My core research centers on the activity and evolution of comets. How are the gases that we see connected to ices in the nucleus? In how far do comets represent left-over building blocks from the disk out of which our planets were formed? I try to answer these questions by combining remote, telescopic observations with in-situ exploration by planetary missions (Rosetta, EPOXI, and Stardust NEXT). One of my primary tools is the Swift Gamma-ray observatory. Swift’s unique capabilities offers a unique perspective on comets.

Evolution of Activity

How does the activity of comets change with age? Dynamically new comets from the Oort cloud show different sublimation behavior before and after they are heated by the Sun for the first time. Swift’s Ultraviolet-Optical Telescope (UVOT) can observe comets at UV wavelengths not accessible from Earth. Its sensitivity in UV wavelengths allows us to detect OH (hydroxyl) when the comet passes Jupiter’s orbit. The OH molecule is an important tracer of cometary activity: it is formed when water molecules are destroyed by solar UV light. For this I have developed a technique that uses Swift’s UVW1 filter, which is centered on the exact wavelength on which OH molecules re-emit sun light. We are currently working on a long-term survey of the evolution of the activity of Oort Cloud comets, including comets ISON, Garradd, and Siding Spring [1].

Heterogeneity of Comet Nuclei

Images of comet nuclei taken with spacecraft such as Deep Impact suggest that comet nuclei consist of smaller building blocks. These cometesimals were formed in the protosolar disk. Chemical heterogeneity of comet nuclei directly links comets to planet formation. Swift’s Ultraviolet-Optical Telescope (UVOT) is equipped with grisms that provides 2D spectral-spatial images of the coma. We have developed tools to analyze grism images of comets to measure the chemical composition of gas in the coma [2].

Like Earth, comets know day and night. Different parts of the nucleus turn active when icy areas rotate in and out of sunlight. By looking for jets and by measuring the composition of the coma as the comet rotates it is possible to look for differences in the ices in the nucleus. Because Swift is in space, it is possible to observe comets almost uninterruptedly for an entire rotation period.


Swift was designed for the rapid follow-up of gamma ray bursts. It can be on-target within 24h, a unique feature for a space telescope. When it was reported that the large asteroid (596) Scheila suddenly showed a giant tail, Swift was the first telescope to search for water. To investigate whether the asteroid was a dormant comet Swift used both grism spectroscopy and filter imaging. No evidence of water was found and we concluded that Scheila was impacted by a smaller, 100-meter sized asteroid [3].

Interaction with the Solar Wind

Swift is equipped with three telescopes that allow it simultaneously detect gamma rays, X-rays, and UV-optical light. Cometary X-rays from the interaction between the solar wind and neutral gas in the coma are explained on a separate page.


[1] ‘The Evolving Activity of Dynamically Young Comet C/2009 P1 (Garradd)’ - D. Bodewits, T. L. Farnham, M. F. A’Hearn, L. M. Feaga, A. McKay, D. G. Schleicher, and J. M. Sunshine, submitted

[2] ‘Swift UVOT Grism Spectroscopy of Comets: a First application to C/2007 N3 (Lulin)’ - D. Bodewits, G. L. Villanueva, M. J. Mumma, W. B Landsman, J. A. Carter, and A. M. Read, Astronomical Journal 141, 12 (2011)

[3] ‘Collisional Excavation of Asteroid (596) Scheila’ - D. Bodewits, M.S. Kelley, J.Y. Li, W.B. Landsman, S. Besse and M.F. A’Hearn. Astrophysical Journal Letters 733, L3, 2011 (ArXiv here)

Left: The UVOT grism produces images that separate incoming light from ultraviolet and visible sources by wavelength. Molecules produce separate images at discrete wavelengths. Comet dust, which reflects sun light, instead produces a broad continuous swath.

Above: Swift observations of Comet 81P/Wild-2, target of the Stardust mission. The bright jet to the left was no longer active during later observations, due to seasonal effects on the nucleus.

BElOW: Swift started observing Comet C/2012 S1 (ISON) while it was still far from the Sun. At a distance of 5 AU (750 million kilometers) from the Sun, the comet already had a clear tail - proving it was already active. Much closer to the Sun at November 2013, Swift mapped the comets enormous gas coma of hundreds of thousands of kilometers across. Swift used its filters to image the water (blue; OH) and dust (yellow) around the comet.