An icy visitor from outer space
posted: February 25, 2015

After the M2 flare on 9 February, the Sun decided to take a break, with only low-level C-class flares during the subsequent two weeks. In fact, for 5 consecutive days (from 13 till 17 February), not even a C-class flare was registered, as can be seen in graph underneath.

So, there seemed to be very little of interest to write about, but fortunately the folks at SOHO alerted the community of a tiny comet passing through the field of view of SOHO's coronagraphs, see their pick of the week. A coronagraph is an instrument that blocks out the solar disk so that the immediate vicinity around the Sun becomes visible, similar to what our Moon does during a total solar eclipse. Though SOHO's coronagraph is used to observe coronal mass ejections (CME), it has also made SOHO the most successful comet hunter so far. Indeed, the newly found dot is the 2875th comet on SOHO's very long list of comet discoveries. Quite a few of those were actually made by amateur astronomers who were examining SOHO's imagery.

As the comet SOHO-2875 was approaching the Sun, it appeared as a fast-moving starlike object on 18 February (see image above on top). It could hardly be distinguished from the starry background, except for its motion of course. Scientists were not expecting much of this ball of ice and dust, until the comet all of a sudden started to display a tiny tail after its swing-by of the Sun. Astronomers are now eagerly awaiting its appearance in the evening sky to see if it can be observed by telescope.

What seems already certain is that -unlike most of the comets discovered by SOHO- this one does not belong to the Kreutz group. This is a group of comets that all follow pretty much the same orbit that brings them very close to the Sun ("sungrazers"). It is believed that they are all fragments of a comet that broke apart several centuries ago. Most of the Kreutz members do not survive their kiss and ride with the Sun, comet Lovejoy (2011) being a notable exception (see sketch of orbit below). That SOHO-2875 does not belong to the Kreutz group is apparent from its motion. The Kreutz members all pass most of their orbit below the ecliptic (the plane created by the yearly movement of the Earth around the Sun), which make them also rather insensitive to gravitational perturbations from the planets. Only during their close encounter with the Sun do they briefly stay above the ecliptic. The completely different orbits can be seen in this movie where the motion of SOHO-2875 is compared to that of comet Lovejoy.

SOHO-2875 seems also to have been just in time to avoid being hit by 2 CMEs. In particular the second CME on 21 February was quite strong and speedy (average plane-of-the-sky speed of about 900 km/s). It originated from the backside of the Sun and concerned the eruption of the very long filament that was visible from Earth just 2 weeks ago, as discussed in this news item. The image underneath (as well as in the movie) is a false color clip showing the comet to the left and the CME from the filament eruption expanding to the lower right. The CME did not have an Earth directed component.

Credits - Data and imagery for the movie clips were taken from SDO, SOHO, NASA/JPL/NEO/Solar System Dynamics, and (J)Helioviewer.