First Hubble telescope images of interstellar comet 3I/ATLAS | Hacker News Reader

First Hubble telescope images of interstellar comet 3I/ATLAS

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109 pointsjandrewrogers29 comments

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?

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.

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...

You're on the money. Hubble isn't too perturbed when it can lock on to guide stars, but some observations are no longer possible in one-gyro mode.

Hubble's slew rate (the rate at which it can change the broad direction its camera points) is about 6 degrees per minute [1], or about 1/10 of a degree per second (I refuse to use the incredibly cursed unit "minutes per second"). It tracks a fine object slower than that, but that gives a reasonable order-of-magnitude estimate.

At even 1 AU of distance, an angular velocity of 1/10 of a degree per second requires a linear speed of about 0.87c. Needless to say, 3I/ATLAS is not moving that fast - if it were, it would be outputting about 100 TW, mostly as heat, just from slamming into the interplanetary medium at relativistic speeds [2].

[1] https://www.pbs.org/deepspace/hubble/diagram.html#:~:text=Th...

[2] https://www.wolframalpha.com/input?i=%2840+*+mass+of+hydroge...

> I refuse to use the incredibly cursed unit "minutes per second"

Man, that is a horrible unit. I've never heard of that, but I can only imagine each semester professors every where have to endure an enterprising student using this to be "clever" in some way

“Arcminutes per second” would be the standard way to express it in astronomy.

FWIW, isn't it only "minutes" if you want to be confusingly short? I'd have said "minutes of arc" or "arcminutes".

With Rubin Observatory coming online recently, we should be able to find many more of this type of event. So, it would make sense to have a more generic probe ready in standby and wait for the next interstellar object discovery. Then, send the probe to the interstellar object for a flyby mission. It would be revolutionary if the probe could directly detect the element and isotope composition of the comet tail during the flyby.

These are moving so fast, that there is nothing we might have on stand-by.

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?

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.

I don't think those are motion-blurred stars. Wouldn't they have to be considerably closer to us - or Hubble going much faster - for that to be the case?

The telescope is panning to keep the comet still in the frame during the exposure.

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.

A lot of motion blur: have they tried adjusting the shutter speed…

At ~100s, it's already at about the minimum for Hubble; often it's 1-2 orders of magnitude longer.

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.

Noob Q: How do they know it's an interstellar comet? With the speed of movement between two frames?

This object was already discovered and known to have an hyperbolic (uncaptured) orbit.

Short answer, yes. But it's many frames, and over a time span of many nights and now weeks.

We know how fast it is moving and how far from the Sun it is. If its velocity is greater than the escape velocity of the Sun at its current distance, it can't have gained that velocity just from its orbit around the Sun (because by definition an object in an [elliptical] orbit is traveling more slowly than escape velocity).

It is possible for a Solar System comet to be perturbed by other effects (like a close passage with Jupiter) into an escape orbit. But in those cases, the speed above escape velocity is small, and the orbit barely escapes. 3I/ATLAS is moving much, much faster than that, too fast for within-the-solar-system effects to explain it. It must therefore be interstellar.

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...

Oh dear, he's already at it with this one too

It'd be more surprising if he wasn't, tbh.

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.

> "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.

I think there's also a sampling bias here? ATLAS, the survey that discovered the comet, is specifically looking for potential Earth impactors. One assumes that would involve looking close to Earth's own orbital plane.

Why would it be cool, though? More like frightening, if the thing was sent on purpose by another civilization.

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