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15 Years of SOHO Comet Observations!

On December 2, 1995, the joint ESA-NASA Solar and Heliospheric Observatory was launched from Cape Canaveral, Florida, on a nominal 2-year mission to study the Sun. Hopes are always high for such missions, but getting into space, and keeping satellites healthy out there, is not easy. But SOHO has survived countless particle storms, meteor showers, hardware malfunctions, software glitches, and even freezing almost solid while "lost in space" for several weeks!

Today, on its fifteenth birthday, the ESA and NASA teams are celebrating the many astounding accomplishments of SOHO. It has revolutionized solar physics, and captured the imagination of countless people around the world with amazing images of the Sun and solar events. But here I want to emphasize one of SOHO's other accomplishments -- not one related to understanding the Sun, but one that is an unintended consequence of SOHO having a unique view of the solar system: comet discovery!

SOHO is, by far, the greatest comet discoverer in history, with over 1,950 comet discoveries to date. That's more comet discoveries than every other discoverer throughout history, combined! That's right -- well over half of all cataloged comets now bear the name SOHO, mainly thanks to the dedicated group of amateur astronomers who routinely report these objects to me on this website. So in this article I am going to give a run-down of 15 Years of SOHO Comet Observations, and share some of SOHO's "unintentional" successes!

1996 -- The Christmas Comet

Surprisingly, after just over a year of operations, only 6 SOHO comet discoveries had been cataloged. (We now average about 140 new discoveries per year!) The comet shown here is what we affectionately refer to as the "Christmas Comet" (even though it was seen on Dec 23rd, not 25th...). It was a particularly bright member of the Kreutz-group, officially named C/1996 Y1 (SOHO), in the LASCO C2 field of view. The LASCO C2 and C3 cameras are responsible for nearly all of SOHO's comet discoveries.

1997 -- The first of a new group

The tiny little object pictured here -- C/1997 L2 (SOHO) -- isn't much to look at, but was an important find. It was the first discovered member of the "Meyer" group of sungrazing comets. The Meyer group is named after comet-hunter and scientist Maik Meyer, who first identified a handful of SOHO objects as being related to each other. This was the first of three previously unknown populations (or "groups") of comets to be discovered by SOHO.

1998 -- A SOHO comet, seen from the ground

The overwhelming majority of SOHO's comets are only ever seen in our images, and typically get vaporized a few hours after discovery. So we get very excited when one is observed through an Earth-based telescope, and particularly excited when it looks like this one! C/1998 J1 (SOHO) was a beautiful comet that was estimated to be as bright as magnitude 0 when it was near the Sun. This is probably SOHO's most 'picturesque' comet discovery to date.

1999 -- SOHO's first major milestone

It's hard to believe, but it took over four years for SOHO to reach 100 comet discoveries. At the time, only a small number of people were looking, and they were not looking very hard, though word was starting to spread of SOHO's comet-finding abilities. This tiny object -- another Meyer-group comet -- was SOHO's 100th discovery.

2000 -- Another new group

Another small, unremarkable object, but still an important one. This is comet SOHO-189, or C/2000 O3 (SOHO), and was the founding member of the "Kracht" group of comets. This group was founded by Rainer Kracht, who recognized several objects in the SOHO data were related. This group, and it's cousin the "Marsden" group (named after Dr. Brian Marsden, who very sadly passed away recently), are SOHO's two other comet populations, and are likely the "children" of the large comet 96P Machholz (see 2002, below).

2001 -- Active Sun, active comet discoveries

2001 marked the year when SOHO comet hunting really started to take off. Word had spread that these images were freely available on the Internet, and the increasing online community soon caught on to this. At this time too, the Sun was peaking in activity, giving us beautiful images, and lots of comet discoveries. This bright Kreutz-group comet was SOHO-367.

2002 -- 96P Machholz: frequent visitor, absent parent

Every now and then, SOHO gets to enjoy a unique view of a large, previously discovered comet making its passage past the Sun. One frequent visitor is comet 96P Machholz. With a ~6yr orbital period, SOHO has seen this comet three times now, but its 2002 passage was the most impressive. The comet is also the parent of the Kracht and Marsden comet groups, and related to at least one metero stream and (maybe) one asteroid. In the picture opposite, you see a large coronal mass ejection (CME) just below the comet. This CME did not hit this comet, but not all comets are this lucky...

