An Ultraviolet Spectrometer for the Rosetta Orbiter

Periodic comet Wirtanen was discovered in 1948 by Carl Wirtanen of Lick Observatory in the course of a stellar proper motion survey. The plate on which the discovery was made was taken on 1948 January 15, about a month and a half after the comet had passed perihelion, at which time it was about 16th magnitude. Pre-discovery plates are known which provide positions of the comet from the beginning of December 1947. Within a month after discovery, the comet was determined to be short-periodic. A moderately close approach to Jupiter in April 1972 reduced the perihelion distance to 1.26 AU and the perihelion distance will remain well below 1.5 AU until the mid 21st century. Long-term integrations show that, other than during the current period, the perihelion distance is typically between 1.5 AU and 2 AU from the 17th through the 24th century. 46P/Wirtanen belongs to the Jupiter family of comets, all of which have aphelia between 5 and 6 AU, and it has a period of 5.46 years. There have been four other, non-periodic comets also called "Wirtanen", discovered during the period 1947-1956.

Not much was known about 46P/Wirtanen prior to its being selected as the target for the Rosetta mission. Since then a number of studies and observations have been made of this object, the results of which are summarized below.

Physical Data

• The average radius of the nucleus, i.e., the radius of a sphere with the same cross-section, is only 600m (assuming a reflectivity like that of comet Halley), with an axial ratio > 1.2 for projected cross-section variation and a rotational period of 6 hours. [Lamy, et al. (1998)]

• Nuclear reflectivity increases 10% per 1000 Angstroms, indicating a moderately red color for the nucleus. [Lamy, et al. (1998)]

• Outgassing activity
  • Water (based on OH): roughly 1 to 2 x 10²⁸ molecules per second or 300-600 kg/s.

    This number is based on an average of the values from Stern, et al. (1998) pL30, Fink, et al. (1998) pL37 and Farnham & Schleicher (1998) pL50. The comet is less active by about a factor 100 than 1P/Halley and by about a factor 1000 than C/Hale-Bopp.

  • Dust Production: 100 kg/s

    This value is an average of the optical continuum measurements by Fink, et al (1998) pL37, Schulz, et al. (1998) pL46, Farnham & Schleicher (1998) pL50, and Jockers, et al. (1998) pL56; scaled to production rate using the empirical relation found by Arpigny (Atlas of Cometary Spectra, Kluwer Academic Publishers, in press).

  These numbers, together with the size deduced above, imply that the comet must be active over nearly its entire surface. The comet has a lower dust/gas ratio than average for comets.

• Farnham & Schleicher (1998) included this plot of molecular production and outgassing rates vs. heliocentric distance.

Orbital Data

The following information comes from the JPL Horizons System and the 1997 Catalogue of Cometary Orbits.

Period	5.46 years (among the shorter periods for Jupiter-family comets)
Perihelion Distance	1.064 AU
Eccentricity	0.657
Argument of Perihelion	356.3°
Longitude of the Ascending Node	82.2°
Inclination	11.7°
Perihelion Passages	27 Aug 2002 02 Feb 2008 09 Jul 2013 = end of Rosetta mission

Images

• This series of ESO/ESA images of P/Wirtanen show some evolution of the comet's structure during its perihelion approach in 1996.

• This color image of P/Wirtanen was kindly provided by the Max-Planck-Institut für Aeronomie.

Light Curves

Charles Morris (JPL) has compiled individual light curves for each of comet Wirtanen's 1986 and 1991 apparitions, and has also generated a combined light curve for both apparitions.

Additional information and references are also available at Mr. Morris's 46P/Wirtanen web page.

UV Spectra

• ALICE will be studying the comet in the ultraviolet (UV) range. These ultraviolet spectra were taken by ALICE team members with additional collaborators using the Hubble Space Telescope.

• Also available is a more detailed version of the last of these spectra, dated 15 January 1997.

References

Stern, S.A., J.W. Parker, M.C. Festou, M.F. A'Hearn, P.D. Feldman, G. Schwehm. R. Shulz, J.-L. Bertaux and D.C. Slater 1998, A&A. 335. L30-L36.

Fink, U., M.D. Hicks, R.A. Fevig, and J. Collins 1998, A&A, 335, L37-L45.

Schulz, R., C. Arpigny, J. Manfroid, J.A. Stüwe, G.P. Tozzi, K. Rembor, G. Cremonese, and S. Peschke 1998, A&A, 335, L46-L49.

Farnham, T.L., and D.G. Schleicher 1998, A&A, 335, L50-L55.

Jockers, K., T. Credner and T. Bonev 1998, A&A, 335, L56-L59.