Astronomers discover 3I/ATLAS – Third interstellar object to visit Solar System

Original Article ↗ Hacker News Discussion ↗

Detection and Recent Surge in Interstellar Objects

  • Commenters note we saw none for millennia and three in a few years; main explanations:
    • Improved surveys, hardware, and GPU-powered algorithms.
    • New dedicated systems like ATLAS and especially the Vera Rubin Observatory, which repeatedly scans the (southern) sky and is expected to reveal many more.
  • Some speculate we might be entering an interstellar debris-rich region, but others point out our local galactic environment is relatively sparse.
  • Several remarks that we probably had the capability earlier but lacked focus, and that statistics with only three objects (N=3) are too poor to say much yet.

Orbit, Dynamics, and Physical Properties

  • 3I/ATLAS has a very high orbital eccentricity (>6), much higher than 1I and 2I, confirming it as unbound and interstellar.
  • Current estimates (if inactive) suggest ~8–22 km diameter, with big uncertainty from unknown albedo; if active, dust could make it appear larger.
  • It is retrograde and passes close to the Solar System’s orbital plane, inside Jupiter’s orbit and briefly inside Mars’s, but not especially close to any planet.
  • Closest solar approach is ~1.35 AU around late October 2025 at ~68 km/s.
  • Discussion clarifies “eccentricity” refers to orbit shape, not object shape, and that mass is not needed to fit the trajectory under gravity.

Impact Scenarios and Energy Calculations

  • Multiple back-of-the-envelope calculations explore the kinetic energy of a hypothetical Mars or Earth impact, with some corrected mid-thread (notably a m/s vs km/s error).
  • Consensus: an Earth impact by an object in this size and speed range would be extinction-level, comparable to or larger than the Chicxulub impactor.
  • For Mars, impacts in this range could release tens of thousands to tens of billions of megatons TNT equivalent; speculation about possible “terraforming” by polar impact.

Observation Infrastructure and Data

  • Explanation of Minor Planet Center circulars, historical punch-card-style formats, and how observations feed into JPL’s Horizons system.
  • Emphasis that large telescopes like ELT are mainly for deep follow-up, while Rubin is optimized for discovery.
  • Some users struggle with orbit viewers and object IDs; others clarify alternate designations (e.g., C/2025 N1).

Frequency, Origins, and Survey Bias

  • A cited paper estimates a low volumetric density of such objects, but still implies roughly one within Saturn’s orbit at any time.
  • Interstellar objects can be ejected from planetary systems via close passes with giant planets, analogous to gravity assists.
  • Detection is biased toward objects near the ecliptic, aligning partly by chance and partly by where surveys tend to look.

Aliens, Culture, and Public Perception

  • Many humorous allusions to alien probes, “passive sensor drones,” Rama, Three-Body Problem “sophons,” and sci-fi scenarios about deceleration stages and fleets.
  • Some criticize media language like “visiting” as feeding alien hype.
  • Side discussions about cosmic scale, public skepticism (e.g., Moon landings), and how hard it is to intuit astronomical distances from everyday experience.

Planetary Defense and Feasibility of Deflection

  • For a large, fast interstellar object on a collision course, commenters are pessimistic about current ability to divert it; DART-like missions are far too small in scale.
  • In principle, a small nudge with long warning could suffice, but detecting, intercepting, and significantly deflecting such a massive, high-velocity body is seen as beyond current capability.