[HN Gopher] First Hubble telescope images of interstellar comet ...
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First Hubble telescope images of interstellar comet 3I/ATLAS
Author : jandrewrogers
Score : 76 points
Date : 2025-07-22 16:41 UTC (6 hours ago)
(HTM) web link (bsky.app)
(TXT) w3m dump (bsky.app)
| hooo wrote:
| While it would be cool if it were alien technology[1], it looks
| like an ancient comet?
|
| [1]: https://avi-loeb.medium.com/is-the-interstellar-
| object-3i-at...
| csours wrote:
| Oh dear, he's already at it with this one too
| meepmorp wrote:
| It'd be more surprising if he wasn't, tbh.
| mcswell wrote:
| Agreed, it would be cool, but. From that article, with my
| commentary (disclaimer: IANAA, I Am Not An Astronomer):
|
| 1) "The retrograde orbital plane... of 3I/ATLAS around the Sun
| lies within 5 degrees of that of Earth... The likelihood for
| that coincidence out of all random orientations is 0.2%." Not
| sure where he comes up with 0.2%. 5/180 = 2.8%. (I use 180
| degrees, rather than 360, because I suspect that if it were not
| retrograde, he'd use the same argument.)
|
| 2) "the brightness of 3I/ATLAS implies an object that is ~20
| kilometers in diameter (for a typical albedo of ~5%), too large
| for an interstellar asteroid. We should have detected a million
| objects below the ~100-meters scale of the first reported
| interstellar object 1I/`Oumuamua for each ~20-kilometer
| object." Huh? We barely detected this object because it's so
| dim. Why should we be detecting interstellar objects two or
| three orders of magnitude smaller?
|
| 3) "No spectral features of cometary gas are found in
| spectroscopic observations of 3I/ATLAS." An article today (22
| July, https://astrobiology.com/2025/07/spectroscopic-
| characterizat...) says "Spectral modeling with an areal mixture
| of 70% Tagish Lake meteorite and 30% 10-micron-sized water ice
| successfully reproduces both the overall continuum and the
| broad absorption feature... 3I/ATLAS is an active interstellar
| comet containing abundant water ice, with a dust composition
| more similar to D-type asteroids..."
|
| 4. "For its orbital parameters, 3I/ATLAS is synchronized to
| approach unusually close to Venus (0.65au where 1au is the
| Earth-Sun separation), Mars (0.19au) and Jupiter (0.36au), with
| a cumulative probability of 0.005% relative to orbits with the
| same orbital parameters but a random arrival time." This
| probability is harder to compute (although 0.65au from Venus is
| nearly the radius of Venus' orbit, 0.72au, i.e. not close). In
| any case, so what? Why would an interstellar probe travel close
| to Mars or Jupiter, if they're interested in Earth? (see next
| point) Later (his point 8), he says the probe comes close
| enough to these planets to launch ICBMs at them. Ok...
|
| 5. "3I/ATLAS achieves perihelion on the opposite side of the
| Sun relative to Earth. This could be intentional..." Sure, if
| they're interested in Earth, stay away from it.
|
| And similarly for the rest of his points.
| teraflop wrote:
| > "The retrograde orbital plane... of 3I/ATLAS around the Sun
| lies within 5 degrees of that of Earth... The likelihood for
| that coincidence out of all random orientations is 0.2%." Not
| sure where he comes up with 0.2%.
|
| This part of the calculation, at least, is basically correct.
| The orientation of a plane in space is defined by its normal
| vector, so the right way to look at probabilities is in terms
| of solid angle. The normal of 3I/ATLAS's orbit falls within a
| cone around Earth's normal vector, having a half-angle of 5
| degrees, and that cone's solid angle occupies about 0.2% of
| the full sphere.
|
| Of course, this is only the chance of a retrograde alignment.
| Presumably, if the comet's orbit was _prograde_ aligned with
| the Earth 's to within 5 degrees, Loeb would be making
| exactly the same claim. So really, the relevant probability
| is 0.4%.
|
| Nevertheless, I agree that the article is basically just a
| bunch of cherry-picked probabilities and insinuations that
| don't add up to much.
