Since its launch in late 1995, the Solar and Heliospheric Observatory (SOHO) has discovered more than 2000 comets. The vast majority of these have been members of the Kreutz group of sungrazing comets, having perihelion distances within 1-2 solar radii, highly inclined orbits (~143 degrees) and periods of 500-1000 years. In addition to the Kreutz group, four new groups of near-Sun comets have been discovered in SOHO images: the Marsden, Meyer, Kracht, and Kracht-II groups, and nearly 100 non-group comets have been discovered. While most SOHO comets are observed for less than 24 hours, the sheer quantity provides a robust dataset. These near-Sun comets present opportunities to probe otherwise unexplored regions of the Solar System, encountering vastly different temperatures and stresses than are experienced by all other known small bodies. By understanding their compositions and physical structure it is hoped that we can learn about the environment in which they formed millions of years ago.

My thesis built on the work by Biesecker et al. (2002) who studied the Kreutz comets which arrived from 1996-1998. Biesecker et al. showed that Kreutz comets behave in a characteristic fashion: brightening as they approach the Sun, reaching a peak in brightness prior to perihelion, then fading as they continue to approach perihelion. They also found that the brightening followed two "universal curves" which peaked at slightly different heliocentric distances. I have updated and improved their photometric routines (presented at DPS in 2004) and have calculated the photometry for all SOHO comets which were discovered by February 2006 (approximately 1100 comets and more than 20,000 images). From this much larger sample of light curves (the original "two universal curves" model was derived from only the 56 brightest comets seen from 1996-1998), it appears that the "two universal curves" are smeared out and become part of a broader distribution of peaks in the brightness which occur from 10-14 solar radii (presented at DPS in 2005). I have investigated the nature of the orange-clear color discrepancy in the SOHO photometry, finding that it can be explained by the presence of sodium emission roughly 300 times the solar continuum at those wavelengths (presented at DPS in 2006), a finding consistent with recent observations of comet C/2004 Q2 Machholz. The sungrazing comets are observed over a large range of phase angles, and individual comets often change phase dramatically in a short time (20-30 degrees in ~24 hours for Kreutz comets, as much as 120 degrees in 37 hours for Marsden comets). This necessitates a correction for phase angle which is based off the work of Marcus (2007a, 2007b) (presented at ACM in 2008).

I have now completed the photometric analysis of the Kreutz comets seen by SOHO from 1996-2005, and it is published in AJ. I previously presented a summary at DPS, and in chapter 3 of my thesis. Key results include: The lightcurves peak from 10.5-14 solar radii (prior to perihelion) and appear to represent a continuum of compositions rather than two distinct subpopulations. The lightcurves have two rates of brightening, typically near r^-7.3 when first observed by SOHO then rapidly transitioning to r^-3.8 between 20-30 solar radii. It is unclear at what distance the steeper slope begins but it likely does not extend much beyond the SOHO field of view. We derive nuclear sizes up to ~50 meters in radius for the SOHO observed comets, with a cumulative size distribution of N(>R)~R^-2.2 for comets from 5-35 meters in radius. The size distribution cannot explain the six largest members seen from the ground, suggesting that either the family is not collisionally evolved or that the distribution is not uniform around the orbit. The total mass of the distribution up to the largest expected size (~500 meters assuming an 800 year orbital period) is ~4e14 g, much less than the estimated mass of the largest ground observed members. After correcting for the changing discovery circumstances, the flux of comets reaching perihelion has increased since 1996, and the increase is seen in comets of all sizes.

In addition to the physical studies of the Kreutz system, I have investigated the dynamical history of the Kreutz, Marsden, and Kracht groups. Using HNBody (Rauch & Hamilton 2002) I have investigated the apparent cascading fragmentation (e.g. Sekanina 2002) and attempted to explain the recent evolutionary history of these groups (presented at DPS in 2007 and AAS in 2008). Dynamical simulations suggest that the orbital distribution of the Kracht group can be explained by low velocity fragmentation events and close approaches to Jupiter over the last 50-250 years. Based on the lightcurves of the the known Kracht and Marsden comets, we estimate that they are all smaller than ~30 meters in radius, although the survival of a number of these comets indicates that many do not disrupt in the SOHO field of view (the assumption inherent in the size estimate). Thus, they are likely larger than the photometry indicates. 7-8 comets in each group may be visible on their next perihelion passage, and Marsden comet C/2010 H3 was recently observed for a third time (previously seen as 1999 J6 and 2004 V9). My photometry was instrumental in identifying this as the same comet (CBET 2256), since its return was 11.5 days earlier than predicted. See chapter 4 of my thesis for more discussion and Rainer Kracht's website for more comprehensive dynamical linkages.

On July 4, 2005 the impactor from the Deep Impact mission struck comet 9P/Tempel 1. In support of this mission I observed the comet in the days before, during, and after impact from Kitt Peak National Observatory in Arizona. We observed the comet in the near-IR using the SQIID instrument on the 2.1-m telescope. This allowed us to observe the comet simultaneously in J, H, and K, and measure the color of the ejecta using aperture photometry. We combined our data with broadband optical (BVRI) data acquired at the same time by Kevin Walsh and colleagues at San Pedro Martir (Mexico), finding that the ejecta "bluened" immediately after impact and returned to their normal color by the following night, suggesting the ejecta were rich in water ice. These results were published in the Deep Impact special issue of Icarus.

I observed comet 73P/Schwassmann-Wachmann 3 (aka. SW3) near it's close approach to the Earth in May 2006. I observed SW3 in the near-IR using the SQIID instrument on the 2.1-m telescope at Kitt Peak. I observed fragments B and C over 8 nights, fortuitously observing fragment B during an outburst in which it brightened by >4 magnitudes from May 7-8. These observations were part of a collaboration in which we observed the comet in the optical and near-IR using multiple observing modes (narrowband imaging, broadband imaging, and spectroscopy) on several telescopes (KPNO 4-m, KPNO 2.1-m, WIYN, and McMath-Pierce Solar) over several weeks leading up to close approach. Preliminary results of this collaboration were presented at DPS [abstract] [abstract] and AAS [abstract] in 2006, and we expected to submit papers for publication in 2008.

For the 2006 COSPAR conference in Beijing, China, I estimated the likely discovery rates of Kreutz sungrazing comets for STEREO and ground based all-sky surveys like Pan-STARRS [abstract]. While the potential for Kreutz discoveries with STEREO was good, the early returns are not promising, as the bandpass is unfavorable (the extremely bright sodium doublet is beyond the range) and the observing sequences tend to wipe faint, fast moving objects like comets before downlinking the data to the Earth. Due to the geometry of the Kreutz orbit, a survey with capabilities comparable to Pan-STARRS located in the Southern Hemisphere would stand a reasonable chance of discovering Kreutz comets several months before they reached perihelion. Without knowing the slope of the brightening beyond the SOHO field of view (we attempted to constrain this by conducting a small survey using the MOSAIC camera on the KPNO 4-m in January 2005) it is impossible to predict the actual number of Kreutz comets that might be discovered by such a survey.

In 2002 I conducted a study (presented at the Mitigation of Hazardous Asteroids and Comets Conference Conference) to determine whether the increase in surveys was doing a satisfactory job of discovering new comets. Contrary to expectations, amateur comet hunters were still discovering comets at nearly the same rate as they were prior to the influx of surveys. Also, the surveys were systematically missing many comets which were first visible in the Southern Hemisphere, bad news in the case of a "killer comet"!