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Qicheng Zhang @cometary.org · 10/07/2026

1. All the 1-orbit-old dust has reconverged to the comet's orbit where they were released, 2. the comet is near perihelion, so dust from last perihelion is returning to the comet's orbit now, and 3. 10P's activity is sharply peaked near perihelion, so nearly all of the dust is near the orbit now.

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Qicheng Zhang @cometary.org · 24/02/2026

Okay, you're right, I forgot about relativistic proper time being different from "Newtonian" time and handling that brings efficiency up to 1 despite the otherwise identical effective potentials.

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Qicheng Zhang @cometary.org · 06/11/2025

Stronger outgassing also corresponds to less of a tail, because it ejects dust faster = makes the tail puffier => a very puffy tail is essentially just a diffuse coma, basically what we see now. Thin dust tails correspond to dust ejected at low speeds = relatively little outgassing.

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Qicheng Zhang @cometary.org · 06/11/2025

Nothing about his claim is sensible. The nongravs are most likely far smaller than the current measurements, due to the astrometry being biased by the asymmetric coma (comet was ~4 sigma away in the JWST obs). Observations (especially in the radio) also show strong outgassing.

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Qicheng Zhang @cometary.org · 02/11/2025

That may be true, but if 3I/ATLAS were instead an Oort cloud with the same properties, it may still stand out in some aspects, but I'd argue (based on the published data so far) would be distinctly less weird than at least of couple of other comets we know of.

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Qicheng Zhang @cometary.org · 31/10/2025

The thing above it is just a star that saturated the detector.

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Qicheng Zhang @cometary.org · 31/10/2025

Yes, it's very close to the JPL ephemeris position.

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Qicheng Zhang @cometary.org · 31/10/2025

Depends a lot on your sky conditions, horizon, and telescope. The first two need to be just about perfect right now to catch it with a typical amateur-class telescope, but it's rapidly becoming easier.

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Qicheng Zhang @cometary.org · 31/10/2025

I think it's bright enough now that even small telescopes should be able to get it, especially with how quickly the comet is moving out from the Sun angularly.

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Qicheng Zhang @cometary.org · 30/10/2025

October 29 data is likely from these ALMA observations: almascience.nrao.edu/aq/?project_... They need highly accurate orbits for their narrow fields of view, so will often bypass the MPC (at least initially) to get their astrometry incorporated into the JPL orbit faster.

almascience.nrao.edu

ALMA Science Archive

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Qicheng Zhang @cometary.org · 30/10/2025

The thought is that if you have some CO2 mixed with H2O ice near the surface, the cooling from CO2 sublimation will stop efficient H2O sublimation. After enough CO2 goes away, some H2O might get warm enough to do so. May or may not be anywhere close to what's happening, but that's the speculation.

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Qicheng Zhang @cometary.org · 25/10/2025

Yes, that range is still valid. A comet of its brightness would usually be on the upper side of that range, but at the moment, there's no evidence/need for it to be bigger than 5 km, as far as I can tell.

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Qicheng Zhang @cometary.org · 25/10/2025

This is also a very weak bound, because comet astrometry tends to have large systematic errors due to coma asymmetry biasing the fitting of the nucleus position, which is hard for orbit fitting software to handle. In practice, comet positions/orbital parameters being 10+ sigma off is not uncommon.

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Qicheng Zhang @cometary.org · 25/10/2025

The easy way to do the calculation would be to put the astrometry into an orbit fitting program with nongravitational parameters, like find_orb. I just did that, and the 3-sigma upper bound on A1 (radial nongrav at 1 au) is ~5e-7 au/d^2 (~2x that of `Oumuamua), so it's bigger than... ~50 m.

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Qicheng Zhang @cometary.org · 16/10/2025

That sounds about right to me. All the iron/nickel comet papers basically came out together in big bunch once they noticed that.

