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Saturday, March 23, 2013


Comet PanSTARRS Does Not Dodge a Coronal Mass Ejection

Comet C/2011 L4 PanSTARRS on 15 March, 2013 as seen from the STEREO H1B imager showing the comet and a billowing Coronal Mass Ejection. It looks like the CME just misses the comet, but this is an illusion due to the spacecraft's perspective, the comet and CME are nowhere near each other. Click to Embiggen. Location of the comet in respect to the Earth and the STEREO B spacecraft simulated in Celestia (click to embiggen). My STEREO orbit files are a little old, so it's position is not exact, but it's not very different from the real position. The CME of 15 March was Earth directed and I've indicated it on the chart. You can see it goes nowhere near the comet.

There is a stunning video from the Sungrazer Comets group that shows comet C/2011 L4 PanSTARRS for the 9th to the 16th as it passes through the field of view of the SETREO B spacecraft's H1 imager. On the 15th, there was a large coronal mass ejection which produced lots of aurora here on Earth on the 17th and 18th. You can see the comet glide past the CME in stunning detail in the video (my less awesome effort is below).

Some commentators on the video around the web are saying the comet "dodged the CME" (in the sense that it just missed the comet). But that,s an illusion of perspective. The spacecraft is is almost behind the Sun from the Spacecrafts perspective looking back at Earth. The CME was Earth directed. fanning out to be sure but faning AWAY from the spacecraft. And the comet, which is above the plane of the Sun (not shown in the Celestia perspective) and over 90 degrees away from the CME Earth axis.

Occasionally someone will claim that comets cause CME's so it's useful to try and think about the geometry and timing of these events. In this case, the CME occurred 5 days after the closest approach to the Sun, and was directed away from the comet.

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