Qicheng Zhang
@cometary.org
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.
November 6, 2025 at 1:03 AM
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.
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.
November 6, 2025 at 1:00 AM
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.
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.
November 2, 2025 at 3:03 PM
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.
The thing above it is just a star that saturated the detector.
October 31, 2025 at 11:41 PM
The thing above it is just a star that saturated the detector.
Yes, it's very close to the JPL ephemeris position.
October 31, 2025 at 10:05 PM
Yes, it's very close to the JPL ephemeris position.
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.
October 31, 2025 at 9:37 PM
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.
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.
October 31, 2025 at 9:27 PM
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.
We already recovered it with the Lowell Discovery Telescope this morning: cometary.org/@qicheng/sta...
Post by Qicheng Zhang, @qicheng@cometary.org
After passing superior conjunction last week, interstellar comet 3I/ATLAS is once again observable from the ground with optical telescopes, now in morning twilight. Here's a view from the Lowell Disco...
cometary.org
October 31, 2025 at 9:22 PM
We already recovered it with the Lowell Discovery Telescope this morning: cometary.org/@qicheng/sta...
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.
ALMA Science Archive
almascience.nrao.edu
October 30, 2025 at 3:42 PM
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.
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.
October 30, 2025 at 3:30 PM
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.
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.
October 25, 2025 at 12:32 AM
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.
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.
October 24, 2025 at 11:10 PM
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.
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.
October 24, 2025 at 11:06 PM
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.
That sounds about right to me. All the iron/nickel comet papers basically came out together in big bunch once they noticed that.
October 16, 2025 at 8:25 PM
That sounds about right to me. All the iron/nickel comet papers basically came out together in big bunch once they noticed that.
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.
October 16, 2025 at 7:46 PM
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.
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.
September 28, 2025 at 8:15 PM
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.
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.
August 28, 2025 at 7:04 PM
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.
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.
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.
www.nature.com
August 28, 2025 at 6:37 PM
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.
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.
August 27, 2025 at 2:35 AM
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.
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.
August 27, 2025 at 12:31 AM
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.
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...
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...
arxiv.org
August 23, 2025 at 2:09 AM
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...
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.
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.
payloadspace.com
August 22, 2025 at 1:15 AM
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.
This Oort cloud comet has pretty much exactly the same kind of coma profile: iopscience.iop.org/article/10.3...
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iopscience.iop.org
August 18, 2025 at 5:30 AM
This Oort cloud comet has pretty much exactly the same kind of coma profile: iopscience.iop.org/article/10.3...
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
August 18, 2025 at 5:28 AM
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.
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.
August 14, 2025 at 7:29 PM
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.