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C/2006 M4 (SWAN)

Comet C/2006 M4 imaged in bright twilight and at full moon as it skirted the NE morning horizon from Great Shefford in early October 2006, 9 days after perihelion. 

Originally detected independently by Rob Matson (USA) and Michael Mattiazzo (Australia) from SOHO/SWAN satellite images in late June 2006, details were circulated to selected observers in the southern hemisphere to try and confirm the new comet, even though it was close to the Sun in the sky and difficult to observe. The comet was announced officially on 12 July 2006 in  IAUC 8729 (subscription required). 

Terry Lovejoy found he had images of the new comet from wide field survey images he had taken on 30 June 2006 with a digital camera and 100mm focal length lens, noting the comet as about ½° diameter while Rob McNaught using the 0.5-m Uppsala Schmidt telescope on July 12 noted a concentrated coma and a short tail less than 1½' long.

The comet was on its way into perihelion at that time, with closest approach to the Sun due on 28 Sep 2006 at a distance of 0.78 AU. However, the elongation of the comet steadily decreased as well and indeed it passed almost exactly behind the Sun - this is the predicted track against the SOHO C2 field of view

(click on image to display a larger image in a new window)

Unfortunately, this time coincided with a "SOHO Keyhole" (when little or no data is obtained from the spacecraft) and together with the comet being predicted to be as faint as 9th magnitude at that time, being twice as far away from us as the Sun during its passage through the field of view, nothing was seen of the comet.

See the SOHO news page here for a full account of the transit and also the discovery story from Rob Matson, including early images of the comet by Rob McNaught and Sebastian Hönig.

With astrometry reported for the comet from 16-27 July 2006 from the southern hemisphere, it was next reported from the Ageo Observatory in Japan as early as 18 September 2006 (after conjunction with the Sun) at 8th magnitude and at a solar elongation of only 25°. Over the next few weeks it gradually pulled away from the Sun and moved into the evening sky, reaching its closest to Earth at the end of Oct 2006 but still 1 AU away. Maximum elongation of 62° from the Sun will be achieved by mid-November 2006 but the comet will then close in to another conjunction with the Sun in Feb 2007. By the time the comet reappears out of the solar glow in April/May 2007 it will be well over 3 AU from both Sun and Earth and likely to be very faint.

The SOHO images were obtained by the LASCO instrument, on the SOHO satellite. The LASCO instrument was built and is operated by the LASCO consortium of the Naval Research Laboratory (Washington D.C.), The Laboratory for Space Astronomy, Marseilles (France), The Max Plank Institute for Aeronomy, Lindau (Germany) and The Department of Space Research, Birmingham (UK). SOHO is a joint ESA/NASA mission of international cooperation.


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