C/2001 A2 (LINEAR)

Lore & Background

The comet was discovered on 15 January 2001 by the LINEAR team as an asteroidal object of apparent magnitude about 16. Precovery images from 3 and 5 January from LONEOS and LINEAR were also found. CCD imaging revealed a coma 10 to 18 arcseconds across, with an apparent magnitude of 17.2. The first orbital ephemeris indicated the comet would brighten to magnitude 10 in late June 2001. It brightened slowly, reaching magnitude 13.1 on 14 March, then experienced an outburst in late March, jumping from magnitude 10.8 on 28 March to 8 on 30 March. By mid-April it was magnitude 7.5, and by the end of the month about 6.3.

On 30 April 2001, Carl W. Hergenrother, Matthew A. Chamberlain, and Y. Chamberlain reported a double nucleus in images, with two equally bright nuclei separated by 3.5 arcseconds. Zdenek Sekanina calculated the splitting occurred on March 29.9 ± 1.6 UT, coinciding with the outburst. Images from the VLT MELIPAL telescope on 14 May showed nucleus B was elongated; two days later it had split into two pieces separated by about one arcsecond. Fragment A was last detected on 19 May. Fragment G, found on 21 May, had its own tail and likely separated from fragment B five weeks earlier. Another outburst in June brought the comet to magnitude 3.3 on 12 June; by 18 June it had faded to 4.3, with a coma 15 arcminutes across and a tail 4.5 degrees long. High-resolution imaging on 16–21 June revealed fragments D, E, and F near fragment B, with separation dates of June 3.5, 9.5, and 11.3 respectively. The closest approach to Earth was on 30 June 2001 at 0.244 AU. On 1 July the magnitude was 4.4–4.5, and on 5 July two tails each two degrees long were observed. A mid-July outburst reached magnitude 4.4 on 13 July, then faded to about 7 by month's end. Smaller outbursts of 0.4 and 0.2 magnitude occurred on July 25.0 and 30.0.

Reader's Guide

C/2001 A2 (LINEAR) is significant for its multiple fragmentation events and the insights they provided into cometary nucleus heterogeneity. The near-infrared spectrum obtained by NIRSPEC at Keck-2 Observatory on 9–10 July 2001 showed enhanced abundances of ethane, acetylene, hydrogen cyanide, and methanol relative to water, while methane, carbon monoxide, and formaldehyde had more typical abundances. The rotational temperature of volatiles was about 100 K, except for HCN at about 60 K. Observations on 4 and 10 August revealed that methane production increased greatly relative to water between July and August, while formaldehyde abundance decreased fourfold between 9 and 10 July, indicating a heterogeneous nucleus. Radio observations with IRAM-30m and Kitt Peak 12-m telescopes found an upper limit HC3N/HCN ratio similar to comets 153P/Ikeya-Zhang and Hale-Bopp, but lower acetylene-to-water and CH3CN ratios. The HNC/HCN production rate ratio increased as the comet approached the Sun. The Arecibo Observatory detected the echo of large dust grains in the coma, which was depolarized—the first such detection in a comet radar echo. The Far Ultraviolet Spectroscopic Explorer on 12 July 2001 detected, for the first time in a comet, the C–X and B–X lines of carbon monoxide, the OI line at 1152 Å, and three hydrogen Lyman series lines. The flux of all lines decreased by a factor of two within 7.5 hours. The CO emission had a cold and a hot component, with the cold component attributed to excitation of CO2.

Did You Know?

• The comet's nucleus split into multiple fragments, with the first splitting calculated to have occurred on March 29.9 ± 1.6 UT.
• Fragment G was discovered on 21 May 2001 and had its own tail.
• The comet was observed by the Far Ultraviolet Spectroscopic Explorer on 12 July 2001, which detected carbon monoxide lines for the first time in a comet.
• The radar echo from the comet's large dust grains, detected by Arecibo Observatory, was depolarized—the first such detection.