Monday, October 1, 2007

Mankind had, untill last September, detected 189 periodic comets, now add one more to that number...
The 190th has been detected by SOHO, ESA/NASA Solar and Heliospheric Observatory, that is something worthy of being registered as this is the first of that kind among the 1350 comets discovered, so far, by the spacecraft.

First seen in September 1999, and then again in September 2003, it was initially thought to be two different comets but, in 2005, Sebastian Hönig, a German PhD student, took a closer look and launched a new hypothesis, the two comets were so similar in orbit that they might, actually, be the same object. He was correct.

Sebastian Hönig, our guest for today, curiously, dedicates his studies not to comets…he is in the infrared interferometry group at the Max-Planck-Institute for Radioastronomy (MPIfR) working on models for the near- and mid-infrared emission of active galactic nuclei (AGN). An AGN is a super-massive black hole in the heart of a galaxy which is actively accreting matter, but that is another story, today will cling to the P/2007 R5 comet.

Due to the fact that a periodic comet is not a common discovery I started by asking Sebastian Hönig what are the major difficulties to detect and prove that same periodicity, the PhD student from the MPIfR started by saying that, in the first place, one has to find two comets that could actually be one.
He indicates that, for ‘standard’ comets, those appearing in the night sky, the orbital elements are, nowadays, of very high quality, this conducts to the fact that the short-period nature is immediately realized at the very first appearance of the comet. In consequence, as Hönig remarks, there are hardly any comets with short periods that have passed through our scrutiny without being detected.
For comets sighted by SOHO, the situation is, although, different. As a result of the small observed orbital arcs and compromised astrometric accuracy, the orbital solutions can be classified as bearing a high level of ambiguity, therefore, parabolic (where eccentricity equals unity) elements, are assumed. In spite of astronomers possessing the knowledge, though an unequivocal proof is still pending, that the members of some comet groups may be periodic, for other comets this is usually not expected.

And what about the newly discovered comet? What are the orbit characteristics of P/2007 R5 (SOHO)?
What might be its path around the Sun and where is it when related to Earth's orbit?
Hönig tells us that the comet’s orbit is interesting in a sense that it has a semi-major axis that lies in the heart of the main asteroid belt. On the other hand, the perihelion distance is very close to the sun, P/2007 R5 is only at 0.056 AU from our star, to have a better idea, the traveller makes its closest passage at about a tenth of Mercury's distance from the sun. As indicated by MPIfR’s student, there is no other known short-period object in the solar system that makes it so close to the sun and manages to survive and tell the story…
It is also worthy of interest, as noted by Hönig, that the comet is close to a resonant orbit with Jupiter as three revolutions of P/2007 R5 correspond to one revolution of Jupiter, also, the associated 4.0 year period results, under spacEurope’s guest, in a kind of pseudo-resonance with the our planet, this meaning that the comet never gets closer than, approximately, 1 Astronomical Unit to the Earth.

I was curious about how did Hönig’s work developed and in what consisted his calculations for a combined orbit, he explained that when the comet was discovered, four years ago, in September 2003, a German amateur astronomer, Rainer Kracht, noted that the apparent track of the comet was very similar to a comet observed in 1999 (1999 R1) and another one in 2002 (2002 R5). This was, later, confirmed by orbital calculations of Brian Masden from the Minor Planet Center in Harvard. Hönig follows, Rainer and Brain speculated about a new comet group, but he had already mentioned the possibility that 1999 R1 and 2003 R5 might actually be the same comet.

Two years ago, in 2005, he tried to represent the 1999 and 2003 objects with a single, short-period orbit, which he managed to do after a while. Hönig discovered that the residuals of the observations were surprisingly small and the orbit appeared to be stable, this permitted him to be confident about his theory, even more when he was able to prove that the 2002 object is different from the 1999 and 2003 comet, has the astronomer indicated, this is a smaller and might be a fragment that separated long time ago. At the end of his work, he stepped forward with a prediction for the 2007 perihelion passage of 1999 R1 = 2003 R5 which revealed to be to be quite accurate.

video
I've made an animation derived from SOHO's LASCO 2 images where the path of the comet can be seen coming from behind the Sun's corona (left) and leaving the stage in the right side of the film. Can you spot it?

P/2007 R5 appears as a Humpty Dumpty comet in what what concerns measurement observations as the size of a comet can be estimated from its brightness, assuming certain surface properties, Sebastian Hönig told spacEurope that, usually, comet nuclei are very dark objects, what leads to the fact that only a fraction of the sunlight hitting the surface is scattered, approximately 5-10%.
P/2007 R5 is indicated has being a small comet, with a size estimated to be around 100 to 200 meters, however, as Hönig remarks, showing the comet little activity, we can only get a rough idea about its actual size when measuring the total brightness of the object, this procedure is valid too for objects that show no signs of activity which is also not the case of this particular comet.
Another presented caractheristic is the fact of the comet not having its own tail and coma, this can be explained by the fact of P/2007 R5 having only a 4 year period, that originates it to come close to the sun quite often, by doing so, and if the comet is in the current orbit for a long time, it should lose its volatiles quite fast. But the total absence of coma it is not totally true, as Hönig points out, a marginal coma was detected, this might be consequence of, being that close to the sun, it is possible that silicate grains on the comet's surface around perihelion, this might be the answer to the observed residual coma, although less pronounced than those where ices have a role.

And what about the comet’s future? What might be its final destiny?
According to our guest for today This is a question hard to answer since P/2007 R5 travels on a dangerous path around the sun. From time to time, it makes close approaches to Jupiter which maychange its orbit significantly. Hönig indicates that the next series of such close approaches willoccur between 2035 and 2043. On the other hand, as he makes reference to, the strong radiation pressure from the sun also acts on the orbit of the comet, not speaking about the erosion or sublimation of the surface material. The future will tell as astronomers keep a close look, monitoring the comet to see what happens.

Speaking of the future...is, in Hönig perspectives, being remembered throughout times, every four years?...
Is it possible that this comet may be named after him?
Hönig don’t know what will happen, the credits for the discovery and recovery of this comet were attributed to Terry Lovejoy (Australia, 1999), Kazimieras Cernis (Lithuania, 2003) and Bo Zhou (China, 2007) but, as he says, several people have suggested that he might have a chance, since the other people who successfully predicted the appearence of a short-period comet got it named after them (1P/Halley, 2P/Encke, and 27P/Crommelin). But the last decision is up to the Commitee of Small Body Nomenclature of the IAU to decide how it is finally named.

You must admit...Hönig Comet sounds good, doesn't it?...



Know more about SOHO.

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