Discovering SOHO comets
Can anyone discover a SOHO comet?
What software do I need?
Most standard photo editing software is fine. You need the ability to display gif images and find the pixel value of any given point in the images. Software that allows you to create an animation and 'flip' through images, is a bonus. If you don't have the software, you can use the java tool on the LASCO javagifs page to measure positions! Nearly all (over 99%) of SOHO's comets are discovered in LASCO C3 or C2 images. Sometimes, bright comets can be seen in both C3 and C2 images. A very small number of comets have been discovered in SWAN images, but these are extremely difficult images to interpret, and are best left to very experienced astronomers!
Where do I find the latest images?
The best place to look for individual images is here on SOHO's homepage. Latest images and movies are also available from the LASCO site. This page has great java movies and a java tool which you can use to measure positions!
I think I have found a comet! How do I report it?
First, make sure it fits all the requirements for being a comet:
- It is visible in at least five consecutive images.
- It moves in a straight line, at a constant speed.
- It is not a star or a planet! Compare the motion of your object to that of the background stars -- if it moves in the same direction as the stars, and at the same speed as the stars, it is a star! Planets are usually much brighter than the stars, but always move roughly horizontally, from left to right, or right to left.
How do I measure the positions of my object? What is the (0,0) position?
The comets are reported in terms of their x,y position in the images. In many photo editing programs, you can often see this by just moving the cursor over the image -- the software displays a pair of numbers in the lower-left corner of the window -- the first number is the x-position, the second number is the y-position. So to measure the positions of your object, just place the cursor over it and read off the numbers. Note: we also need to know your (0,0) position, or origin. This differs depending on the software you are using, but it is easy to find out. Place your cursor in each of the four corners of your images -- which ever corner has an x,y value of 0,0 (or close to 0,0), that is your (0,0) position! In most Windows applications, this will be the upper-left corner.
When will my claim get checked?
The time varies. The sungrazer webmaster will take anywhere from a few hours to two weeks to check claims. Be patient! Your claim will get checked! When the webmaster has checked all the claims, they will post a message to the reports page in red font color. If your comet is not listed there, your claim has been denied.
If I find a comet, do I get to name it?
Sadly, no. Only discoveries made from the ground are named after the discoverer. Discoveries of objects made by satellites are named after the satellite itself. Comets discovered by SOHO are first given an unofficial naming by the sungrazer webmaster, eg SOHO-900. This name is purely to make it easy to refer to certain SOHO comets, and to keep track of the number of SOHO comets discovered. The official name of the comet is designated by officials at the Central Bureau for Astronomical Telegrams (CBAT). An example of an official SOHO comet name is C/2004 H5 (SOHO).
Is there a guide to comet hunting?
Yes! It is a new addition to the website and can be found here.
The LASCO images
What are the white dots I see moving from left to right?
They are stars. Stars always move horizontally from left to right in a straight line. Planets occasionally go right-to-left. Check the transits page for a list of expected planet transits in the LASCO field of view (FOV).
What are the white streaks I can see on the images? Are they comets?
No, the white streaks are caused by tiny particles (cosmic rays) hitting the camera. Some examples of cosmic rays can be seen in this image.
Why aren't the images up-to-date? What is a 'keyhole period'?
We are often not in continuous contact with SOHO, so you will often see that the images have not been updated for some time. The schedule of spacecraft contact can be found here. Every few months, we go into a 'keyhole' period, in which we have very limited contact with SOHO. It is during this time that SOHO performs its roll manouver -- this is necessary to keep SOHO's high-gain antenna pointed towards Earth. See this page for more details on the keyhole and SOHO's roll.
What is the definition of a sungrazing comet?
There is no definition of a sungrazing comet. All of the comets discovered by SOHO pass close to the Sun, and could be regarded as "sungrazers", but the Kreutz group of comets are the definitive sungrazers. They get as close as 50,000Km from the Sun's surface (the 'photosphere')... an encounter they rarely survive!
How big are the SOHO comets?
The exact answer to this is not really known, but researchers have estimated that the Kreutz-group comets seen in SOHO images are typically a few tens of meters in diameter.
Are the SOHO comets visible from the ground?
Mostly no. There have only been five comets discovered by SOHO that have later gone on to be visible from the ground. Most of the comets are far too small and too close to the Sun to be seen from Earth. (NOTE: Never point a telescope or binoculars at, or near, the Sun without using a special solar filter! Failure to do so will almost certainly result in blindness!)
Why don't all of SOHO's comets have tails?
Actually, most of SOHO's comets do have tails. It's just that the comets are so small, their tails do not show up very well in the images. Upon close inspection, most of the Kreutz-group comets do look slightly elongated, even if they don't have an obvious tail.
Do the SOHO comets hit the surface of the Sun?
None of SOHO's comets are known to have hit the surface of the Sun. The majority of the comets we discover, namely those belonging to the Kreutz group, are believed to burn up in the solar atmosphere, instead of hitting the Sun's surface. This is not to say, however, that Kreutz-group comets do not hit the Sun. It is theorized that a very small number of Kreutz comets may have approched close enough to the Sun to strike its visible surface (photosphere). However, the uncertainty in the orbit of the Kreutz comets means we have no conclusive proof of this.
Do the comets cause coronal mass ejections (CMEs) or flares?
It has been suggested that comets "hitting the Sun" cause solar flares and CMEs. Indeed, if you watch movies of many of our comets, you will see that they do indeed coincide with CMEs. However, there is no relationship between the two, and it is purely coincidence that we see this. Around the peak of the solar cycle, we often see ten or twenty CMEs per day, and on average SOHO discovers a new comet once every three days. So it is really no surprise that we frequently see the two occur at the same time. It is also important to keep in mind the sizes of the objects involved. The Sun is enormous, and SOHO comets are tiny (tens of meters). A few simple calculations can show that a SOHO comets striking the Sun is, dimensionally speaking, approximately the equivalent of a tennis ball striking Australia... and the tennis ball has the density of a soft scoop of ice cream and Australia is at least thousands of degrees Celcius! The tiny comets are of no significance to the Sun, and would plunge in completely unnoticed (and would likely evaporate long before reaching the surface anyway).