Holmes in the Sky of Israel - an interim summary

It's been a few days since it was reported that the comet exploded and brightened to several hundreds of thousands of meters. The chairman of the Israeli Astronomical Society and director of the astronomy forum at YNET, Yigal Fatal summarizes an interim summary of what happened, what we think happened and what we think will happen

The chairman of the Israeli Astronomical Society, and director of the astronomy forum at YNET, Yigal Fatal, says that these days there is an unexpected guest in the evening sky. It is a comet called Holmes. The comet has been in the sky for many months, but until this week large telescopes were required Especially to see it and the interest in it did not go beyond the usual interest in dozens of comets like it that are discovered every year and do not reach consciousness the collective

However, this week (24/10) it was reported by astronomers in Japan and Europe that the brightness of the comet increased by about half a million times and overnight it became a sky gram easily visible to the unaided eye, within the constellation Perseus that was written about in this month's article. Just as the sudden brightening of the comet was not expected, so is its behavior after the eruption and it is highly desirable to follow the comet every night.

We do not know for sure, but it is likely that the cause is the eruption of a gas pocket. Comet Holmes was at its closest point to the sun in its orbit about six months ago, in May. When the comet approaches the sun it heats up and this is the reason for the spectacular halo and tail.

However, the story does not end here. The heat wave created as a result of the comet's surface being heated by the sun continues to penetrate even after the comet moves away from the sun. Usually, the heat wave continues to fuel the comet's activity for some time after the perihelion until the activity almost dies out as it recedes. However, a situation is possible where the heat wave reaches a very large pocket of gas, trapped inside the ice. When the gas heats up and increases its volume, in some cases it may explode the comet.

A similar phenomenon was observed in Halley's Comet after its last pass near the Sun in 1986. Such an explosion can tear the comet apart or tear it into small pieces, depending on the location of the gas pocket in the comet, its size and the physical characteristics of the comet. In experiments at the laboratory for the study of comets in the Department of Geophysics and Planetary Sciences at Tel Aviv University, conducted by a team led by Prof. Akiva Bar-Nun and with the participation of Dr. Diana Lauper and your faithful servant, a similar behavior of a heated ice sample with a pocket of gas was observed.

Comet Holmes is a periodic comet - 17P/Holmes that orbits the Sun once every 6.88 years, when during the perihelion it reaches a distance of 2 astronomical units from the Sun (one astronomical unit is the distance from the origin between the Earth and the Sun). At the time of aphelion, the comet is 3.61 astronomical units away from the Sun, slightly more than the asteroid belt (although its orbit is inclined at an angle of 19 degrees with respect to the plane of Milka). Such data are suitable for comets from the Jupiter family, which were probably captured by Jupiter's gravity and diverted to a more closed orbit than their original orbit. They usually originate in the Kuiper Belt.

The comet reached perihelion in May this year. Comets develop a significant halo approximately at a distance of about 3 astronomical units from the Sun with a significant increase in the evaporation of water and consequently the release of gases and dust trapped in ice, and it grows as the comet begins to approach the Sun. The heat of the sun warms the surface of the comet, the water evaporates and together with it more and more gases and dust trapped in the ice are released. These increase the aura and its opacity. At the same time, the solar wind acts on the halo, pushing it further away from the comet and thus the dust tail and the ion gas are formed (the ion gas remains trapped in the magnetic field lines of the sun while the dust tail continues away from the sun, but its shape is determined due to the dynamics of the particles in the tail that actually continue to circle the sun in countless orbits similar to those of the comet).

Why did the comet explode?

Such events are typical for post-perihelion comets. Halley's Comet exploded when it was 10 AU from Earth, Hale Boop at 6.4 AU from Earth. Eruptions are also characteristic of comets whose perihelion distance is far from the Sun and a good example is comet 29P/Schwassmann Wachmann which erupts almost regularly while increasing the brightness up to 100 times.

The eruption of Comet Holmes is massive in its power - there are estimates between half a million and a million times. It is necessary to understand what the comet's heating mechanism is in order to understand the cause. Near the perihelion, the surface of the comet was heated by the sun. Part of the energy was directed to evaporate the ice and part of the sun's heat moved inward into the comet. The progress of the heat wave causes an additional release of gases from deeper places in the comet. However, it is possible that under the surface (in the case before us, probably at a depth of several tens of meters) there is a large pocket of frozen gases. When the heat wave reaches the gas pocket, the gas heats up, increases its volume and explodes. The gas explosion blows a lot of material around the comet, among them dust that was trapped in ice and water ice that the energy of the explosion causes to evaporate. All of these dramatically increased the halo as well as its density and hence the large increase in its brightness.

What next?

Assuming that there will be no further explosions, the halo will disperse, and the brightness of the comet will depend on the rate of decrease in the density of the halo and, on the other hand, on the rate of its expansion. Presumably, for some time (days, weeks? I currently have no data to calculate this) the brightness of the comet will be more or less stable and then begin to fade more steeply. It is very possible that the eruption caused part or parts of the comet to be torn apart in this case it is important to continue watching the comet to see if there is more than one focus. This will become clearer as the aura becomes more transparent. It is very possible that the explosion greatly destabilized the time of the comet and it may disintegrate completely. A good example of a comet that disintegrated was comet Biala in the 19th century. So we will see this later or towards the next perihelion.

a tail

It should be remembered that the existence of a tail depends on the interaction with the solar wind. The great distance of the comet from the Sun greatly reduces the effect of the solar wind but still does not eliminate it, so there may be a chance for a tail, especially when the halo is so large. On the other hand, there is a geometric consideration which is the direction of the tail in relation to the line of sight. The comet's tail is best seen when it is perpendicular to the line of sight. In the case before us, if there is a tail, in addition to the fact that it is almost in the line of sight, the halo of the comet will be between us and it (we must remember that the solar wind pushes the tail away from the comet and after the perihelion the tail precedes the comet) and unless its size is very significant it may be hidden on her hands Either way, it will be interesting and important to follow the comet.

When and where to see it

The Barkat observatory in Maccabi-Reot said that it can be observed with binoculars in the northwest direction. The best time - three o'clock in the morning Comets and asteroids have a different movement in the sky which distinguishes them from the movement of the stars. Therefore we can expect to find the comet in a different place every night and watching through a telescope we can even notice its movement for a few hours.

The image of the comet was taken using a special method, in which the robotic internet telescope at the observatory tracked the movement of the comet, so the nucleus of the comet can be seen clearly. The images of the comet were taken at the Barakat Observatory using the Internet Telescope. The telescope sits on a robotic tripod and can be controlled from anywhere in the world via the Internet. It is used by students and researchers from all over the world. More information about the Internet Telescope, including demonstration videos can be found on the observatory website.