|
| Also:
|
| > "the brightness of 3I/ATLAS implies an object that is ~20
| kilometers in diameter (for a typical albedo of ~5%), too
| large for an interstellar asteroid."
|
| But to justify this, Loeb cites his own work showing that the
| object is _either_ a large asteroid, _or_ a comet with a
| small nucleus. And then he seems to have looked at some
| earlier spectra and jumped to the conclusion that 3I /ATLAS
| couldn't be a comet, so it must be a large asteroid. But of
| course, follow-up observations have debunked this point and
| clearly shown it to be a comet.
| imafish wrote:
| Why would it be cool, though? More like frightening, if the
| thing was sent on purpose by another civilization.
| dylan604 wrote:
| The slew rate for tracking comets is something that I have not
| had to mess with before, but I adjust my little EQ mount when I'm
| tracking the moon vs deep sky objects. How accurate is Hubble
| now? How many of its reaction wheels does it have left? I seem to
| remember it being down to just one at one point. Does that add
| difficulty in tracking this object with its very high velocity?
| pinko wrote:
| I suspect, at ~4.5AU distance, even though 3I/ATLAS is moving
| at a relative speed of ~60 kms, its angular velocity across the
| sky is manageable for Hubble's current one-gyro pointing
| system, given non-sidereal tracking and short (~100s)
| exposures.
| teraflop wrote:
| I'm no Hubble expert, but a bit of research turned up the "HST
| Primer" [1] which is apparently up-to-date for the current
| observing cycle, and which says:
|
| > HST is capable of tracking moving targets with the same
| precision achieved for fixed targets. This is accomplished by
| maintaining FGS Fine Lock on guide stars and driving the FGS
| star sensors in the appropriate path, thus moving the telescope
| to track the target. Tracking under FGS control is technically
| possible for apparent target motions up to 5 arcsec/s.
|
| According to JPL Horizons, the current angular motion of
| 3I/ATLAS across the sky is <0.03 arcsec/s, so it's well within
| Hubble's capabilities.
|
| My understanding is that the Hubble's one-gyro mode mainly
| complicates the process of quickly moving from one target to
| another. Once the telescope is pointed at a target, the
| stabilization and tracking is done using guide stars without
| relying on gyros.
|
| Anyway, in absolute terms, 3I/ATLAS isn't moving _that_ fast.
| Its orbital speed is about 3x that of Mars, but it 's farther
| away, and (for now) much of that motion is directed inward
| towards the sun.
|
| [1]: https://hst-docs.stsci.edu/hsp/the-hubble-space-telescope-
| pr...
| rkagerer wrote:
| Dumb question: Is it the smaller one (that moves, along the same
| axis as the background stars) or the bigger one (that's fairly
| static). What's the other one?
| exitb wrote:
| The static lines are motion blurred stars (even the bright
| one), the small dots are radiation noise, the one that moves,
| with a coma is the comet.
| mcswell wrote:
| About the bigger one that doesn't seem to move: I think it does
| move, it's just that it's so bright (for Hubble) that its
| brightness overwhelms the slight elongation of its image. In
| other words, it's (apparently) moving just like the other
| stars, it's just hard to tell.
| throw0101b wrote:
| A lot of motion blur: have they tried adjusting the shutter
| speed...
| pinko wrote:
| At ~100s, it's already at about the minimum for Hubble; often
| it's 1-2 orders of magnitude longer.
| mcswell wrote:
| If I know what you're referring to, the motion blur is the
| stars, not the comet. That's because Hubble is tracking
| (pointing at) the comet, not the stars. The comet is therefore
| not blurred in its direction of travel, while the stars appear
| to be moving in the direction opposite of the comet's travel.
| To the extent that the comet appears blurred, that's presumably
| its coma.
| amrrs wrote:
| Noob Q: How do they know it's an interstellar comet? With the
| speed of movement between two frames?
| baggy_trough wrote:
| This object was already discovered and known to have an
| hyperbolic (uncaptured) orbit.
| vikingerik wrote:
| Short answer, yes. But it's many frames, and over a time span
| of many nights and now weeks.
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