Hubble Space Telescope Observations of the Interstellar Interloper 3I/ATLAS (2508.02934v2)

Abstract: We present high angular resolution observations of the third known interstellar interloper, 3I/ATLAS, from the Hubble Space Telescope. The object is clearly active at 3.8 au pre-perihelion, showing dust emitted from the hot Sun-facing side of the nucleus and a weak, radiation pressure swept tail away from the Sun. We apply a simple model to estimate the mass loss rate in dust as dM/dt = 6 sqrt(a) kg/s, where a is the mean particle size in microns. With 1 < a < 100, we infer dM/dt = 6 to 60 kg/s. A fit to the surface brightness distribution of the inner coma limits the effective radius of the nucleus to be r < 2.8 km, assuming red geometric albedo 0.04. Conversely, the nucleus cannot be smaller than 0.16 km in radius if its coma is supplied by sublimation of carbon monoxide, and must be larger if a less volatile molecule drives the mass loss.

• The paper constrains 3I/ATLAS's nucleus size using photometry and convolutional brightness fitting, setting an upper limit of less than 2.8 km.
• It characterizes the coma morphology with a dominant sunward plume, quantifying dust ejection speeds that highlight weak gas-dust coupling.
• The study estimates dust mass loss rates of 6–60 kg/s and compares these findings with 1I/'Oumuamua and 2I/Borisov to refine interstellar object statistics.

Hubble Space Telescope Observations of the Interstellar Interloper 3I/ATLAS

Introduction

This paper presents high angular resolution HST/WFC3 observations of 3I/ATLAS, the third confirmed interstellar object to traverse the solar system, following 1I/'Oumuamua and 2I/Borisov. 3I/ATLAS exhibits a highly hyperbolic orbit ($e=6.143$, $i=175.1^\circ$, $q=1.357$ au) and an extreme velocity at infinity ($\sim$60 km s$^{-1}$), making it the most dynamically energetic interstellar interloper detected to date. The paper aims to constrain the nucleus size, characterize the dust activity, and compare 3I's properties to those of previous interstellar objects.

Observational Constraints on Nucleus Size

HST imaging reveals that 3I/ATLAS is active at 3.8 au pre-perihelion, with no discernible central condensation attributable to the nucleus. Photometric analysis within a 0.2" aperture yields $V=20.09$, corresponding to $H=14.42$ and a scattering cross-section of $6\times10^7$ m$^2$ (assuming $p_V=0.04$), which sets an upper limit of 4.4 km for the nucleus radius. More stringent constraints from convolutional surface brightness profile fitting, which models the nucleus as a Dirac delta function and extrapolates the coma profile inward, yield $r_n<2.8$ km ($H>15.4$). This upper limit is significantly larger than the nuclei of 1I/'Oumuamua ($\sim$0.1 km) and 2I/Borisov ($\sim$0.4 km), but the actual nucleus is likely much smaller, as the coma dominates the optical cross-section.

Coma Morphology and Dust Dynamics

The coma of 3I/ATLAS is characterized by a broad sunward plume and a weaker anti-solar tail, with the dust preferentially ejected towards the Sun. The surface brightness profile follows $\Sigma(p)\propto p^{-m}$, with $m\sim1$ for $p<0.4"$, steepening to $m\sim1.5$ at $p\sim1"$ and further at larger distances. The $m=1$ regime is consistent with steady-state expansion, while $m=1.5$ reflects radiation pressure acceleration. The steepening at larger $p$ may indicate progressive destruction of ice grains via sublimation, supported by the detection of a weak 2 $\mu$m water ice band in the coma.

Dust ejection speeds are derived from the extent of the sunward coma and the width of the radiation pressure tail. For dielectric spheres, $\beta\sim1/a_\mu$ (with $a_\mu$ in microns), and the sunward ejection speed is $V_\parallel=22\,a_\mu^{-1/2}$ m s$^{-1}$, while the perpendicular speed is $V_\perp=7.5\,a_\mu^{-1/2}$ m s$^{-1}$. These speeds are much lower than the thermal speed of sublimated gas ($V_{th}\sim460$ m s$^{-1}$ for H$_2$O at 183 K), indicating poor gas-dust coupling, likely due to large particle sizes, weak gas flow, or limited source regions.

