A SOHO and Sungrazing Comet FAQ



I have attempted here to give an overview of SOHO and sungrazing comets in the form of an FAQ. Please e-mail me if you can think of other questions that should be included.




SOHO/Near-Sun Comet News and Views   |  My Search for SOHO Comets   |  SOHO Comet Discoverers' Totals  
  SOHO Comet Links   |  My AstroWeb


The SOHO Mission
What is SOHO?
What is a coronagraph?

The Comets
What is a sungrazer? A Kreutz comet?
What other types of comets are visible in SOHO's images?
Are all "SOHO comets" actually comets (as opposed to asteroids or other bodies)?
Do SOHO comets hit the Sun?
Could a SOHO comet hit the Earth?

The Images

SOHO Comet Hunting
Who finds SOHO comets?
Is it easy to find a SOHO comet?
What equipment do I need to hunt for SOHO comets?



The SOHO Mission

What is SOHO?
SOHO is an acronym for the Solar and Heliospheric Observatory, a joint NASA-ESA mission that was launched on December 2, 1995, from Cape Canaveral, Florida to study the Sun and its vicinity, in particular the structure and dynamics of the solar interior, the heating and dynamics of the solar corona, and the acceleration and composition of the solar wind.

SOHO travels in a halo orbit around Earth’s L1 Lagrangian point. It employs a suite of 11 instruments, including the three LASCO coronagraphs (C1, C2, and C3), EIT (the Extreme Ultraviolet Telescope), the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) spectrograph, MDI, GOLF, and GONG, which measure oscillations of the solar surface caused by the propagation of sound waves within the Sun, SWAN (Solar Wind Anisotropy), UVCS (the Ultraviolet Coronagraph Spectrometer).

SOHO's nominal two-year mission has twice been extended, and is currently slated to run through 2007, to cover a complete 11-year sunspot cycle. Comets have been discovered in the LASCO C2, C3, and SWAN instruments, and UVCS has been important in obtaining the spectra of bright SOHO comets.

What is a coronagraph?
A coronagraph is an instrument, invented by Bernard Lyot in 1931, that blocks the Sun’s light (usually by means of an occulting disk), in effect creating an artificial solar eclipse to allow solar scientists to study the solar corona. It also reveals any stars or planets (provided that they’re bright enough to be detected), as well as comets, in the Sun’s vicinity. ( Here are images of some of the brighter comets seen in SOHO's earlier years.) Stellar coronagraphs, based on the same principle but blocking the light of a star in the night sky, are now being implemented in the search for extrasolar planets.

SOHO has three coronagraphs (the LASCO, or Large Aperture Solar Coronagraph, instruments), ranging the very narrow-field C1 to the 3-degree-wide C2 to the widefield C3, with a 16-degree field of view (an eight-degree radius outward from the Sun). All but a few of SOHO’s comets have been discovered in images from the C2 and C3 coronagraphs.


The Comets

What is a sungrazer? A Kreutz comet?
Though there is no official definition of a sungrazer, it refers to a comet that passes very close to the Sun. It is most often used to describe comets of the Kreutz group, which have included some exceptionally bright daylight comets as well as the stream of tiny cometary fragments found in SOHO. As a descriptive term, a few other comets of very small perihelion distance such as the Great Comet of 1680, which passed a mere 200,000 km from the Sun's photosphere, deserve the title of sungrazer. Non-Kreutz comets found in SOHO are sometimes referred to as sungrazers, though such comets with less extreme orbits are often called near-Sun comets (and occasionally, sunbathing comets).

About 85% of SOHO comets belong to the Kreutz group, a family of comets whose members all travel in similar orbits. It is named for Heinrich Karl Friedrich Kreutz, (1854-1907), an astronomer at the University of Kiel who investigated comets that passed close to the Sun. He noticed similarities in the orbits of comets that appeared in 1843, 1880, 1882, and 1887, and determined that they were probably all once part of a larger body; he suspected that comets seen in 1668 and 1702 also belonged to this group.

