APOD: Comet PanSTARRS and a Crescent Moon (2015 Jul 20)

[APOD Robot]

Comet PanSTARRS and a Crescent Moon

Explanation: A comet has brightened quickly and unexpectedly. Discovered last year, Comet C/2014 Q1 (PanSTARRS) is expected to be visible now for a few days to the unaided eye, just after sunset, from some locations. The comet rounded the Sun on July 6 and apparently has shed quite a bit of gas and dust. Today it is now as close as it will ever get to the Earth, which is another factor in its recent great apparent brightness and the large angular extent of its tails. In the featured image taken two days ago, Comet PanSTARRS is seen sporting a short white dust tail fading to the right, and a long blue ion tail pointing away from the recently set Sun. A crescent moon dominates the image center. Tomorrow, Comet PannSTARRS will pass only 7 degrees away from a bright Jupiter, with even brighter Venus nearby. Due to its proximity to the Sun, the comet and its tails may best be seen in the sunset din with binoculars or cameras using long-duration exposures.

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[bkebackstrom]

Why does a comet leave a dust trail in the first place? Are not all the comet's content travelling with the same speed?

[Ann]

Why does a comet leave a dust trail in the first place? Are not all the comet's content travelling with the same speed?

No, not all comets travel at the same speed, no more than all planets travel at the same speed.

So why does a comet leave a dust tail in the first place?

Hubblesite reference desk FAQ wrote:

There are two types of comet tails: dust and gas ion. A dust tail contains small, solid particles that are about the same size found in cigarette smoke. This tail forms because sunlight pushes on these small particles, gently shoving them away from the comet’s nucleus. Because the pressure from sunlight is relatively weak, the dust particles end up forming a diffuse, curved tail.

So the dust tail contains small, solid particles. They are left in the comet's trail, and they remain in orbit around the Sun. The dust tail becomes invisible when the particles become too scattered, but they keep orbiting the Sun. When we experience meteor showers on the Earth, it is because the Earth passes through such the remnants of a dust tail left behind by a comet.

The dust tail is affected both by the motion of the comet and the solar wind from the Sun. It reflects the yellow-white color of the Sun.

A gas ion tail forms when ultraviolet sunlight rips one or more electrons from gas atoms in the coma, making them into ions (a process called ionization). The solar wind then carries these ions straight outward away from the Sun. The resulting tail is straighter and narrower.

The gas ion tail is made of ionized gases, not solid particles. It always points directly away from the Sun. It is also bluish in color.

Ann

[Chris Peterson]

Why does a comet leave a dust trail in the first place? Are not all the comet's content travelling with the same speed?

Well, yes. Mainly. But not exactly.

When a comet gets near the Sun, there's enough energy to volatilize its ices, which results in trapped dust and rock being ejected. This creates a cloud of debris around the nucleus. In the absence of other forces, this cloud would continue expanding outward, generally following the same orbit as the comet, but distorted by the fact that particles drifting outside the comet's orbit would lag, and particles drifting inside the comet's orbit would lead (Kepler's Second Law).

However, there are other forces acting on the ejected material- a number of them. The simplest is radiation pressure. This affects very small particles- typically 1 µm or less- and pushes them outward. This is the dominant force creating a cometary dust trail (which always lags the comet, being in a larger orbit). These small particles are an important source of interplanetary dust. Larger particles, however, are not pushed outward, but rather experience a form of drag called the Poynting-Robertson effect. This results in their moving inward, and eventually into the Sun. But since small particles are much more numerous than larger ones, we mainly see those, and therefore the typical tail.

Quite a lot of larger material stays in nearly the same orbit as the parent comet for many orbits, only slowly dissipating due to the above mentioned forces. This material produces meteor showers in the case of comets which have orbits intersecting Earth's. And as the cometary material dissipates, it regenerates the interplanetary dust cloud (which we observe directly in zodiacal light and gegenschein, in many meteors, and instrumentally in IR light), which is constantly being swept into the Sun or out of the Solar System.

[Mikado]

"Due to its proximity to the Sun, the comet and its tails may best be seen in the sunset din with binoculars or cameras using long-duration exposures."

A typo? Just wondering what is meant by sunset din

[bystander]

A typo? Just wondering what is meant by sunset din

Not a typo. Din generally means a loud, continuous noise. In this case we are talking about light instead of sound. The background "noise" from the light of the sunset make the comet difficult to see.

[starsurfer]

This is a lovely and magical image, quite romantic in some respects. On a slightly related note, not heard from Moonlady for quite a while now?

[LocalColor]

Very nice image. Recently we watched Professor Carolin Crawford's excellent lectur (2013) on comets from Gresham (London.)

http://www.gresham.ac.uk/lectures-and-e ... lar-system

[Pianosorplanets]

Quite a lot of larger material stays in nearly the same orbit as the parent comet for many orbits, only slowly dissipating due to the above mentioned forces.

Do I understand you to be saying that larger fragments can break off a comet and follow it all the way around its orbit possibly numerous times? Wow! That is extremely cool. Thanks for sharing that one!

[Chris Peterson]

Do I understand you to be saying that larger fragments can break off a comet and follow it all the way around its orbit possibly numerous times? Wow! That is extremely cool. Thanks for sharing that one!

A comet ejects all sizes of material, from submicron dust to boulders (following a power law, such that number of particles of any given size falls off as the size increases). Material that survives for a long time in an orbit similar to the comet nucleus needs to be large enough that it is only weakly affected by radiation pressure or Poynting-Robertson drag, which means it must be on the order of a millimeter or larger. That, of course, is precisely the size where we start seeing meteors.

Over time, debris spreads out along the entire orbit, but is generally densest close to the nucleus. That's why we usually see annual showers no matter where the comet happens to be in its orbit, but we often see stronger showers when the comet has recently crossed Earth's orbit (for example, the Leonids tend to be very strong every 33 years, when Comet 55P/Tempel–Tuttle is near perihelion).