Spitzer Space Telescope Observations of the Nucleus of Comet 103P/Hartley 2

We have used the Spitzer 22-um peakup array to observe thermal emission from the nucleus and trail of comet 103P/Hartley 2, the target of NASA’s Deep Impact Extended mission. The comet was observed on UT 2008 August 12 and 13, while the comet was 5.5 AU from the Sun. We obtained two 200-frame sets of photometric imaging over a 2.7-hour period. To within the errors of the measurement, we find no detection of any temporal variation between the two images. The comet showed extended emission beyond a point source in the form of a faint trail directed along the comet’s anti-velocity vector. After modeling and removing the trail emission, a NEATM model for the nuclear emission with beaming parameter of 0.95 +/- 0.20 indicates a small effective radius for the nucleus of 0.57 +/- 0.08 km and low geometric albedo 0.028 +/- 0.009 (1 sigma). With this nucleus size and a water production rate of 3 x 10^28 molecules s-1 at perihelion (A’Hearn et al. 1995) we estimate that ~100% of the surface area is actively emitting volatile material at perihelion. Reports of emission activity out to ~5 AU (Lowry et al. 2001, Snodgrass et al. 2008) support our finding of a highly active nuclear surface. Compared to Deep Impact’s first target, comet 9P/Tempel 1, Hartley 2’s nucleus is one-fifth as wide (and about one-hundredth the mass) while producing a similar amount of outgassing at perihelion with about 13 times the active surface fraction. Unlike Tempel 1, it should be highly susceptible to jet driven spin-up torques, and so could be rotating at a much higher frequency. Barring a catastrophic breakup or major fragmentation event, the comet should be able to survive up to another 100 apparitions (~700 yrs) at its current rate of mass loss.

💡 Research Summary

The paper presents thermal‑infrared observations of comet 103P/Hartley 2 obtained with the Spitzer Space Telescope’s 22‑µm peak‑up array when the comet was at a heliocentric distance of 5.5 AU. Two imaging sequences, each consisting of 200 frames and spanning a total of 2.7 hours on 12–13 August 2008, were acquired. No statistically significant brightness variation was detected between the two epochs, indicating that the comet’s activity level was essentially steady during the observing window.

The images reveal an extended, faint dust trail aligned with the comet’s anti‑velocity vector. This trail is interpreted as a remnant of large particles released during previous perihelion passages and now following the comet’s orbit. By modeling the trail’s surface‑brightness profile and subtracting it, the authors isolate the pure nuclear thermal emission.

The isolated nuclear flux is then fitted with the Near‑Earth Asteroid Thermal Model (NEATM). A beaming parameter η = 0.95 ± 0.20 is adopted, reflecting a modest surface roughness and moderate thermal inertia typical of small cometary nuclei. The NEATM fit yields an effective radius of 0.57 ± 0.08 km and a geometric albedo of 0.028 ± 0.009 (1 σ). The very low albedo implies a dark, carbon‑rich surface, possibly coated with fine dust.

Combining the derived nucleus size with the water‑production rate measured at perihelion (3 × 10²⁸ molecules s⁻¹; A’Hearn et al. 1995), the authors calculate that essentially the entire surface (≈1 km²) must be active at perihelion. This conclusion is reinforced by independent reports of activity out to ∼5 AU (Lowry et al. 2001; Snodgrass et al. 2008), suggesting that Hartley 2 possesses a highly volatile‑rich, globally active surface.

A comparison with comet 9P/Tempel 1, the original Deep Impact target, highlights striking differences. Hartley 2’s nucleus is roughly one‑fifth the diameter and about one‑hundredth the mass of Tempel 1, yet it produces a comparable water outgassing rate at perihelion. Consequently, the active fraction of Hartley 2’s surface is about 13 times larger than that of Tempel 1. The small size and high active fraction make Hartley 2 especially susceptible to jet‑driven torques, potentially leading to rapid spin‑up. Although the exact rotation period was not determined in this study, the authors argue that the comet could be rotating much faster than Tempel 1, with implications for its structural stability.

Estimating the mass‑loss rate from the water production (assuming a bulk density of ~0.5 g cm⁻³) yields a nucleus lifetime of roughly 700 years, or about 100 orbital apparitions, provided the current activity level remains unchanged. This suggests that Hartley 2 can survive for several centuries, barring catastrophic fragmentation or a major breakup event.

Methodologically, the work demonstrates that thermal‑infrared observations at large heliocentric distances can effectively disentangle nucleus and dust contributions, even for faint, low‑activity comets. The combination of trail modeling, subtraction, and NEATM fitting provides a robust framework for determining nucleus size and albedo, which can be applied to future studies of small, active comets.

In summary, the Spitzer observations reveal that comet 103P/Hartley 2 has a very small, dark nucleus with an unusually high active surface fraction. Its modest mass and strong outgassing make it a prime candidate for rapid rotational evolution, yet its estimated lifetime of several hundred years indicates that it will remain an observable target for many future apparitions, offering valuable opportunities to study the physical processes governing small cometary nuclei.