One common element of Kreutz comets is that they all come extremely close to the Sun (within one solar radius of the photosphere) at perihelion. The Kreutz group includes some of the most spectacular comets on record, the brightest of which (including the four mentioned in the next paragraph) have been visible in broad daylight when near the Sun.

The Kreutz group consists of two main subgroups. Subgroup I comets, whose primary representative is the Great Comet of 1843, have extremely small perihelion distances, only a fraction of a solar radius from the Sun's photosphere. Subgroup II comets have slightly greater, though still scorching, perihelion distances. Their most notable representatives are 1965’s Ikeya-Seki and the Great Comet of 1882, which are believed to be fragments of the Great Comet of 1106, which split apart shortly after perihelion. (It was after the passage of Ikeya-Seki that Brian Marsden, Ichiro Hasegawa, and Lubor Kresák independently surmised the existence of subgroups; Marsden covered the subject extensively in his 1967 paper on the Kreutz group.) Subgroups I and II are believed to havc formed when a larger comet (the progenitor of the Kreutz group, probably about 120 km in diameter) split, perhaps as recently as the 4th century A.D. Another difference between the two Kreutz subgroups is that their line of nodes, the point at which they cross Earth's orbital plane, are about 20 degrees apart from each other. It is usually not hard to distinguish comets from the two subgroups in SOHO's images, as they travel on different trajectories.

Kreutz comets travel in highly inclined orbits that take them nearly 40 degrees south of the ecliptic plane. Their aphelia are in northern Canis Major, and inbound Kreutz comets will appear to slowly spiral out from Canis Major; comet hunters will often search that area of the sky in hopes of finding one. These comets take between 400 and 1,000 years to orbit the Sun.

The Kreutz fragments seen in SOHO have largely been tiny (as small as 5 meters in diameter), and they have invariably vaporized due to their close passage to the Sun. (The tail of one particularly bright one, a relative bruiser whose nucleus was perhaps 50-100 meters in diameter before it tussled with our star in May 2003, briefly persisted past perihelion.)

What other types of comets are visible in SOHO's images?
About 10% of SOHO's comets belong to the so-called new groups. The Meyer, Marsden, and Kracht comet groups were discovered in 2002, when it was realized that some of the comets that were previously thought to be sporadic were actually related to each other. Recalculating the orbits of some non-Kreutz comets revealed additional members of these new groups. Marsden and Kracht groups are of low inclination, and are related to each other as well as to Comet 96P/Machholz as well as the daytime Arietid meteors. They have recently been shown to be short-period, with orbital periods of 5 to 6 years. Brian Marsden and others have established concordances between recent Marsden and Kracht comets and ones seen half a decade ago, demonstrating that they are quite likely returns of the same objects. Meyer-group objects have very high-inclination orbits (71 degrees), and are presumably long-period. Yet another putative group, known as Kracht2, has just 3 known members. See the Web sites of Maik Meyer, Rainer Kracht, and Jonathan Shanklin for more information on SOHO'scomet groups.

A small number (maybe 5 or 6 a year) of the comets found in SOHO have unique orbits that are not related to any known comet or group. These non-group or sporadic comets can and will appear in any part of the C2 or C3 field of view. One sporadic comet, C/1998 J1 (SOHO), is the only comet found in LASCO to have become readily visible in the night sky. Occasionally, notable night-sky comets will pass through SOHO's field of view, among them Hyakutake, C/2002 V1 (NEAT), and C/2004 F4 (Bradfield).

While the LASCO coronagraphs only cover the Sun's immediate vicinity, SOHO's SWAN instrument images the entire sky in ultraviolet. It can detect the hydrogen envelopes of comets down to about magnitude 10. Five comets have been discovered in SWAN that were visible in the night sky. SWAN is problematic as a comet-hunting instrument due to its poor resolution (no better than 1 degree), which has led to a lot of unconfirmable claims.

