3I/ATLAS (C/2025 N1): Direct Spacecraft Exploration of a Possible Relic of Planetary Formation at "Cosmic Noon" (2508.15768v1)

Abstract: The interstellar object 3I/ATLAS (also C/2025 N1 (ATLAS), henceforth, 3I), discovered by the ATLAS Chile telescope on 2025 July 1, was rapidly revealed to be the third known interstellar object (ISO) transiting the solar system, with an incoming velocity at infinity of 57.9763 $\pm$ 0.0044 km s$^{-1}$. An examination of 3I's pre-encounter kinematics shows that it is likely to be an object from the galactic thick disk, and thus a remnant of the Galaxy's ``cosmic noon'' period of intense star formation $\sim$9 - 13 gigayears ago. This kinematic assignment of 3I to the thick disk can be tested observationally in the transit of 3I through the solar system. Unfortunately for terrestrial observers, the 3I perihelion will happen when it is on the other side of the Sun as seen from Earth, at a solar elongation of 12.80 degrees, rendering observation from Earth (or near-Earth space telescopes) hard or impossible. With a retrograde orbit inclined 175.114 degrees (only 4.886 degrees from the ecliptic plane), and a trajectory passing inside the orbit of Mars, 3I will pass relatively close to a number of already launched interplanetary spacecraft. We find a strong science case for observations in the periods of the close approaches of the Psyche spacecraft on 2025 September 4, at 0.302 AU, the martian spacecraft array on 2025 October 3, and the Juice spacecraft on 2025 November 4. In addition, the Europa Clipper, Hera and even the more distant Lucy spacecraft may pass through 3I's cometary tail in the period after its perihelion passage, potentially directly observing the conditions and composition there. Spacecraft observations could, to the extent they are possible, provide the only source of spectral and imaging data during the 3I perihelion passage.

• The paper presents the kinematic classification of 3I/ATLAS, identifying it as a thick disk relic formed during the Galaxy’s cosmic noon.
• It details innovative spacecraft observation strategies, including synthetic tracking and multi-wavelength imaging, to capture the object's coma and tail dynamics.
• The study uses PAH diagnostics and infrared spectroscopy to predict compositional properties, informing models of planetary formation in ancient galactic environments.

Direct Spacecraft Exploration of Interstellar Object 3I/ATLAS: Implications for Planetary Formation at "Cosmic Noon"

Introduction and Discovery Context

The paper presents a comprehensive analysis of the interstellar object 3I/ATLAS (C/2025 N1), the third confirmed ISO to traverse the solar system. Discovered at 4.52 AU from the Sun by the ATLAS Chile telescope, 3I/ATLAS exhibits a hyperbolic trajectory with an incoming velocity at infinity of $57.98 \pm 0.0044$ km/s. Early observations revealed active cometary behavior, including a substantial dusty coma ($\sim$25,000 km diameter) and a red spectrum, with possible water ice features in the infrared. The nucleus size is constrained to $0.16 \leq R \leq 2.8$ km, with a rotation period of $16.79 \pm 0.23$ h, precluding rotational disruption.

The perihelion geometry renders Earth-based observations nearly impossible during the most scientifically valuable period, as 3I will be at a solar elongation of $12.8^\circ$. However, its trajectory brings it into proximity with several interplanetary spacecraft, enabling unique opportunities for direct observation and characterization.

Galactic Kinematics and Origin

A detailed kinematic analysis using astrometric data and galactic UVW velocity components demonstrates that, unlike 1I/'Oumuamua and 2I/Borisov (which are associated with the Pleiades and Wolf 630 stellar streams, respectively), 3I/ATLAS does not match any major thin disk stream. Instead, its velocity is statistically consistent with the thick disk population, characterized by slower galactic rotation and significant excursions from the galactic plane ($a \sin i \sim 911$ pc, near the thick disk scale height).

The thick disk is a relic of the Galaxy's "cosmic noon" ($\sim$9–13 Gyr ago), a period of intense star formation. The assignment of 3I to the thick disk is supported by $\chi^2$ analysis of its velocity components, though some contradictory metallicity estimates exist. The possibility of a halo origin is considered but deemed unlikely given the kinematic evidence.

Compositional Predictions for Thick Disk Planetesimals

Thick disk objects are expected to be iron-poor and $\alpha$-element enhanced, with [$\alpha$/Fe] up to $+0.45$ and [Fe/H] between $-0.3$ and $-1.2$. Rocky exoplanets from the thick disk exhibit lower densities, implying smaller iron cores and potentially weaker magnetic fields. The formation environment was likely warm ($\sim$300 K) and highly irradiated by FUV/EUV, leading to depletion of supervolatiles (CO, CO$_2$, N$_2$, CH$_4$) and a red surface coloration. The presence of amorphous ice is hypothesized as the energy source for coma activity at large heliocentric distances.

