Is this the end of comet ISON?
Comet ISON has failed to survive its perihelion intact? Yes, but something is still there.
A few signs of unhealthy behaviour were observed in the days before it made its closest approach to the Sun. Being so close to the Sun must have been very “stressful” and what is left (see image) could disappear to our eyes very soon. For the time being, let’s keep watching.
Gas emission from comet ISON dropped dramatically, however the nucleus was still intact. Astronomers working with the IRAM telescope (Spain) found a drop in hydrogen cyanide (HCN, a molecule often found in comets) production from November 19 to November 25 by factor of 20. The comet then started to fade dramatically. A new orbit with a large non-gravitational forces was published by the Minor Planet Center. This change in orbital parameters results from the strong effect of jets of outgassing material on the small nucleus of a comet.
ISON started to brighten again. Later on, the IRAM telescope detected marginal HCN emission coming from the expected position of the nucleus; in 24 hours the comet brightened by a factor of 40 on images taken by the STEREO-A and SOHO spacecrafts. Still, no ion tail was formed again, confirming that the large contribution to total brightness was due to dust. Another feature that appeared only at this time was a thin tail moving along the comet’s orbit which was identified as a synchronous feature. The fast analysis of Z. Sekanina noted that large dust grains of this feature were released at about a distance of 20 AU from the Sun (where 1 AU is Earth’s distance from the Sun).
Apparently the large dust production during yesterday’s outburst was the product of the terminal disruption of the entire remaining nucleus and nearly 10 hours before perihelion passage the comet started to fade dramatically. During just 6 hours the comet faded by a factor of 40 and its head disappeared. Extensive heating by the Sun caused evaporation of small dust grains, but some large debris survived closest approach and appeared again on the other side of Sun as a diffuse, thin, bright line. Is a small nucleus still there? Likely yes. We’ll have to wait and see, but the chances of a spectacular show in the sky are very small.
This remnant is visible to telescopes and is offering a great opportunity for astronomers to study it in detail. These follow-up observations may also be crucial to precisely identify the time and the size of a destructive event that happened to the original nucleus of this comet. The remnant is only visible from the northern hemisphere.
Sadly, Comet ISON will likely fail to become a ‘great’ comet, but recently, another comet was discovered – C/2013 R1 (Lovejoy). A few days ago it reached its minimal distance to Earth and in the following weeks it will pass its perihelion. Now it is already visible to the naked eye from dark sites. The comet should remain visible to the naked eye at magnitude 5 through to the second half of December. It is visible only to northern hemisphere observers and on images it shows an almost 8° long gas tail.