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Comets ISON & PanSTARRS: Comets in the "X"-Treme
Comet ISON

  • Chandra observed Comet ISON and Comet PanSTARRS when these comets were relatively close to the Earth.

  • The X-ray emission is produced when a wind of particles from the Sun strikes the comet's atmosphere.

  • The Chandra data was use to estimate the composition of the solar wind, finding values that agree with independent measurements.

For millennia, people on Earth have watched comets in the sky. Many ancient cultures saw comets as the harbingers of doom, but today scientists know that comets are really frozen balls of dust, gas, and rock and may have been responsible for delivering water to planets like Earth billions of years ago.

While comets are inherently interesting, they can also provide information about other aspects of our Solar System. More specifically, comets can be used as laboratories to study the behavior of the stream of particles flowing away from the Sun, known as the solar wind.

Recently, astronomers announced the results of a study using data collected with NASA's Chandra X-ray Observatory of two comets -- C/2012 S1 (also known as "Comet ISON") and C/2011 S4 ("Comet PanSTARRS").

Chandra observed these two comets in 2013 when both were relatively close to Earth, about 90 million and 130 million miles for Comets ISON and PanSTARRS respectively. These comets arrived in the inner Solar System after a long journey from the Oort cloud, an enormous cloud of icy bodies that extends far beyond Pluto's orbit.

The graphics show the two comets in optical images taken by an astrophotographer, Damian Peach, from the ground during the comets' close approach to the sun that have been combined with data from the Digitized Sky Survey to give a larger field of view. (The greenish hue of Comet ISON is attributed to particular gases such as cyanogen, a gas containing carbon and nitrogen, escaping from the comet's nucleus.)

The insets show the X-rays detected by Chandra from each comet. The different shapes of the X-ray emission (purple) from the two comets indicate differences in the solar wind at the times of observation and the atmospheres of each comet. Comet ISON, on one hand, shows a well-developed, parabolic shape, which indicates that the comet had a dense gaseous atmosphere. On the other hand, Comet PanSTARRS has a more diffuse X-ray haze, revealing an atmosphere with less gas and more dust.

Scientists have determined that comets produce X-ray emission when particles in the solar wind strike the atmosphere of the comet. Although most of the particles in the solar wind are hydrogen and helium atoms, the observed X-ray emission is from "heavy" atoms (that is, elements heavier than hydrogen and helium, such as carbon and oxygen). These atoms, which have had most of their electrons stripped away, collide with neutral atoms in the comet's atmosphere. In a process called "charge exchange," an electron is exchanged between one of these neutral atoms, usually hydrogen, and a heavy atom in the solar wind. After such a collision, an X-ray is emitted as the captured electron moves into a tighter orbit.

The Chandra data allowed scientists to estimate the amount of carbon and nitrogen in the solar wind, finding values that agree with those derived independently using other instruments such as NASA's Advanced Composition Explorer (ACE). New measurements of the amount of neon in the solar wind were also obtained.

The detailed model developed to analyze the Chandra data on comets ISON and PanSTARRS demonstrates the value of X-ray observations for deriving the composition of the solar wind. The same techniques can be used, together with Chandra data, to investigate interactions of the solar wind with other comets, planets, and the interstellar gas.

A paper describing these results appeared in the February 20th, 2016 issue of The Astrophysical Journal and is available online. The authors are Bradford Snios and Vasili Kharchenko (University of Connecticut), Carey Lisse (Johns Hopkins University), Scott Wolk (Harvard-Smithsonian Center for Astrophysics), Konrad Dennerl (Max Planck Institute for Extraterrestrial Physics) and Michael Combi (University of Michigan).

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.

Fast Facts for Comet ISON:
Credit  X-ray: NASA/CXC/Univ. of CT/B.Snios et al, Optical: DSS, Damian Peach (damianpeach.com)
Release Date  April 18, 2016
Scale  Main image is about 40 arcmin across. (X-ray image: 3.7 arcmin)
Category  Solar System
Observation Dates  13 Oct - 6 Nov 2013
Observation Time  10 hours
Obs. IDs  15673-15675, 16493-16495
Instrument  ACIS
References Snios, B. et al, 2016, ApJ, 818, 199; arXiv:1601.06622
Color Code  X-ray (Purple); Optical (Red, Green, Blue)
Optical
X-ray
Distance Estimate  0.95 Astronomical Units (AU)
distance arrow
Fast Facts for Comet PanSTARRS:
Credit  NASA/CXC/Univ. of CT/B.Snios et al, Optical: Damian Peach (damianpeach.com)
Release Date  April 18, 2016
Scale  Main image is about 2 degrees across. (X-ray image: 9 arcmin)
Category  Solar System
Observation Dates  17-23 Apr 2013
Observation Time  12 hours 30 min
Obs. IDs  15627-15629, 15639-15641
Instrument  ACIS
References Snios, B. et al, 2016, ApJ, 818, 199; arXiv:1601.06622
Color Code  X-ray (Purple); Optical (Red, Green, Blue)
Optical
X-ray
Distance Estimate  1.44 Astronomical Units (AU)
distance arrow
Visitor Comments (1)

I have a theory I'd like to share. Seems comets could be a modular or multifunctional object. Mostly an object that comes to the sun to gather energy and transport it back to its origin. I wonder what is the origin of the different comets. Seems the sun reacts to sun grazers. Not sure if the flare is a protective reaction or a mechanism that charges the comets. considering that it sometimes kills the comet, it is either a defensive feature or the comet was at the end of its usefulness. I have always loved science physics and space. Thank you for sharing informative pictures.

Posted by Gipson on Tuesday, 02.21.17 @ 08:36am


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