> 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...
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...
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
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.
hooo•1d ago
[1]: https://avi-loeb.medium.com/is-the-interstellar-object-3i-at...
csours•1d ago
meepmorp•1d ago
mcswell•1d ago
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•1d ago
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.
rachofsunshine•20h ago
imafish•1d ago