Comet LINEAR 1999 S4
Imaging Results from After Breakup

Starting on July 21, observers around the world reported that LINEAR was exhibiting activity that was unusual, even by this comet's standards. This activity culminated in the complete disruption of the nucleus.

First, a long, well-defined plasma tail was seen, lasting for about two days before it disappeared. Shortly after that, the bright central condensation started to elongate and fade, until on July 27 there was no longer a bright peak defining the nucleus.

Our Images

We obtained images from Lowell Observatory and Perth Observatory (the comet was rapidly moving south), on eight nights between July 26 and Aug 10. The full sequence of these images shows the evolution of the coma and tail after the comet disintegrated, and allowed us to constrain many of the characteristics of the comet and its destruction.

Our images from after the breakup consist primarily of measurements with broadband filters. We did obtain some images with narrowband filters (see the photometry page for more information) so we could look at the morphology of the gas species, but the gas production dropped so rapidly that all that is visible in these images is the dust. Because the broadband filters also show the dust, but at a higher signal-to-noise, we have used the broadband filters for our analysis.

Breakup Sequence

July 26, 2000
LINEAR on July 28 (after the breakup)

July 28, 2000
LINEAR on August 2 (after the breakup)

August 2, 2000

This figure shows a representative sample of images showing the evolution of the tail after the comet disintegrated. These pictures show how the central condensation faded and became more elongated with time, with the residual debris slowly dissipating down the tail. Altough HST images from August 4 showed at least 16 fragments 50-120 meters (160-400 feet) across, the spatial resolution in our images was not sufficient to resolve any of these subnuclei.

These broadband R images were obtained on July 26, and 28 and August 2. Each box is about 10⁵ km (60,000 miles) long, with the sun to the right.

Dust Tail Modeling

In order to learn about the structure of the comet, we produced models of the dust tail in the post-breakup images.

The Model: We used a Finson-Probstein model that computes the motions of dust particles under the influence of gravity and solar radiation pressure. Light acts as a force that pushes the dust grains away from the sun. Because small particles are pushed faster than large ones, radiation pressure acts to "sort" the different sized particles into different regions of the tail. We can model the tail to determine how many particles of different sizes are present. This gives us information about what the structure of the comet is like.
Limitations of the Model: Because of the nature of light scattering by small particles, very small dust grains are most prominent in the tail, even though larger grains contain the majority of the mass. (A 1 cm grain contains the same amount of mass as 10¹² micron-sized grains, but the ensemple of smaller grains scatters 10⁴ times as much light.) For this reason, the modeling can only determine the properties of the less massive dust particles, even though the majority of the mass is in larger grains.
Input Constraints: We used a series of four images for the dust tail models: July 27 and 30 and August 4 and 10. The changes that take place between these dates provide information about the motions of the dust, adding further constraints to our models.
Model Results

Particle size distribution: In general, the tail after the breakup has a lower slope than more typical comets. This means that the ratio of large to small particles is higher than normal, which is not unexpected given the breakup.
Emission velocity: The emission velocity was also typical, at a value of about half a kilometer per second, until shortly before the breakup, when it dropped rapidly.
Dust production rate: The production rate started to increase about July 19-20, and for the larger particles, peaked about July 21. Small particle production continued for another 2-3 days beyond that.

Conclusions from Model Results

The rise in production rates indicates that the onset of the breakup started on July 19.
The rise in production rates over several days suggests that the comet "unraveled" over several days, rather than experiencing a single explosive event.
The difference in peak emission for large and small grains means that there was significant sub-fragmentation. Large grains broke into smaller and smaller particles over a several day period.
All of the particles visible in the tail contain a mass equivalent to about one of the 100 meter cometesimals seen in the HST images. Thus the amount of mass in the tail was only a very small fraction of the total.
Adding up all the mass in the HST fragments plut that seen in the tail is only a small fraction needed to produce a 0.44-km nucleus. We therefore infer that the vast majority of the comet's mass ended up in fragments between 1 millimeter and 50 meters in size (the size range that is difficult to see).
Using the amount of dust we compute, and the amount of ice computed from other studies (published in the May 18 2001 edition of Science) we find that the comet had very little ice compared to the amount of dust (the ratio of dust to ice may be as high as 400).

Why did LINEAR break up?

At present it is not known exactly what caused LINEAR to break up. Several theories have been proposed for why they break up, and we can look at these individually:

The comet was hit by a small asteroid: This is an interesting idea, but there is no evidence for it. LINEAR was exhibiting outbursts (including fragments coming off the nucleus) for two months before the final breakup. These precursors suggest that other mechanisms were responsible for these events.
A thermal wave propagated into the interior: In this scenario, heat from the sun propagates into the interior, heating a pocket of volatile material, which caused pressure to build up inside the nucleus until it was evetually blown apart. Again, this is unlikely because of the precursor outbursts and the fact that the comet seemed to unravel rather than exploding.
Rapid rotation caused LINEAR to Spin apart: If the nucleus were spinning fast enough, then centripetal acceleration could cause it to come apart. We don't have any solid evidence for the rotation period of the comet (other than our constraint of less than 12 hours), so this mechanism can't be rejected offhand. However, the rotation rate would have to be extremely fast for it to cause the total disruption of the nucleus (a period of less than an hour). This rapid a rotation is difficult to accept, because of the fragment seen breaking off in early July. If a piece breaks off the nucleus, it takes angular momentum with it, thus causing the nucleus to slow down.
The nucleus ran out of ices: This seems to be the most likely explanation, but it does not explain why LINEAR is different from other comets in that respect. It is generally believed that ices act as the glue that holds the components in a nucleus together. LINEAR had little ice compared to the amount of dust, and once the ice was vaporized, there was nothing left to hold the comet together. This raises the question of why more comets are not observed to disintegrate. Is LINEAR different from other comets, or are all comets essentially the same and others just need a few more passes around the sun before they come apart?