Orbit Determination for Comet C/2011 W3 (Lovejoy) Using a New Technique

On Nov. 27, 2011, a bright, new member of the Kreutz system of sungrazing comets was discovered, designated C/2011 W3 (Lovejoy). During the 18 days remaining before perihelion, 116 ground-based astrometric observations were made, along with a several dozen from spacecraft observing the Sun. Unfortunately, pre-perihelion data alone was not sufficient for an accurate determination of the orbital period, and the spaceborne astrometric observations were not sufficiently accurate to help. Surprisingly, the comet survived perihelion, but it clearly underwent major changes: the nuclear condensation completely disappeared within days, and a narrow spine tail formed. Post-perihelion ground-based astrometry from Rob McNaught was referenced to the sunward tip of the spine tail, but it could not be used successfully in orbit solutions. We show that the spine tail was a synchronic feature which originated from the terminal disintegration of the nucleus, on Dec. 17.6 ± 0.2 UT (Sekanina & Chodas, submitted). In a new technique, we derive astrometric positions of the missing nucleus via two constraints: first, that it would lie on the extrapolated spine tail, and second, that it would lie on a line of orbital-period variation, obtained by forcing a range or orbital periods to sets of elements based on pre-perihelion astrometry. The resulting osculating orbital period is 698 ± 2 years, which shows that C/2011 W3 cannot be a fragment of any sungrazer observed since the 17th century, and must be a member of the expected new 21st-century cluster of bright Kreutz-system sungrazers, predicted by Sekanina & Chodas (2007).