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STEREO/SECCHI Comets: The First 5 Years

October 26 2011

Comet McNaught as seen by SECCHI HI-1B on January 11, 2007. This was the very first image we took with this instrument! (Click for larger)
Comprised of two near-identical satellites, the revolutionary NASA STEREO mission launched on October 26, 2006 from Cape Canaveral, Florida, on a Delta II rocket. Shortly after launch, the twin satellites -- Ahead and Behind -- separated from one another, marking the beginning of an epic journey that would take them to opposite sides of the Sun, and beyond. On board each STEREO spacecraft was a suite of telescopes known as the Sun-Earth Connection Coronal and Heliospheric Investigation, or SECCHI. Each of these two NRL-led instrument packages contained one EUV imager, two coronagraphs (COR-1 and COR-2), and two heliospheric imagers (HI-1 and HI-2), and between them all they image the entire heliosphere from the solar surface out to beyond the Earth. That's a pretty large volume of space so, needless to say, we see a lot of stuff going on in our images: solar flares, coronal mass ejections (CMES), planets, stars, asteroids... and lots of comets!

So to celebrate STEREO's fifth "birthday", here's a run-down of the most notable STEREO cometary appearances of the past five years.

2007: And the first thing we saw was...

A consequence of a fortuitous spacecraft roll angle on January 11, 2007, the very first thing we saw when we opened the telescope doors on the SECCHI-B Heliospheric Imager (HI) was the magnificent Comet McNaught. (At first we thought there was something wrong with the camera until realization dawned on me and I excitedly exclaimed to the crowded SECCHI operations room that it was Comet McNaught!) Over the coming days, we followed the comet through the HI-1 imagers on both A and B spacecraft, with the HI-1A instrument in particular capturing what I still claim is the most beautiful sequence of images of a comet ever recorded (30Mb .mov file). McNaught went on to gain fame as one of the few recorded "daylight" comets, visible to the naked eye next to the Sun on a cloudless day. But we will always fondly remember it as the very first thing that we saw!

Bonus comet: For 2007 there has to be an honorable mention for Comet Encke, which while much smaller than Comet McNaught, suffered the terrible fate of having its tail ripped clean off by a passing CME in the HI-1A field of view. Fortunately for Encke, the tail soon grew back!

STEREO's third comet discovery, and one of five found in just five days in February 2008. (Click for larger. Image courtesy of Alan Watson.)

2008: STEREO's First Comet Discovery

We knew prior to launch that STEREO/SECCHI would see and likely discover comets. And indeed that prediction came true when, in February 2008, Australian amateur astronomer Alan Watson discovered a previously unknown Kreutz-group comet in the SECCHI HI-1A images. Within five days, a further three Kreutz-group comets were discovered in the data, and STEREO was off the mark. Almost fifty new comets have been discovered in STEREO/SECCHI images since then. This number could have been higher as it certainly observes a large number of Kreutz-group comets, but the SOHO data typically arrives at Earth one to three days ahead of the SECCHI data, and hence SOHO frequently gets the discoveries.

2009: A down-to-Earth discovery

Much like SOHO/LASCO, the vast bulk of STEREO's comet discoveries are only ever seen by the STEREO (and SOHO) satellites. However, in April 2009, Chinese amateur astronomer Jiangao Ruan reported a small comet moving through the SECCHI HI-1B images. This itself wasn't all that unusual, but immediately we knew that this object was not a Kreutz-group comet and was moving away from the Sun, not towards it. I quickly made some measurements and submitted the observations to the Minor Planet Center. Just a few hours after publication, a group of Japanese astronomers turned their telescopes to the sky and, to everyone's delight, they saw the comet (now officially named C/2009 G1 (STEREO)). While never a tremendously spectacular comet, C/2009 G1 did nonetheless grace the southern hemisphere skies for several weeks and was, for a while, the brightest comet in the sky with a peak magnitude of around m9. This comet was not believed to be periodic, so we will likely never see this one again, but it does get to carry STEREO's name back off into the vastness of the solar system.

2010: The tail of two comets...

STEREO-37 and its forked tail in SECCHI COR2-B. This was the first Kreutz-group comet seen to clearly have both ion and dust tails.(Click for larger.)
2010 saw several bright comets, and indeed an historic late-year "comet storm" in the SOHO data, but there are two STEREO Kreutz-group comet discoveries early in the year that stand out in particular. The first was STEREO-23, discovered on January 1st in the SECCHI HI-1A data by Alan Watson, and was visible in those images for almost two days. This comet was notable as the first space-based Kreutz-group comet ever observed to have an obvious, flowing ion tail. And when I say flowing, I mean flowing! You can see in this movie that as it neared the edge of the HI-1A field of view, its tail grew tremendously and can be seen waving and wiggling furiously as it gets buffered by the solar wind. Movies of the this comet can be downloaded here as an .avi (10MB),.wmv (1.3MB), or .mov (<1MB)

The second comet was STEREO-37, another bright Kreutz-group comet first seen in SECCHI HI-1A. This comet also exhibited an ion tail, but this time we could see it along with the classical dust tail in the SECCHI COR2-B images. Once again, we had never seen this behavior in any of the previous 1,600+ Kreutz-group comets observed by the space-based SOLWIND*, SMM, LASCO or SECCHI missions. This comet was also viewed simultaneously by SOHO/LASCO and the SECCHI COR2-A telescopes, though neither of these two were able to detect the second tail because of the angle at which they were viewing the comet. Movies of this comet are available as a 22MB .mov or a 2Mb .mp4 file.

Comet Elenin (small diffuse blob, center-right, moving left to right) seen from a distance of just over 7-million miles... almost 5 times closer than it would ever get to Earth! (Click for larger.)

2011: The End of the World as we know it? (We feel fine...)

Doomsday loomed this year. A giant comet was going to pass within 35-million miles of the Earth this month (October), become a second Sun in the sky, rain fire and ice upon us all, and set forth a series of devastating earthquakes that would ultimately destroy life as we know it**. However, before raining fire and ice-balls upon us, the interstellar destroyer-of-worlds known as Comet Elenin would fly just 7-million miles from the STEREO-B spacecraft! Fortunately the mighty comet was no match for the delicate STEREO-B spacecraft, and over a period of a few days we successfully rolled the spacecraft and took some eagerly anticipated images of the comet as it raced past us. The data we returned painted a rather less frightening picture or a small, condensed and somewhat diffuse comet that was clearly not living up to the hype. By mid-September, i was fading dramatically and, by the time it reached its closest point to Earth in mid-October, Comet Elenin was little more than an elongated cloud of dusty/icy debris.

* In 1979 P-78/SOLWIND became the first ever satellite to discover a comet, which was spotted by four NRL solar physicists (two of which are still here at the Lab!). SOLWIND also discovered Coronal Mass Ejections.
** I got emails, and/or read forum threads, that threatened these consequences, plus more.