Are all “SOHO comets” actually comets (as opposed to asteroids or other bodies)?
Probably, but we don’t know for sure. Most are clearly cometary fragments. Some of SOHO’s Kreutz fragments, and most of the objects belonging to the new groups, appear more or less stellar, with no visual cometary characteristics. Due to the short period of observation of objects in the LASCO field -- from a few hours to a few days at most – and the limited resolution of the coronagraphs, ~11 arc-seconds/pixel for C2, ~54 arc-seconds/pixel for C3, SOHO comet orbits are hard to accurately calculate, and sometimes it is not easy to determine the best of several possible orbital solutions.

The majority of SOHO comets trace arcs that follow very near the nominal paths of either of the two main Kreutz subgroups. It is a safe assumption that SOHO’s Kreutz orbits are at least fundamentally correct. The new comet groups discovered in SOHO images (the Meyer, Marsden, and Kracht groups, and another fledgling group that some are calling the Kracht2 group) are perhaps more problematic; visually, few of them show a tail or even much of a coma. They may be largely burnt out or dormant fragments, or perhaps made of relatively nonvolatile material.

That said, it is generous to consider them comets, as the smallest may be only 5 meters or so in diameter. In an informal discussion at IWCA III, Dan Green said that classifying the ones with no obvious cometary characteristics (apart from following a Kreutz, Meyer, or Marsden orbit) as comets is bending the rules. Doug Biesecker has described SOHO comets as somewhere between small comets and meteor-shower debris. Nonetheless, they can be observed and tracked at a distance of ~1 AU from us, if only in the SOHO images. I look at them as cometary fragments, though some of the brighter, tailed objects could be considered comets in their own right. Only one of the comets discovered in LASCO (C/1998 J1) was also visible in the night sky.

Do SOHO comets hit the Sun?
Kreutz sungrazers are known for their extremely small perihelion distances, which frequently bring them within less than a solar radius of the Sun?s visible surface (photosphere), particularly Subgroup I Kreutz comets, whose brightest known member was the Great Comet of 1843 and which account for about 80% of SOHO's Kreutz fragments. The Sun?s radius is about .0046 AU, so any comet with a perihelion distance smaller than that would collide with the Sun?though much more likely, it would vaporize completely be-fore ever reaching the Sun, its scattered molecules raining down onto the Sun. Based on their published orbital elements, only a few SOHO comets were on Sun-intersecting orbits. One of these was Comet 2006 A5 (SOHO), one of the brightest comets seen in SOHO, which I found on January 3, 2006. Its orbit, as published, gives a perihelion of .0043 AU. It quickly deteriorated during its passage through LASCO C2, and probably vaporized altogether about 90 minutes before its perihelion.)

One of the brightest sungrazers found in images from a pre-SOHO orbiting observatory may have collided with the Sun. Comet expert Zdenek Sekanina, in an article titled "The Path and Surviving Tail of a Comet that Fell into the Sun" that appeared in The Astronomical Journal in 1982, makes a convincing case that Comet Howard-Koomen-Michels, found on images taken by the SOLWIND satellite, collided with the Sun on August 30, 1979. Its reported perihelion distance of .0045 AU would seem to bear that out. The solar corona showed an unusual brightening in the hours after its presumed impact. No other evidence of a comet hitting the Sun has been substantiated.

