The sungrazer, will it shine or shatter?

Here is a slightly improved version of our space view of the extraordinary path of comet C/2024 S1 ALAS.

You may have to look closely (see the end note about how to enlarge images) to make out the tiny yellow dot of the Sun at true scale, and, in red, the comet’s path hooking tightly around the Sun on a single day: October 28.

In the name by which we have to refer to thus comet, “ATLAS” means that its discoverer was the team operating the Asteroidal Terrestrial-impact Last Alert System’s robotic telescope at Haleakala Observatory in Hawaii, on 2024 Sep. 27. And “S1” means that it was the year’s first discovery of a comet of the kind classified, because of its orbit, as “sungrazing.” (The punctuation of this term is debatable.)

Suppose that a chunk of solid matter, far out from the Sun in a vast slow orbit like millions of others, comes to a standstill. It can’t just do that – can’t say “Stop the bus, I want to get off!” – it has to have felt a gravitational nudge, perhaps from a star passing relatively near. Coming to a standstill means it has no forward motion to keep it in orbit. It has to drop toward the Sun. With relentless increasing speed. Many years later, at colossal speed, it plunges into, and becomes part of, the Sun.

If it was initially slowed rather than brought to a standstill, it does not go into that straight-line drop but curves into a parabolic or elliptic orbit, like the many other comets that have come down among the planets. If it was slowed very close to a standstill, it drops almost straight but not quite. A path like a long thin pencil, just missing the Sun’s surface, whipping around through a perihelion – nearest-to-Sun point – on the farther side, and back out. Back out, if it survives.

Sungrazers of the past have been among the few most spectacular of all comets: bright enough to be seen in the daytime sky, long backward-curving tails driven outward from the Sun. But the cause of this – the close rush past the Sun – can also cause the comet’s tiny solid body to disintegrate.

C/2024 S1’s orbit has a calculated perihelion distance of 0.0082 AU (astronomical units, the Sun-Earth distance). That is about 1,230,000 kilometers.  The Sun’s radius is about 696,000 kn. So the comet will skim about 530,000 km above the Sun’s surface.

The comet does literally “drop” in the gravitational field toward the Sun, like a stone dropping down a well.  I thought of trying to work up a version of the diagram with south up, so that the dropping motion would look more familiar to us – but, enough.

The comet’s motion relative to the ecliptic is obliquely northward and then southward. Now here is how it translates into motion on our (equatorially based) map of the sky: a long sweep eastward in September and October, then back westward in November and December.

This comet is south of the ecliptic plane – therefore south of the Sun – for all but a few hours on a single day. So, not surprisingly, people on the southern part of Earth are going to have better chances of seeing it.

Here is a pre-dawn scene for a southerly location (an approximation of Australia and New Zealand) on October 12.

The arrow through the comet shows its motion, relative to the starry background, from 2 days earlier to 2 days later.

It would be interesting to hear from friends down-under whether they can find the comet, whether they can estimate its magnitude, and whether it has a discernible tail. Much about it is still uncertain!

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This weblog maintains its right to be about astronomy or anything under the sun.

ILLUSTRATIONS in these posts are made with precision but have to be inserted in another format. You may be able to enlarge them on your monitor.

One way: right-click, and choose ”View image” or ”Open image in new tab”, then enlarge. Or choose ”Copy image”, then put it on your desktop, then open it. On an iPad or phone, use the finger gesture that enlarges (spreading with two fingers, or tapping and dragging with three fingers). Other methods have been suggested, such as dragging the image to the desktop and opening it in other ways.

Sometimes I make improvements or corrections to a post after publishing it.  If you click on the title, rather than on ‘Read more’, I think you are sure to see the latest version. Or, if you click ‘Refresh’ or press function key 5, you’ll see the latest version.