Dust Mass Loss Rate and Particle Size Constraints

Photometry within a 1" aperture yields $V=17.95$ ($H=12.47$), corresponding to a scattering cross-section of $3.8\times10^8$ m$^2$. Assuming optically thin dust with $\rho=10^3$ kg m$^{-3}$, the total dust mass is $M=5.1\times10^5\,a_\mu$ kg. The steady-state mass loss rate is $dM/dt=6\,a_\mu^{1/2}$ kg s$^{-1}$, with $a_\mu$ in the range 1–100, implying $dM/dt=6$–60 kg s$^{-1}$. The dust-to-gas production ratio $\psi$ is estimated as 0.5–2, and spectroscopic limits on OH production ($Q_{OH}<8.2\times10^{26}$ s$^{-1}$) constrain $a_\mu$ to 4–70 $\mu$m, with a nominal value of $\sim$20 $\mu$m.

Sublimation models for H$_2$O, CO$_2$, and CO ices at 3.8 au yield maximum rates of $1.2\times10^{-5}$, $1.4\times10^{-4}$, and $3.1\times10^{-4}$ kg m$^{-2}$ s$^{-1}$, respectively. The required exposed ice areas to sustain the observed dust mass loss are modest, setting strong lower limits on the nucleus radius: $r_n\gtrsim0.16\,a_\mu^{1/4}$ km for CO, $0.23\,a_\mu^{1/4}$ km for CO$_2$, and $0.80\,a_\mu^{1/4}$ km for H$_2$O.

Comparison with Other Interstellar Interlopers

Scaling mass loss rates to 1 au ($r_H^{-2}$ dependence), 3I/ATLAS exhibits dust mass loss rates $>10^5$–$10^6$ times higher than 1I/'Oumuamua and comparable to or exceeding those of 2I/Borisov. The nucleus of 3I is inferred to be sub-kilometer, similar to 2I/Borisov, and much smaller than early estimates. The total mass loss is negligible compared to the nucleus mass.

The high entry velocity of 3I suggests a long interstellar residence time, with potential irradiation by galactic cosmic rays leading to the formation of a refractory mantle. The observed preferential day-side mass loss implies that any irradiation mantle is thinner than the diurnal thermal skin depth ($\sim$1 cm for comet-like rotation periods), or that the nucleus has a high obliquity with a spin pole directed towards the Sun, allowing deeper heat penetration.

Implications for Interstellar Object Population Statistics

The dominance of coma scattering in 3I/ATLAS complicates the interpretation of its detection and the derivation of interstellar object number densities. The HST upper limit on nucleus size alleviates the galactic mass budget problem posed by earlier overestimates. However, the activity-driven brightness enhancement means that similar-sized but inactive interlopers would likely escape detection, rendering population statistics uncertain and dependent on the unique case of 1I/'Oumuamua.

Conclusion

HST observations of 3I/ATLAS provide stringent constraints on the nucleus size ($r_n<2.8$ km), dust ejection speeds ($V_\parallel=22\,a_\mu^{-1/2}$ m s$^{-1}$, $V_\perp=7.5\,a_\mu^{-1/2}$ m s$^{-1}$), and mass loss rates ($dM/dt=6$–60 kg s$^{-1}$). The coma is dominated by sunward dust emission, with a weaker anti-solar tail, and the dust is likely composed of large particles with poor gas-dust coupling. The nucleus is inferred to be sub-kilometer, and the observed activity can be sustained by modest areas of exposed ice. The results highlight the diversity of interstellar interlopers and underscore the challenges in constraining their population statistics due to activity-driven detection biases. Future observations, particularly those capable of resolving the nucleus or detecting rotational modulation, will be critical for advancing our understanding of the physical properties and origins of interstellar objects.