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Qicheng Zhang @cometary.org · 16/10/2025

That's because even though it was recorded on those earlier comets, it wasn't actually noticed apart from on sungrazing comets until that paper.

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Qicheng Zhang @cometary.org · 28/09/2025

C/2025 K1 was >10 deg to the west of C/2025 R2 at the time of this image, whereas 3I looks to be at about the right spot ~4 deg away.

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Qicheng Zhang @cometary.org · 28/08/2025

It may be at least a little weird, but hard to say by how much as there's only been 1 comet where the ratio has been measured at greater distances from the Sun.

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Qicheng Zhang @cometary.org · 28/08/2025

Fig. 2 of this paper shows the ratio is generally on the order of ~1 for solar system comets not super close to the Sun: www.nature.com/articles/s41... Fig. 3 also suggests the actual line strengths of the two tend to be somewhat similar.

nature.com

Iron and nickel atoms in cometary atmospheres even far from the Sun - Nature

High-resolution ultraviolet and optical spectra of a large sample of comets show that Fe i and Ni i lines are ubiquitous, even when the comets are far from the Sun.

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Qicheng Zhang @cometary.org · 27/08/2025

The thing is, one could say the same about any of the thousands of known comet or asteroids, or even about birds. Perhaps one day, all the penguins will just suddenly fly themselves to Mars. But until then, there's no good reason to seriously consider how they might all be aliens in disguise.

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Qicheng Zhang @cometary.org · 27/08/2025

The problem is by this logic, we can *never* rule out anything as being aliens. Birds? Could be aliens spying on us. Clouds? Might be their nanobot formations. Lightning? Could be their communications. There's just as much evidence for those claims as there is of 3I/ATLAS being from aliens.

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Qicheng Zhang @cometary.org · 23/08/2025

I'd forgotten about SPHEREx, but this new paper seems to support that the Swift OH detection is probably not real, with a far lower upper limit on water: arxiv.org/abs/2508.15469 Pretty convincing CO2 though. But obviously alien spaceships are gas powered. Or breathing. Or something...

arxiv.org

SPHEREx Discovery of Strong Water Ice Absorption and an Extended Carbon Dioxide Coma in 3I/ATLAS

In mid-August 2025, 0.75-5.0 micron SPHEREx imaging spectrophotometric and ancillary NASA-IRTF SpeX 0.7-2.5 micron low-resolution spectral observations of Interstellar Object 3I ATLAS were obtained. T...

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Qicheng Zhang @cometary.org · 22/08/2025

Going by this article (payloadspace.com/reflect-orbi...), it sounds like their plan is based off having "thousands" of 55-m mirrors. From some quick math, they'd need ~10^4 of those pointed at one spot to match the Sun.

payloadspace.com

Reflect Orbital Raises $20M Series A

Reflect is planning to launch its first sat next spring to illuminate 10 locations around the world and boost public interest in nighttime lighting.

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Qicheng Zhang @cometary.org · 18/08/2025

The previous figure in the exact same paper (arxiv.org/pdf/2508.02934) clearly shows the inner coma closely following a 1/rho profile just like a normal comet with steady dust loss. It rolls off farther out as normal, usually from activity beginning recently + radiation pressure + dust evolution.

arxiv.org

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Qicheng Zhang @cometary.org · 14/08/2025

I'd expect no less. It's the broken clock / monkey-on-typewriter strategy of being right; send out enough nonsense, and eventually some of it'll accidentally turn out to be correct.

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Qicheng Zhang @cometary.org · 14/08/2025

Also not particularly convincing to me. It looks to me like it just assumes the rock/non-water component is a certain color matching a particular meteorite, and then attributes the entire observed color difference to water (vs. any of the many other differences that could affect the color).

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Qicheng Zhang @cometary.org · 14/08/2025

Honestly, I'm very skeptical of the claimed OH detection as it doesn't really look the way OH should (i.e., a very diffuse fuzz, not a point). Also a huge jump from an upper limit from just weeks earlier. There's since been JWST observations that'll give better numbers, whenever that's published.

