4000 Comets for SOHO - Cité de l'Espace

4000 Comets for SOHO

4000 Comets for SOHO

The Solar and Heliospheric Observatory (SOHO) involves the European Space Agency and NASA so that the Sun can be scanned on a permanent basis. Its instruments also help detect the comets which pass near our star.

The operational adventure of SOHO started with its launch in 1995. The following year, the space observatory reached the Lagrange point of gravitational balance L1 between the Earth and the Sun. From there, 1.5 million kilometres from our planet, it scans the Sun and has since surpassed its two-year operational life by far! On 15 June 2020, the data which it collected enabled it to discover its 4000th comet.

Finding Comets by Watching the Sun

Loaded with instruments, SOHO has carried out many measurements and, even if other observatories dedicated to the Sun have since been sent into space, the fruit of this co-operation between ESA and NASA continues its mission. The images from its LASCO instrument (Large Angle and Spectrometric COronagraph) show what happens in the immediate environment of our day star. Very useful for examining eruptions and the solar corona, LASCO also sees the so-called sun-grazing comets, those which during their orbit cross near our star. SOHO has proved itself to be very productive at this little game, since it reached the landmark on 15 June of 4000 comets discovered.

Ce cliché issu de l’instrument LASCO de SOHO montre les 3999ème et 4000ème comètes détectées grâce à l’observatoire spatial de l’ESA et la NASA. Le rond rouge en haut simule la place et la taille du Soleil dans cette image. L’instrument est doté d’un coronographe (rond bleu autour de l’image du Soleil) qui bloque l’éclat de notre étoile ce qui permet de suivre l’activité autour de celle-ci sans éblouir les capteurs. Crédit : ESA/NASA/SOHO/Karl Battams

This image from SOHO’s LASCO instrument shows the 3999th and 4000th comets detected thanks to the ESA’s and NASA’s Space Observatory. The red circle at the top simulates the position and size of the Sun in this image. The instrument is provided with a coronograph (a blue circle around the Sun’s image) which blocks the sun’s brilliance enabling us to follow the activity around the Sun without dazzling the sensors.
Credit: ESA/NASA/SOHO/Karl Battams

The discoveries of comets with SOHO are frequently made by “citizen scientists,” people who voluntarily study closely the freely accessible data of space missions. Thus, Trygve Prestgard noted the presence of this comet, registered as SOHO-4000, on 15 June. A student of Geophysics at the University of Grenoble in France, he knew SOHO’s “meter” was approaching 4000, but he did not think it would be himself who would be lucky enough to happen on this symbolic figure.

The video from NASA’s Goddard Center below brings together “4 of our favorite comets discovered by SOHO.” It concludes with SOHO-4000.


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According to the first estimates, SOHO-4000 has quite a small core of 5 to 10 m wide. As SOHO-3999 and SOHO-4000 approached the Sun only 1.5 million kilometres apart, astronomers do not rule out that they may have been linked until very recently.

Maquette taille réelle de SOHO à la Cité de l'espace de Toulouse (France). Crédit : Olivier Sanguy

A life-size model of SOHO at Cité de l’espace in Toulouse (France). Credit: Olivier Sanguy