Comet C/2001 A2 (LINEAR) was discovered as part of the Lincoln Laboratory Near-Earth asteroid survey (LINEAR) on 16 January 2001. Observing conditions were favorable at Arecibo July 5-15 2001, and observing time for both spectroscopy of 18-cm OH lines and S-band radar observations were arranged. The comet nucleus was observed to split into two pieces on April 30. Component A broke into two pieces May 16, and faded away within a week. Component B, now C/2001 A2-B LINEAR, continued with nearly the original predicted brightness. Three additional fragments were seen to separate from A2-B in early June, at which time there was an increase in gas and dust emission.

The Arecibo observations of the OH lines took place 5-6 July 2001. The OH in the cometary coma absorbs solar radiation, which can be re-emitted as microwaves under favorable conditions. The emission strength depends on the radial heliocentric velocity of the comet and on the amount of OH in the coma. The 1612, 1665, and 1667 MHz OH lines were observed, but only the 1667 MHz line was detected. In addition to looking at the nucleus, we also observed one beam width away (4.1 arcmin), in a hexagonal pattern around the nucleus, oriented along the sun-tailward direction. The figure shows the nucleus spectrum and the sum of all six of the offset positions. The strengths of these lines will be compared to model predictions to determine the outflow velocity, identify asymmetries, and to search for signs of collisional quenching. In very active comets, such as Hale-Bopp, the OH lines were thermalized or ``quenched'' in the inner 500,000 km of the coma. One of the goals of this investigation is to determine the extent of quenching in more ``normal'' comets with lower gas production rates.ˆà

The spectrum of Comet LINEAR A2-B shows the 1667 MHz OH line in emission.

Because the comet passed within 0.25 AU of the Earth in early July, when it was visible from Arecibo Observatory, it was fairly well suited to radar observation. We detected the comet with the S-band (2380 MHz) radar in continuous wave (CW) mode on July 7-10. The spectrum shows a broad profile of return echo power from the cometary coma. There is no clear central spike, indicating return from the nucleus, contrary to what has been seen on other comets (Harmon et al 1999). Some return was also seen in the unexpected or same-sense polarization, at 20% of the expected sense. This indicates that the particles in the coma are comparable in size to the radar wavelength (12.6 cm). Given the fragmentation of this comet in the previous months, it may not be surprising to find large-sized debris in the coma. The gas and dust production from this comet is close to the prediction for a several-km nucleus. We attempted to image the comet on July 9 and 10, and those data are still being analyzed.ˆà

This CW echo from Comet LINEAR A2-B shows a broad return from the coma, in both opposite-circular (upper plot) and same-circular (lower plot) polarization. This indicates that the particles are comparable in size to the radar wavelength (12.6 cm) No clear return from the nucleus is seen.