By Steve Campbell April 11, 2020
Comet 2019 Y4 (hereinafter called simply “Y4”) is still approaching the inner Solar System and still being observed. It occurs to me that I have not posted an actual image of this comet, So, that will be figure 1:
I promised a diagram to explain Y4’s path through the Solar System. To quote my previous comment:
“ I’ll include a diagram in the next update. But, for now, imagine yourself as very small and standing on an old-school LP record. The gaps between songs are the orbits of the planets. You are on the third gap and the comet is at the 4th gap but high above the LP. It will come in past all the rings and dive into the record inside Mercury’s orbit (1st gap) on May 30th. It will emerge on the bottom side and make a similar, but mirrored, exit path. It is moving at a tremendous velocity because it has been falling toward the Sun since about 1844. -Steve”
With this verbal imagery and the diagram below (figure 2), I hope the situation is well-explained.
I have plotted the position of the Y4 on March 13th and April 10th. Those dates were chosen from the a magnitude chart because the comets was at the same brightness on both. In the meantime, the comet has moved much closer to the Sun. Please see the magnitude chart below and pick up the story below that.
Comets don’t shine like stars, they only reflect the sunlight that shines on them. Before I go further, please remember that magnitude is a smaller number for brighter objects.
If the comet were unchanging, then the fact that it is closer should mean that the brightness would increase from magnitude 8.7 to about 7.9 (just take my word for that:-). Clearly, something did change, because the comet brightened too quickly and then dimmed again. What happened is why comets are so unpredictable.
The comet body, warmed by the sun will begin to “evaporate”. Frozen gasses are vaporized and form a gas cloud around the “nucleus” and are blown away by the radiation from the Sun and the rush of sub-atomic particles called the “solar wind”. That is what makes the comet’s “tail”. The bigger ball of gas reflected more light and so the brightening.
The dimming? Maybe:
- The vaporizing gas may have “unglued” the body of the nucleus and it started falling apart.
- The stuff beneath the frozen gasses is darker rock and reflects less light.
- The ice that was volatile at this temperature has all vaporized.
What will happen next? Maybe:
- The comet may fall apart completely and disappear.
- Less volatile ices may vaporize when the comet is closer – and therefore hotter – and that will renew the brightness
That last option at least leaves open the possibility that the show is not over.