I’ve cited this before elsewhere as the limit case of staging. The pro’s here don’t need the reference but we amatuers might learn something from it: What If - Model Rockets
I’ll point out the OP’s requirement wasn’t to rendezvous with the object, but simply to get close enough long enough to gather useful data. A single high speed crossing path might do it. Where Earth in a more propitious spot in its orbit when the object went by the delta-V might be vastly less. 3i/ATLAS is kinda worst case for launching a probe to get anywhere near it.
You will not get the kind of sequence you imagine because propulsion systems and structures don’t just scale that way in real life even if they do so in “Kerbal Space Program”. Having worked with many different types of space launch and sounding rockets as well as developed numerous fairly detailed ‘toy models’ of alternative launch and spacecraft propulsion systems with more physical detail than KSP acknowledges, I can say with a lot of confidence that once you start scaling down to the low end your inert mass fraction just keeps climbing.
You couldn’t power a mass spectrometer with just betavoltaics but you don’t really need to, either. The interplanetary environment is full of both ionizing radiation and charged particles that will interact with the coma, and as long as you can fly near that you can use a passive spectrometer to get a reasonable estimate of composition. But frankly, it’s just as easy to do that remotely (at least, if your observatory isn’t on the far side of the Sun when the object makes its closest pass) without the hassle and risk of a probe that isn’t going to do much else.
If we had enough response time we could send an impactor (at least, if you are confident that it is not an alien vessel that would react adversely to such an apparent attack) or a flyby probe to pass through the tail and sample the coma emissions but catching up with such a fast moving object is basically outside the realm of feasibility even for electric propulsion. Fortunately, we’ve just started looking for these objects and given that we’ve found three in the last seven years means that they are probably pretty common, although 3i/ATLAS would have been interested because of its apparent origins in the ‘thick disc’ of our galaxy or perhaps even from outside of it,
While true, there’s a reason I stopped at a few tens of kilograms, and that’s because Rocket Lab has already demonstrated a functional kick stage/bus at that rough size level. They’re a bit secretive about the performance, but the Curie/HyperCurie engine is believed to have an Isp of ~320 s, which is the number I used in my hand-waving math. The platform isn’t a perfect fit for this application, but they are advertising an interplanetary version, and in any case I think it demonstrates that the basic performance is achievable. I wouldn’t trust any numbers too much below this, though. You’re obviously correct that the scaling does break down at a certain point.
More to the point, it just isn’t really necessary.
I worked on an unsolicited proposal to NASA for an array of solar-orbiting satellites that, among other things, would house optical telescopes and spectrometers to identify extraplanetary bodies and their composition. This would be perfect for this kind of work without having to send a probe to intercept individual objects as well as supporting telemetry for outer planets mission, but like most such proposals it didn’t even get short-listed for consideration.
Some more information in the composition of the coma from the SPHEREx mission:
On August 6, 2025 , JWST leveraged its infrared powers to capture the first detailed spectra of 3I/ATLAS, revealing its chemical makeup in unprecedented detail. NASA reports that JWST’s NIRSpec captured spectra across 0.6–5.3 µm that reveal a coma dominated by carbon dioxide (CO₂), along with water (H₂O), carbon monoxide (CO), carbonyl sulfide (OCS), water ice, and dust.
Infrared spectroscopy lets astronomers identify molecular “fingerprints” in the comet’s coma. JWST’s capabilities allowed scientists to distinguish faint gases and ices that would remain invisible to lesser instruments.
This is the kind of supercharged observation only JWST can pull off—and it’s already delivering jaw-dropping data.
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