are composed of ice, snow, dust and rocks. Surprisingly
these "snowballs" are surrounded by a halo of X-ray producing gas. Since
X-rays usually imply extremely high temperatures, it may be that comets
simultaneously represent some of the coldest and some of the hottest
material in the solar system. These surprising cometary X-ray "halos" were
first
detected by ROSAT, while a Chandra
observation of X-rays around the comet C/1999 S4 (LINEAR) showed
that the cometary X-ray emission was produced by free electrons from the
solar wind that were captured by by cold atoms and molecules from the comet, not by hot gas surrounding the comet. Now an
X-ray halo around the comet 3I/Atlas has been observed by the
XMM-Newton X-ray observatory - an extraordinary observation, since Comet 3I/Atlas is a visitor from beyond our solar system, speeding through our neighborhood at 130,000 miles per hour (the highest velocity ever recorded for a solar system visitor). The XMM-Newton false-color image (shown above) was obtained on December 3 and shows the comet's X-ray halo in red. At the distance of the comet, 5 arcminutes corresponds to about 200,000 miles. Note that the black stripe across the comet is a camera artifact. The
measured spectrum of the X-ray emission and its brightness variations provide a rare glimpse of how the solar wind interacts with the extra-solar gas and dust making up the comet. Comet 3I/Atlas will make its closest approach to earth on Friday, December 19, when it passes within 167 million miles of our planet.
Published: December 15, 2025
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Page Author: Dr. Michael F. Corcoran
Last modified Monday, 22-Dec-2025 11:10:14 EST