46P/Wirtanen

46P/Wirtanen has been observed at 9 appearances (1947, 1954, 1961, 1967, 1974, 1986, 1991, 1997 & 2002). The comet was discovered on January 15th 1948, from Lick Observatory, as a 17th magnitude object that was receeding from perihelion. Due to the small number of observations made and the short arc covered the orbit was initially very uncertain and it was a full year before it was recognised as being of short period.

The comet was recovered in 1954 by Carl Wirtanen himself at Lick, very close to the predicted position, although a rather fainter object than in 1948. From then the comet was observed at each return until 1974. At that apparition the comet was extremely faint and had just suffered a major perturbation by Jupiter that sharply reduced its orbital period from 6.7 to 5.9 years and the perihelion distance from 1.61 to 1.26AU.

The 1980 apparition was similarly unfavorable to the circumstances in 1974 but, despite the comet's decrease in perihelion distance, this time it could not be recovered. A new Jupiter encounter brought the period and perihelion distance sharply down again, this time to 5.5 years and 1.08AU allowing the theoretical possibility of very close approaches to the Earth.

The comet's current period is 5.44 years and the perihelion distance has decreased slightly further to 1.06AU.

This comet was the chosen target for the ESA ROSETTA mission that was to be launched in January 2003 and encounter the comet in 2014 after two asteroid fly-bys, and gravity assists from Mars and the Earth. Unfortunately, the Ariane 5 failure in December 2002 led to the suspension of all ESA launches and the very tight launch window being missed. A mission profile still exists that would permit an encounter, but only of the more powerful Russian Proton booster is used. This would imply a launch in late 2004 and an encounter in 2014, some 2 years from perihelion. One of the attractions of 46P/Wirtanen is its small size and moderate activity. The nucleus is estimated to be only 1.4km across. Both the main probe and the lander are designed to operate in this small gravity field, so a mission profile that allows 46P to be encountered still, rather than a larger comet, is an attractive option.

The 2002 apparition

46P/Wirtanen passed perihelion on August 26th and peaked at magnitude 10. Since then it has been poorly placed for observers in the dawn sky, but appears to have suffered a major outburst as the data (left) from Giovanni Sostero show. Visually the comet may have already reached magnitude 8. Studies of the light curves of periodic comets show that objects that have large decreases in perihelion distance are often unusually active for a number of returns afterwards. The light curve activates very suddenly and shows an extremely rapid rate of brightening (a power law around 18th power, against the "standard" 4th power law).

 

  

 

CCD observations in a 10 arcsecond aperture by:

  • Ramón Naves & Montse Campàs - MPC 213
  • Rolando Ligustri - MPC 235
  • Albert Sánchez – MPC 442
  • Juan Rodríguez – MPC 620

CCD total magnitude estimates in apertures of 0'.3, 0'.6, 1'.0, 1'.2 and 1'.3 by.

  • Rolando Ligustri
  • Giovanni Sostero

Última actualización 18/05/2003  
Por M.R.Kidger