Polycyclic Aromatic Hydrocarbons and Infrared Diagnostics

PAHs are ubiquitous in the ISM and are sensitive tracers of formation environment metallicity and radiation field. The thick disk metallicity ($\sim$0.58 $Z_\odot$) predicts a PAH-to-dust luminosity ratio of 2–15%. The 3.3/11.2 $\mu$m band ratio is a robust indicator of PAH molecular size, but only JWST can observe the full PAH suite, and not during perihelion due to solar elongation constraints. Coordinated IR observations from spacecraft (e.g., JUICE/MAJIS, Hope/EMIRS) are recommended to probe PAH emission and constrain the formation conditions of 3I.

Spacecraft Observation Strategies

Given the unfavorable geometry for Earth-based telescopes, the paper identifies three primary spacecraft campaigns:

• Psyche (Sep 4, 2025): Closest approach at 0.302 AU, enabling phase angle measurements of coma dust with simultaneous observations from Earth, Mars, and other spacecraft.
• Mars Orbiter Array (Oct 3, 2025): Closest approach at 0.195 AU; high-resolution imaging and UV/IR spectroscopy (HiRISE, HiRIC, EMUS, EMIRS) can resolve coma structure, monitor water production via Lyman-$\alpha$, and search for fragmentation events.
• JUICE (Nov 4, 2025): Closest approach at 0.428 AU, optimal for visual/IR/UV spectrometry during perihelion, when Earth-based observations are impossible.

Synthetic tracking and high dynamic range imaging are required due to the high apparent sky velocity of 3I. Multi-vantage astrometry from spacecraft will improve orbital solutions and enable detection of non-gravitational accelerations at the $10^{-13}$ AU day$^{-2}$ level.

Coma and Tail Characterization

Spacecraft observations will enable:

• Coma phase angle response: Determination of dust size distribution and composition via multi-angle photometry.
• Spectral diagnostics: Search for silicate/carbonaceous grains, water ice, and low-temperature volatiles. Detection of Ni I and Fe I lines will test thick disk compositional predictions.
• Fragmentation monitoring: High-cadence imaging near perihelion to detect and characterize possible splitting events.
• Tail passage: Potential in situ sampling by Hera, Lucy, and Europa Clipper if they traverse the plasma/dust tail, with magnetometer and plasma instruments available for direct analysis.

Observational Constraints and Campaign Recommendations

Terrestrial and space-based telescopes (HST, JWST, ALMA, Chandra, XMM-Newton) are subject to stringent solar elongation limits, resulting in a blackout period for 3I observations during perihelion. The burden of observation thus shifts to interplanetary spacecraft, which are less constrained and can provide unique data on coma evolution, fragmentation, and compositional diagnostics.

A coordinated campaign is advocated, leveraging the armada of spacecraft at Mars, Psyche, and JUICE, with emphasis on multi-wavelength and multi-vantage observations. The combination of spacecraft and terrestrial data will enable direct testing of the thick disk origin hypothesis and provide insight into early galactic planetesimal formation.

Implications and Future Directions

The identification of 3I/ATLAS as a probable thick disk relic offers a rare opportunity to probe planetary formation conditions during the Galaxy's "cosmic noon." Confirmation of its compositional and kinematic properties will inform models of exoplanet formation, disk chemistry, and the prevalence of habitable environments in ancient stellar populations.

The paper highlights the necessity of interplanetary spacecraft for ISO characterization, especially when Earth-based observations are precluded by geometry. The methodologies developed here—multi-vantage astrometry, synthetic tracking, coordinated spectroscopic campaigns—will be essential for future ISO encounters.

Statistical estimates suggest that thick disk ISOs are rare ($\sim$6% of the population), implying that decades may pass before another such object is discovered. The scientific imperative is thus to maximize the observational return from 3I/ATLAS, as it may remain the only thick disk sample for the foreseeable future.

Conclusion

3I/ATLAS presents a unique opportunity for direct spacecraft exploration of an interstellar object likely formed during the Galaxy's peak star formation epoch. The paper provides a rigorous framework for kinematic classification, compositional prediction, and observational strategy, emphasizing the critical role of interplanetary spacecraft in ISO science. The results will have broad implications for galactic archaeology, exoplanetary science, and the understanding of planetary system formation in diverse galactic environments.