The SOHO/LASCO coronagraph has enabled the discovery of more than five throusand near-sun comet! However most are tiny meter-sized sungrazers that quickly fall apart just before perihelion. As such, they are almost never recovered from Earth. There are however exceptions, as was the case of C/2015 D1 (SOHO)! Indeed, although the comet ultimately fell apart, it made it passed perihelion and could be tracked over over a week as it moved into darker skies. In fact, beautiful images were obtained of the remaining dust cloud before it faded and dissipated beyond detection.
Fig. 1: Animation of comet C/2015 D1 (SOHO) during its transit across SOHO/LASCO C3 images. Note that at the beginning of the animation the comet appears rather condensed, while it appears significantly more elongated as it exits the FOV. Image credit: ESA/NASA SOHO/LASCO C3.
C/2015 D1 (SOHO) was a relatively bright non-group comet found by Worachate Boonplod (Thailand) in SOHO/LASCO C3 images of February 18th, 2015. When discovered, the comet was near the (northern) edge of the C3 FOV, shining faintly at around mag +10, and brightening (Fig. 1). Hence, it was clear that the comet would probably become a relatively bright object. Indeed, over the course of the next 24 hours, the comet brightened considerably, shining at around mag +5 by the late hours of February 19th. In regards to its morphology, it displayed a condensed appearance with a very faint tail. Many expected the comet to disintegrate, as is the fate of most SOHO comets.
As the comet entered the C2 FOV, it remained condensed, still displaying a faint tail opposite its [rapid] motion. However, rather than fading, it rapidly brightened as it cruised past perihelion. In fact, the comet’s brightness saturated the C2 imager (Fig. 2), suggesting that it had probably reached mag +2. It hds also developed a faint (curved) tail that was progressively seen growing (possibly, in part, due to the angle of observation).
Fig. 2: SOHO/LASCO C2 image extract showing C/2015 D1 (SOHO) on February 19th, 2015, only an hour before it left the FOV. Notice the saturation spikes, indicative that the comet probably reached was shining around mag +2 in this image. Image credit: ESA/NASA SOHO/LASCO C2.
During this period, the comet had also picked-up in two images by the STEREO/SECCHI COR2-A telescope (see Fig. 4). Unfortunately, due to the low quality of the images at the time (side-lobe operations), these could not be used to further contribute to establishing better orbital elements.
Fig. 3: One of two STEREO/SECCHI COR2-A image showing C/2015 D1 (SOHO). The images were taken during side-lobe operations hence the low resolution of this image. Als note Jupiter to the left of the Sun, and a massive CME. Image credit: NASA/SSC STEREO/SECCHI COR2-A.
The comet faded slowly, and left C3 on February 21st, 2015. At first glance, it seemed relatively “healthy”, although it was noted that it did appear slightly more elongated (Battams et al., 2015). Amazingly, it only took four days before ground-based instruments caught this small comet. The first to recover it was Justin Coward (USA) using where they noticed that it appeared only as a faint elongate “smudge”, clearly indicating that the comet had fully disintegrated inbetween the time it left SOHO/LASCO. As a consequence of its disintegration, the comet was not recovered in SOHO/SWAN.
Fig. 4: Extract of Justin Cowart’s recovery image of C/2015 D1 (SOHO). This was the first ground-based image of the comet. (c) Justin Cowart.
The remaining cloud that was once comet SOHO kept dissipating and fading, as is seen when comparing Michael Jäger’s images from February 28th and March 7th, 2015 (Fig. 5 and 6). The fact that this comet quickly fell apart is indicative of C/2015 D1 being an intrinsically faint comet, much like nearly all near-sun comets.
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