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Qicheng Zhang @cometary.org · 06/08/2025

If the coma were made of very small grains, it might be possible to tell in MIR as dust grains don't thermally radiate efficiently at wavelengths more than an order of magnitude or so larger than their size. However, many distant comets have ~0.1 mm grains, which emit MIR just like a big nucleus.

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Qicheng Zhang @cometary.org · 30/07/2025

Better to get to writing those before he scoops you!

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Qicheng Zhang @cometary.org · 25/07/2025

Should note this new paper says C/2017 K2's nucleus is actually < ~8 km in diameter (whereas I assumed ~12 km in that calculation): iopscience.iop.org/article/10.3... If C/2017 K2 is really ~7-8 km in diameter, that shifts the scaled perihelion brightness of C/2014 UN271 a bit upward to ~11.

iopscience.iop.org

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Qicheng Zhang @cometary.org · 25/07/2025

Interestingly, it actually still looks quite on track for my rough target of mag ~12 at perihelion from 2021, treating it as a scaled up C/2017 K2: groups.io/g/comets-ml/...

groups.io

Outdated Browser

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Qicheng Zhang @cometary.org · 22/07/2025

You might find the DRC images a little nicer to work with as they've had a correction applied to remove the long trails attached to every bright source.

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Qicheng Zhang @cometary.org · 17/07/2025

Any optical gas coma this far away from the Sun should be very large/diffuse (many arcminutes wide). I'd guess the blue fringe is most likely an artifact from seeing variation across the different colors and/or a slight color channel misalignment (or if real, variation in dust color over the coma).

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Qicheng Zhang @cometary.org · 04/07/2025

The HiRISE specs (www.uahirise.org/specs/) indicate it's 30 cm/px from 300 km away, or 0.2 arcsec/px. For a comet 0.2 au away, that becomes 30 km/px.

uahirise.org

HiRISE | Camera Technical Specifications

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Qicheng Zhang @cometary.org · 06/05/2025

Dan Green did a reanalysis of C/1532 R1 in Chapter 7 of his thesis (core.ac.uk/reader/6116550) which seems to indicate a rather bigger difference between the orbits of the two. Still somewhat similar, but perhaps closer to the level of C/1680 V1 and C/2012 S1.

core.ac.uk

Assessment of early-modern observations of comets and supernovae: Focus on pre-telescopic European astrometric and physical data - CORE Reader

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Qicheng Zhang @cometary.org · 04/04/2025

C/2025 F1 is already assigned to this ATLAS comet: www.minorplanetcenter.net/mpec/K25/K25... So probably F2.

cbat.eps.harvard.edu

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Qicheng Zhang @cometary.org · 17/01/2025

Comet C/2024 G3 (ATLAS) looks a bit like a will-o'-the-wisp with its fan of many tails as it makes its way across STEREO-A's HI1 field of view. Here's an animation of the comet on Jan 14, zoomed in on the upper right quadrant of HI1 that captured the comet.

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Qicheng Zhang @cometary.org · 11/01/2025

I could do it manually from that in theory, but it would need some extra calculations that JPL Horizons normally takes care of/makes convenient.

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Qicheng Zhang @cometary.org · 11/01/2025

JPL Horizons still hasn't loaded any SOHO ephemerides for 2025 yet for some reason, but approximating SOHO as being at Earth, it should enter around 10 UT +/- probably a couple hours or so.

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Qicheng Zhang @cometary.org · 08/01/2025

Personally, I want to see it split into a few big pieces. Haven't had a comet this bright do that since C/1975 V1 (West), and that seems to be the secret sauce turning a bright comet into a really brilliant one.

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Qicheng Zhang @cometary.org · 15/12/2024

The 1910 January comet had a tail curving in the other direction while bright, so this must have been a different comet, unless the painting is mirrored?

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