APOD: Comet A3 Through an Australian Sunrise (2024 Sep 25)

[APOD Robot]

Comet A3 Through an Australian Sunrise

Explanation: Comet Tsuchinshan-ATLAS is now visible in the early morning sky. Diving into the inner Solar System at an odd angle, this large dirty iceberg will pass its closest to the Sun -- between the orbits of Mercury and Venus -- in just two days. Long camera exposures are now capturing C/2023 A3 (Tsuchinshan–ATLAS), sometimes abbreviated as just A3, and its dust tail before and during sunrise. The featured image composite was taken four days ago and captured the comet as it rose above Lake George, NSW, Australia. Vertical bands further left are images of the comet as the rising Sun made the predawn sky increasingly bright and colorful. Just how bright the comet will become over the next month is currently unknown as it involves how much gas and dust the comet's nucleus will expel. Optimistic skywatchers are hoping for a great show where Tsuchinshan–ATLAS creates dust and ion tails visible across Earth's sky and becomes known as the Great Comet of 2024.

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[johnnydeep]

So what's the mass of this comet, and how big is the nucleus? I don't see that mentioned in any of the links.

[Chris Peterson]

So what's the mass of this comet, and how big is the nucleus? I don't see that mentioned in any of the links.

Mass and size are unknown. These are not easy to determine.

[johnnydeep]

So what's the mass of this comet, and how big is the nucleus? I don't see that mentioned in any of the links.

Mass and size are unknown. These are not easy to determine.

I'd think it would be enough to know it's orbit. Isn't that how masses are generally found out?

[Chris Peterson]

So what's the mass of this comet, and how big is the nucleus? I don't see that mentioned in any of the links.

Mass and size are unknown. These are not easy to determine.

I'd think it would be enough to know it's orbit. Isn't that how masses are generally found out?

The mass of an object does not have any impact on its orbit around a much more massive body. (That's why the debris that a comet sheds continues to orbit in the same path as the parent body, other than being impacted by other forces like radiation pressure and solar wind. Or more graphically, why a space walking astronaut outside the ISS doesn't go ripping off into a new orbit!) The Keplerian formula for orbital period depends only on the mass of the central body.

The mass of comets is estimated if we know their size, sometimes by optical observation, better by radar (a problem since losing Arecibo), sometimes by assumptions based on optical magnitude. The only bodies where we very accurately know the mass are those we've visited with probes, where orbital aberrations in those probes allow the mass of the comet to be calculated.

[johnnydeep]

Mass and size are unknown. These are not easy to determine.

I'd think it would be enough to know it's orbit. Isn't that how masses are generally found out?

The mass of an object does not have any impact on its orbit around a much more massive body. (That's why the debris that a comet sheds continues to orbit in the same path as the parent body, other than being impacted by other forces like radiation pressure and solar wind. Or more graphically, why a space walking astronaut outside the ISS doesn't go ripping off into a new orbit!) The Keplerian formula for orbital period depends only on the mass of the central body.

The mass of comets is estimated if we know their size, sometimes by optical observation, better by radar (a problem since losing Arecibo), sometimes by assumptions based on optical magnitude. The only bodies where we very accurately know the mass are those we've visited with probes, where orbital aberrations in those probes allow the mass of the comet to be calculated.

Ok, thanks, I see. Yeah, clearly there could be both a ping pong ball and a 2 ton satellite in the exact same geosynchronous orbit.