Finding comets on SOHO images


Updated 2017 January 16






This tutorial relates to the presentation on the same subject.






SOHO, the Solar & Heliospheric Observatory, is a project of international collaboration between ESA and NASA to study the Sun from its deep core to the outer corona and the solar wind. SOHO was launched on December 2, 1995. The SOHO spacecraft was built in Europe by an industry team led by prime contractor Matra Marconi Space (now EADS Astrium) under overall management by ESA. The twelve instruments on board SOHO were provided by European and American scientists. Nine of the international instrument consortia are led by European Principal Investigators (PI's), three by PI's from the US. Large engineering teams and more than 200 co-investigators from many institutions supported the PI's in the development of the instruments and in the preparation of their operations and data analysis. NASA was responsible for the launch and is now responsible for mission operations. Large radio dishes around the world which form NASA's Deep Space Network are used for data downlink and commanding. Mission control is based at Goddard Space Flight Center in Maryland.


As shown in Figure 1 below the spacecraft is located at the First Lagrangian Point (L1) approximately 1.5 million km from Earth in the direction of the Sun.



Figure 1, Location of the SOHO spacecraft


Of interest to comet hunters are the two Large Angle and Spectrometric Coronagraph Experiment (LASCO) cameras, C2 and C3. There was a third, C1, but this ‘died’ in 1988 when contact with SOHO was lost for a while.



Figure 2. C2 image, 1.5 to 6 solar radii



Figure 3. C3 image, 3.5 to 30 solar radii


Sungrazers, etc.


There are four groups of Sungrazing comets;

- Kracht, Marsden and Meyer groups, sungrazers, proper, have perihelion distances <2 solar radii and do not survive perihelion passage

- Kreutz group, sunskirters, have perihelion distances between 6 and 12 solar radii and do survive perihelion passage

- Sunplungers, an informal term for those comets which, as the name suggests, do actually collide with the Sun.


Returning comets will therefore belong to the Kreutz group and it is believed that this group are fragments of  a comet first observed in 214 BC. A paper by Zdenek Sekanina and Paul Chodas can be found at


Comet hunting


The homepage of the Sungrazer Project is here and the “Official Guide” to SOHO Comet Hunting is here.


The basic steps are;

a) knowing where to look

b) knowing what to look for

c) finding a comet

d) measuring its coordinates

e) checking for previous observations

f) reporting observations


(a) Knowing where to look


The tracks of sungrazer, etc comets vary depending on the time of year. The tracks can be seen at (with an example below each web address);

Figure 4. Kreutz group tracks for June

Figure 5. Meyer group tracks

Figure 6. Meyer group tracks for January


b) Knowing what to look for


To get a good idea of typical comets it is useful to look at recent reports and then call up the relevant images – these comet are mostly quite faint and therefore forewarned is forearmed.


Recent reports can be found here (or linked to from the Sungrazer Project page). A typical report is shown in Table 1


Jun 17 2014 13:01:38

Peiyuan Sun



Further to my post of...
Images: C2; 1024x1024 images.
(0,0) Upper Left.
20140617     11:58:36
Kreutz group comet.
1604  873  977
1635  872  958
1648  870  951
--Peiyuan Sun


Jun 17 2014 16:40:10

Rob Matson



Confirming comet of:
Images: C2; 1024x1024 images.
(0,0) Upper Left.
20140617     11:58:36
Kreutz group comet.
--Rob Matson


Table 1. Discovery report and subsequent confirmation.


Figures 7 and 8 are stills from the movie showing the above reported comet which can be accessed as follows;

From the Sungrazer Project page;

- under Images and Other Links select Real Time images

- under Real time Images select Search and Download Data

On the Search and Download SOHO Near Realtime Data page input the following;

- Image type, LASCO C2

- Resolution, 1024

- Display, Movie

- Start Date, 2014-06-17 (leave End Date and Latest n images blank)

- Search

When the movie has loaded;

- click on Stop

- Enter 72 in Frame Start and 74 in Frame Stop

- click on Start (use the various buttons to vary the image rate, reverse, start, stop, etc)


Figure 7. Screen shot of  comet in LASCO C2 image (see Figure 7 for enlargement of bottom right-hand corner of image)


SOHO comet

Figure 8. Portion of LASCO C2 image – position of comet is circled.


Images can be loaded into any software package which will allow coordinates to be measured. I have used Astroart which measures from the bottom left – Figure 9.


Figure 9. LASCO C2 image loaded into Astroart for measuring position of suspect comet.


Discovering a comet


Using latest images


From the home page, under Images and Other Links, select Real Time Images.

On The Very Latest SOHO Images page under Data/Archive, Real Time Images select ‘Search and Download Data

On the Search and Download SOHO Near Realtime Data page input the following;

- Image type, LASCO C2 or LASCO C3

- Resolution, 1024

- Display, Movie

- enter number of images required in Latest n images box

- Search


If you think you have discovered a comet measure its position on at least five images and report it as described in ‘Reporting a potential discovery’ below


Using the web based tool


From the home page select Report a SOHO Object and select ‘Click here to use a web based tool to measure positions on real-time movies’ to bring up the page shown in Figure 10.


Figure 10. Select a LASCO/EIT Real-Time Movie


You will need to enable the Java Console which (in Windows XP) is done from Control Panel/Java/Advanced tab/Java console/enable Show console button.


To display the latest 25 images select;

- File Type JPG (1024 x 1024)

- On-Screen Controls, Yes

- Camera: C2 or C3

- Submit


Measure the positions of the suspected comet on at least 5 images as described below the LASCO C2 or C3 images. Your measurements will be listed in the Java Console window, Figure 11, and can then be entered in the on-line form at the bottom of the Report a SOHO Object page


Figure 11. Java Console window


Reporting a potential discovery


Once the positions of the comet in at least five images have been measured a report can be made using the on-line form at the bottom of the Report a SOHO object page – Figure 12. It is recommended that you check Recent Reports and report accordingly e.g. Potential comet or Confirming comet of… if  verifying a previous observer’s report.


SOHO report

Figure 12. Example partial report


and finally… I came, ISON, I was no more


Figure 13 below shows the fateful perihelion passage and movies of that event can be seen here



Figure 13, Comet ISON images by the SOHO LASCO C3 camera