Comet LINEAR Update



With just 5 days to go to perihelion, Comet LINEAR continues to behave in a fairly unpredictable way. The Full Moon has severely impaired observations for the last few nights. Closest approach to the Earth is late on July 22nd and perihelion at 04:16 on July 26th.

There are a number of interesting items that have come up. Here, in no particular order, are a few:


The light curve should have been brightening about 0.1 magnitudes per day over the last week. Observations though show that there is was an apparent step in the light curve in late June when the brightness seems to have faded more than a magnitude while invisible at Full Moon. Latest observations seem to show a very rapid rate of brightening again, but from a much fainter base.

The comet should still get up to about magnitude 5.5 but, at this stage, all bets are off. In fact, a magnitude estimate made last night by a fairly experienced Spanish amateur (Carlos Segarra, in Southern Spain) shows the comet to be magnitude 7.5 in both 10x50 binoculars and with a 25cm reflector at x30. This looks distinctly unpromising, although the naked eye limit for the observation was only 4.7, so it is possible that a lot of outer coma was lost. More encouraging is that the coma was strongly condensed (DC=7) against the estimates made by most observers in May and early June that showed a very low degree of condensation. Generally, strongly condensed comets are the best performers as this implies a high level of current activity. In contrast, a very diffuse object is usually one where current activity is very low and the coma slowly disperses into space - these comets often just fade out and disappear, sometimes even before they reach perihelion.


The latest light curve fits show the comet to be clearly well below average absolute magnitude for a new comet. Depending on how you fit the light curve the absolute magnitude comes up between 7.5 and 8.5 (an "average" new comet has an absolute magnitude of 6.5), around 20 times fainter than 1P/Halley. The indicated diameter of the nucleus is around 2km. This is actually as large as, or larger than Comet Hyakutake (C/1996 B2 Hyakutake) as estimated by radar, although Comet Hyakutake was around 3 magnitudes brighter intrinsically, suggesting that its surface was extremely active.


Comet Hyakutake








An interesting item is that although Comet Hyakutake was very active, its non-gravitation terms (i.e. jet forces from the nucleus) were fairly small. In contrast, Comet LINEAR has very large non-gravitational terms. For Comet LINEAR the A1 term is four times as large as it was for Comet Hyakutake indicating that there is extremely strong jet activity from the nucleus, although there have been no reports of bright jets being observed to date. This is definitely an item that I wish to pursue in the observations that I'll be making next week on the 1-m telescope.


Initially, based on a comparison with Comet Hyakutake, I expected a tail that could reach 15 or 20 degrees (or more) depending on the activity of the comet, particularly as we see it close to side-on at perihelion. In fact, the longest tail estimate that I have seen is still only 1 degree - equivalent to a very disappointing 1.4 million kilometers. Tail activity generally is much greater after perihelion than pre-perihelion and this tail length must surely increase to 5-10 degrees in the next week or two.

However, expect the tail to be very faint and tenuous still and best visible photographically, or with wide-field binoculars from dark site.

Curiously though, the tail does not seem to be seen as much larger now than when it was being observed almost head-on a few weeks ago.


Ó Mark Kidger