2003 -- A very NEAT comet!

This was exciting! A huge snowstorm shut down NASA Goddard facility (SOHO headquarters) but a small group of determined scientists made it there to enable SOHO to take some special images of comet C/2002 V1 (NEAT). Their efforts were not in vain, and we got to witness a large CME swipe through the tail of the comet as it flew by the Sun! This led to some of the very many scientific papers that have resulted from SOHO observations of comets.

2004 -- A pretty little SWAN

This isn't so well known, but not all of SOHO's comets are named SOHO. Some of them are called "SWAN", named after the Solar Wind Anisotropy experiment on SOHO. The SWAN instrument (now no longer in operation) used ultraviolet observations to detect solar Lyman-alpha photons, and was used to detect active regions on the far side of the Sun. But surprisingly, it could also detect bright comets, and actually found eight comets during its lifetime. These comets are still included in SOHO's comet count, even though they are not named "SOHO". Opposite is an image of a SOHO/SWAN comet (C/2004 V13) passing through the LASCO C3 field of view.

2005 -- A major milestone

A small pair of comets, but an important pair. These were SOHO's 999th and 1000th comet discoveries. Both were Kreutz-group comets, found by Toni Scarmato (Italy) in August 2005. We were astounded -- and delighted -- that SOHO had reached 1,000 comets, and we made a point of making a tribute to the many amateur astronomers who made it possible.

2006 -- Yet another milestone

Again, a small, faint comet, but this one was also important -- it was SOHO's 1000th Kreutz-group comet discovery. Over 85% of SOHO's comets come from this group which, prior to 1978, only had a few (admittedly huge) members.

2007 -- A repeat offender

In 2005, comet hunter and scientist Dr. Sebastian Hoenig, published a scientific paper linking two of SOHO's previously seen objects, and making a rather bold prediction that they were the same object and would return on September 11, 2007 at ~06:18UT. (The prediction was bold because it is extremely difficult to make good orbital predictions from the SOHO data.) Well, Sebastian was wrong... It returned on September 10, instead! It returned on September 11,2007 at ~07:40UT, instead! This object became SOHO's first officially periodic comet -- P/2007 R5 (SOHO).

2008 -- Many eyes in the sky

In late 2006 we launched the NASA STEREO mission -- a pair of spacecraft that watch the Sun from opposite sides of the solar system. STEREO does not discover, or see, quite as many comets as SOHO does, but it does see the brighter ones and allows us -- for the first time -- to simultaneously observe a single comet from multiple locations in space. (This makes a huge difference when it comes to computing orbits for these objects.) Opposite is SOHO-1476, another bright Kreutz comet, seen by six telescopes on three different satellites.

2009 -- Another group reaches 100

Small comets can be important too! The tiny thing opposite is SOHO-1619 aka C/2009 E3 (SOHO), is the 100th member of the Meyer-group of comets, and was found by a Polish high-school student. (Remember: anyone, anywhere, can hunt for SOHO comets!) The Meyer-group is poorly understood. It's probably periodic, but that orbital period could be decades or centuries -- we don't know. There is also still no known "parent" for this group, making it quite intriguing.

2010 -- Many bright comets

This year has been a strange one for SOHO comets. We haven't had any really major milestones or discoveries, but we have seen what appears to be an unusual number of bright Kreutz-group comets. There is speculation that an increase in bright Kreutz could be a precursor to a 'great' comet heading into the solar system. It's just speculation though. This comet is probably the pick of this year's bright Kreutz, noteable mainly for its "forked" tail. This one was actually discovered by STEREO, but of course observed by SOHO, too. The image opposite is from the STEREO/SECCHI COR-2 instrument.

So that's where we currently are, and all eyes now turn to the future. We don't know how long SOHO will keep going (it is old!), but we hope it has many more years to come. And we do have at least one upcoming highlight to look forward to: we are only weeks away (by my estimate) from SOHO's 2000th comet discovery! So stay tuned for all the latest SOHO comet discoveries and developments.

Oh, and Happy Birthday, SOHO!