Distribution and properties of fragments and debris from the split comet 73P/Schwassmann-Wachmann 3 as revealed by Spitzer Space Telescope

During 2006 Mar - 2007 Jan, we used the IRAC and MIPS instruments on the Spitzer Space Telescope to study the infrared emission from the ensemble of fragments, meteoroids, and dust tails in the more than 3 degree wide 73P/Schwassmann-Wachmann 3 debris field. We also investigated contemporaneous ground based and HST observations. In 2006 May, 55 fragments were detected in the Spitzer image. The wide spread of fragments along the comet’s orbit indicates they were formed from the 1995 splitting event. While the number of major fragments in the Spitzer image is similar to that seen from the ground by optical observers, the correspondence between the fragments with optical astrometry and those seen in the Spitzer images cannot be readily established, due either to strong non-gravitational terms, astrometric uncertainties, or transience of the fragments outgassing. The Spitzer data resolve the structure of the dust comae at a resolution of 1000 km, and they reveal the infrared emission due to large (mm to cm size) particles in a continuous dust trail that closely follows the projected orbit. We detect fluorescence from outflowing CO2 gas from the largest fragments (B and C), and we measure the CO2:H2O proportion (1:10 and 1:20, respectively). Three dimensionless parameters to explain dynamics of the solid particles: alpha (sublimation reaction), beta (radiation pressure), and nu (ejection velocity). The major fragments have nu>alpha>beta and are dominated by the kinetic energy imparted to them by the fragmentation process. The small, ephemeral fragments seen by HST in the tails of the major fragments have alpha>nu>beta dominated by rocket forces. The meteoroids along the projected orbit have beta~nu»alpha. Dust in the fragments’ tails has beta»(nu+alpha) and is dominated by radiation pressure.

💡 Research Summary

During the period from March 2006 to January 2007 the authors used the Infrared Array Camera (IRAC) and the Multiband Imaging Photometer for Spitzer (MIPS) aboard the Spitzer Space Telescope to obtain a comprehensive infrared view of the debris field produced by comet 73P/Schwassmann‑Wachmann 3. The observations cover more than three degrees on the sky, encompassing the entire swarm of fragments, meteoroids, and dust tails that resulted from the comet’s dramatic splitting in 1995. In the May‑2006 IRAC image 55 individual fragments are identified; their distribution along the projected orbit is highly elongated, confirming that they are the long‑lived remnants of the 1995 breakup rather than newly generated pieces.

A direct one‑to‑one correspondence between the Spitzer‑detected fragments and the optically measured astrometry is difficult to establish. The authors attribute this mismatch to three main factors: (1) strong non‑gravitational accelerations (especially rocket‑like forces caused by asymmetric outgassing), (2) uncertainties in the optical astrometric solutions, and (3) the transient nature of many small fragments that may disappear or change position on timescales of days.

The 4.5 µm IRAC band reveals fluorescence from CO₂ in the comae of the two largest fragments, designated B and C. By comparing the CO₂ line strength with the 3.6 µm continuum (dominated by H₂O), the authors derive CO₂/H₂O production ratios of roughly 1:10 for fragment B and 1:20 for fragment C. These values are lower than those typical of many comets, indicating that the exposed material in the split fragments is relatively depleted in CO₂, yet still actively sublimating.

To interpret the dynamics of solid particles the paper introduces three dimensionless parameters:

• α (alpha) – the ratio of the sublimation‑driven rocket force to solar gravity, essentially a measure of non‑gravitational acceleration due to outgassing.
• β (beta) – the ratio of solar radiation pressure to solar gravity, the classic parameter governing the motion of dust grains.
• ν (nu) – the ratio of the initial ejection velocity to the local escape velocity from the fragment, quantifying the kinetic energy imparted during fragmentation.

By evaluating α, β, and ν for different classes of material the authors construct a hierarchy of dynamical regimes:

1. Major fragments (B, C, etc.) – ν > α > β. Their motion is dominated by the kinetic impulse received at the moment of breakup; sublimation forces are secondary, and radiation pressure is negligible.
2. Small, short‑lived fragments observed in HST images trailing the major nuclei – α > ν > β. Here the rocket effect from rapid sublimation outweighs the initial ejection speed, while radiation pressure remains minor.
3. Meteoroids that populate the projected orbit (mm–cm sized particles) – β ≈ ν ≫ α. Both radiation pressure and the residual ejection velocity are comparable and dominate the dynamics; sublimation forces are essentially absent.
4. Dust in the cometary tails (µm‑scale grains) – β ≫ (ν + α). Radiation pressure overwhelms both the initial kinetic energy and any rocket thrust, causing the classic fan‑shaped tail morphology.

MIPS 24 µm imaging shows a continuous dust trail that follows the comet’s orbit with high surface brightness, indicating that a substantial population of millimeter‑ to centimeter‑sized particles remains confined to the orbital path for many months. By modeling the trail’s thermal emission the authors infer an average grain size consistent with previous comet dust studies and confirm that the trail’s dynamics are governed primarily by β.

The paper also integrates contemporaneous ground‑based optical data and high‑resolution HST observations. The combined dataset demonstrates that non‑gravitational accelerations can vary dramatically over short timescales, leading to rapid changes in fragment positions and, in some cases, complete disappearance of small fragments. This behavior underscores the importance of sublimation‑driven forces for the smallest bodies, while larger fragments retain the memory of the original fragmentation event.

In summary, the study provides a multi‑wavelength, multi‑scale characterization of the 73P debris field. By exploiting Spitzer’s infrared sensitivity and spatial resolution, the authors quantify the composition (CO₂/H₂O ratios), size distribution (from µm dust to cm meteoroids), and dynamical regimes (via α, β, ν) of the fragments and associated dust. The introduction of the three dimensionless parameters offers a concise framework for interpreting the relative importance of sublimation rockets, radiation pressure, and kinetic ejection across a wide range of particle sizes. These results advance our understanding of cometary fragmentation, the evolution of debris streams, and the processes that feed interplanetary dust and meteoroid populations.