Comet C/1999 S4 LINEAR


Discovered by Lincoln Near Earth Asteroid Research observatory, USA, this comet is the brightest since Hale-Bopp in 1997. However, it was never expected to match for Hale-Bopp as was forecast to reach only magnitude 5.5 (on 23 Jul 2000), though in the event failed to reach even naked eye brightness. It was, however, a very interesting comet for large telescopes as pieces were observed to break off the nucleus during July and the comet disintegrated completely towards the end of July.

Observations up to 20 Jul 2000 (from Oxted)

Prior to this date repeated searches have drawn a blank. With the near full moon these last few days and the lights of London to the north I have not conclusively seen the comet, even with binoculars. The last few nights have been reasonably clear but haze and thin clouds have added to the difficulty.

On the 20th, with the moon past full and rising sufficiently late I caught my first glimpse of LINEAR in 7 x 56 binoculars at 1130BST. Even with these LINEAR was not easy found in the sky glow above London. It appeared as a fuzzy blob of about the 8th magnitude with a 1/4 degree tail of pointing to about 20 degrees right of the vertical.

Observations up to 21 - 26 Jul 2000 (from Oxted)

All nights up to 26th July, and disappointingly those around when LINEAR was predicted to reach its maximum brightness (the 23rd) proved either cloudy, or if clear overhead, too hazy lower down to the north. The 26th was also hazy but just clear enough to permit a sighting at 1030BST when the comet was usefully located just 2 degrees west of the star Upsilon Ursa Major. The difficulty of sighting these last few days has not been helped by the fact that the comet has been crossing an area of sky devoid of nearby marker stars.

On the 26th it as a difficult object even in binoculars and little detail could be discerned through the haze, lit by the London glow. It appeared even fainter than on the 20th, though this was probably due to its lower altitude of about 25 degrees putting it further into the haze and lights. I could not reliably determine any hint of a tail.

In the next few days LINEAR heads rapidly south, fading all the while, and will be lost to UK observers during nighttime hours.

LINEAR starts to disintegrate on 25 Jul 2000

As LINEAR reached perihelion at 114 million km from the sun on 26th July it began to disintegrate. Pieces have been breaking off since the beginning of July leading to brief episodes of brightening. It now seems that the comet is disintegrating completely and that the comet will now fade even more rapidly than expected as the remains evaporate and disperse.

Fragment breaking off Comet LINEAR: 3 views between 5 & 7 Jul 2000 courtesy of Hubble Space Telescope.

 LINEAR outburst on 5 - 7 Jul 2000

Unfortunately I have no more sightings to report. Although 1st Aug was clear I think the comet was too low by the time I looked at about 2230BST, and I shall try earlier next time.

The fact that LINEAR had already begun to disintegrate when I saw it on the 26th goes some way to explaining why it was so difficult to see, as the break up was leading to a dispersal of the coma (the comet's inner atmosphere) reducing its brightness relative to the sky glow.

Coma becoming elongated and dispersed. Note that a central nucleus is no longer visible.

LINEAR showing elongated coma on 28 Jul 2000

Many comets show a significant brightening as they break up, but LINEAR's failure to do so indicates that the nucleus was a lot smaller than anticipated. The early promise of a bright comet seems to derive from LINEAR being a first approach comet (i.e. one that has never previously visited the inner solar system) with a surface crust of volatiles. These boiled off while it was still beyond the orbit of Mars, and the cloud of gas and dust so created gave a false impression of the comet's ultimate brightness. Once it had all evaporated and dispersed the remaining small nucleus could not sustain the same rate of evaporation, so the comet failed to brighten as originally expected as it approached the sun.

External Links...

Link to Comet Linear home page

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