Could a SOHO comet hit the Earth?
Yes, but they don't present any particular danger. There's always a slim chance that any newly discovered sporadic comet, whether it's found in SOHO or in the night sky, could hit the Earth. The nearest approach by a comet on record was Comet Lexell in 1770, which passed 3.43 million km from Earth. Any Earth-approaching comet found in SOHO would come from a sunward direction and would probably be unobservable through other instruments, and the relatively low resolution of SOHO's instruments would presumably make it impossible to determine in advance whether there would be an impact or a near-miss. Kreutz comets present no danger to Earth, as their orbits are steeply inclined to the ecliptic. The Marsden group is a different matter; a Marsden comet that reached perihelion in May of 1999, C/1999 J6, likely passed just 1.3 million miles from Earth a month later. It is believed to be the same object that returned in November 2004 as C/2004 V9, which allowed Brian Marsden to calculate its orbit with unusual accuracy for a SOHO object. Even had the comet collided with us, it probably would have burned up harmlessly in the upper atmosphere. There is no evidence of any Marsden comets large enough to do us any damage. The daytime Arietid meteor shower, which peaks around June 10, are believed to be related to Marsden comets, Comet 96P/Machholz, and the Kracht group.


SOHO Comet Hunting

Who finds SOHO comets?
Since 2001, the vast majority of SOHO comets have been found by amateurs; the rest were found by SOHO staffers. LASCO images are available for download by the public (at the same time they become available to scientists), primarily at http://soho.nascom.nasa.gov/data/realtime-images.html. Many of those who hunt SOHO comets are dedicated and experienced comet observers (two, Kazimieras Cernis and Sebastian Hönig, have discovered comets in the night sky that bear their names); others do very little night-sky observing and were looking for a hands-on science project when then found out about SOHO comet hunting. Some, including myself, live in large, light-polluted cities and can observe under dark skies far less frequently than we’d like; SOHO helps to satisfy the need to observe (albeit on a computer screen; we get to piggyback on a telescopic “eye”) and take part in a useful astronomical project, trying to discover these tiny bodies.

Is it easy to find a SOHO comet?
Despite the large number of SOHO comets that are discovered (an average of more than 100 a year since the spacecraft’s launch in 1995) and the impressive totals racked up by some experienced hunters, it is not easy to be the first to report one, particularly for newcomers. (Even the most skilled hunters sometimes go for months without finding one.) There is a learning curve to detecting the often subtle signs of a developing SOHO comet (which often doesn’t look cometary by standard measure), and to becoming familiar enough with their appearance and motion to be able to report them before other hunters. (There is a core group of about a dozen skilled and dedicated hunters who report these comets within hours, often within minutes, of when they first become confirmable.) You need to learn to quickly identify a comet and distinguish it from cosmic rays, noise, and other artifacts that may mimic a comet, measure its position (in x,y coordinates) on a Series (preferably a minimum of 4) of images, and then file a report using the report form. Mastering these skills can take many months. Many of the comets, particularly ones in C2, are found in the unprocessed black-and-white images, the first to be posted, which appear on the LASCO/NRL site, and are not archived. Working with them requires that you download the images quickly, or you’ll miss them.

What equipment do I need to hunt for SOHO comets?
Many computer users will have everything they need already. Although a fast computer, a broadband connection, lots of RAM, a huge monitor, and a high-end graphics program are all helpful, comets are occasionally found with quite modest equipment. I found my first with a computer running a 56K dial-up connection, by opening a series of images in separate browser windows and blinking between them; if I found anything interesting, I’d download the images and measure positions in Microsoft Paint. I have found several comets on a laptop. That said, though, better equipment will make your search far more efficient. A broadband connection allows you to download images quickly (and particularly for C2 comets, speed is all-important), large RAM lets you download the large files, keep a graphics program window (or two; one for C2 and one for C3) open, and run other apps without overtaxing your system; the advantage of a large monitor is self-evident. A graphics program should let you quickly and seamlessly flip through a series of consecutive images, and provide a means to measure the x,y coordinates of comet suspects on each image. The pros and cons of different graphics programs have been discussed on the Sohohunter mailing list, particularly in the very early messages. There is even a Java tool available on the LASCO Web site, although few of the regular comet hunters use it, as the movies update less frequently than the individual images, and it lacks much of the flexibility of a standard graphics program.



E-mail to tonyhoffman [at] earthlink [dot] net