At the start of this series (April 2019 blog) we very briefly defined a comet and said that we would look only at the asteroids. Comets and asteroids are on distinctly different types of orbits and generally, comets have Tisserand criterion (Tj) of <3.0 compared to asteroidal values of Tj > 3.0. Comets exhibit different observations and physical properties and are smaller (in mass) than asteroids. However, the situation in the solar system, as in many things in life, is not that clear-cut and there is a graduation of objects from comets and asteroids in both physical and orbital perspectives.
In 1996 the, previously inert, and in no way special, main belt asteroid 1979 OW7 (a: 3.16AU; e: 0.16; P: 5.62years) was observed to have a faint dust trail, persisting for a few weeks. The trail was detected by the Belgian astronomer Eric Elst (b.1936) using a photograph taken by Guido Pizarro at ESO’s (European Southern Observatory) La Silla Observatory when the asteroid was near its perihelion point. A reoccurrence of the dust trail (again persisting for several weeks) was seen during the asteroid’s 2002 perihelion passage and the object (also known as asteroid 7968 Elst-Pizarro) is now considered to be a comet (designated 133P/Elst-Pizarro). Considerable interest in 7968 focused on whether the object was a comet, where its orbit had (co-incidentally) evolved to now being wholly within the main belt; or whether the object was a ‘disrupted’ asteroid.
In 1996 the, previously inert, and in no way special, main belt asteroid 1979 OW7 (a: 3.16AU; e: 0.16; P: 5.62years) was observed to have a faint dust trail, persisting for a few weeks. The trail was detected by the Belgian astronomer Eric Elst (b.1936) using a photograph taken by Guido Pizarro at ESO’s (European Southern Observatory) La Silla Observatory when the asteroid was near its perihelion point. A reoccurrence of the dust trail (again persisting for several weeks) was seen during the asteroid’s 2002 perihelion passage and the object (also known as asteroid 7968 Elst-Pizarro) is now considered to be a comet (designated 133P/Elst-Pizarro). Considerable interest in 7968 focused on whether the object was a comet, where its orbit had (co-incidentally) evolved to now being wholly within the main belt; or whether the object was a ‘disrupted’ asteroid.
Au début de cette série (avril 2019) nous avons défini une comète. Les comètes et les astéroïdes se trouvent sur des orbites très différentes, et les comètes ont un critère de Tisserand de <3.0 en comparaison avec les valeurs d’astéroïde de Tj > 3.0. Les comètes montrent des observations et des propriétés physiques différentes, et sont plus petites (en masse) que les astéroïdes. Cependant, la situation dans le système solaire n’est pas tout à fait claire, et il y a une graduation d’objets parmi les comètes et les astéroïdes.
En 1996 on a observé que l’astéroïde 1979 OW7 (a: 3.16AU; e: 0.16; P: 5.62ans) de la ceinture principale avait une traînée de poussière qui persistait pendant plusieurs semaines. Cette traînée a été observée par l’astronome belgique Eric Elst (ne.1936) en utilisant une photo prise par Guido Pizarro à l'observatoire de La Silla de l'ESO (European Southern Observatory) quand l’astéroïde était près de sa périhélie. On a vu une recurrence de la traînée de poussière (qui durait plusieurs semaines) pendant le passage périhélie de l’astéroïde (en 2002), et l’objet (connu comme l’ astéroïde 7968 Elst-Pizarro) est maintenant considéré une comète (désignée 133P/Elst-Pizarro)
En 1996 on a observé que l’astéroïde 1979 OW7 (a: 3.16AU; e: 0.16; P: 5.62ans) de la ceinture principale avait une traînée de poussière qui persistait pendant plusieurs semaines. Cette traînée a été observée par l’astronome belgique Eric Elst (ne.1936) en utilisant une photo prise par Guido Pizarro à l'observatoire de La Silla de l'ESO (European Southern Observatory) quand l’astéroïde était près de sa périhélie. On a vu une recurrence de la traînée de poussière (qui durait plusieurs semaines) pendant le passage périhélie de l’astéroïde (en 2002), et l’objet (connu comme l’ astéroïde 7968 Elst-Pizarro) est maintenant considéré une comète (désignée 133P/Elst-Pizarro)
A disrupted asteroid is one which has undergone significant change due to collision – the production of a cometary-tail in 7968 may have been due to the exposure of sub-surface ices and volatiles by means of the collision, and these were being sublimated at/near perihelion. A disrupted asteroid producing a comet-like trail is said to be an ‘active’ asteroid. Active asteroids can become active through collisions; rotational spin-ups (see our March 2020 blog on the YORP effect). They may also become active by gravitational (tidal) disruption during / after close encounters with other massive objects, or the effects of heat/infrared energy upon the asteroid either via solar radiation or radioactive decay of isotopes within the asteroid.
101955 Bennu imaged on 6th January 2019 by the Osiris-Rex probe.
Notice the release of small particles to the lower left. Courtesy of Nass/Goddard
Notice the release of small particles to the lower left. Courtesy of Nass/Goddard
The re-occurrence of the trails in 2002 however effectively ruled out the disrupted asteroid hypothesis for this object. The probability of two collisions co-incident with the asteroid’s perihelion producing similar trails is extremely small. However, dynamically it was very difficult to see how a ‘normal’ comet’s orbit could be changed to fit 7968’s current orbit.
This asteroid/comet was the first of a new classification of very rare object: the MBC (Main Belt Comets) to be identified. In 2005 two further MBCs were detected (118401 1999 RE70 and 238/P Read) and up to today, just thirty-nine confirmed MBCs have been identified. All the objects thus far detected have dimensions of less than 5km. The most likely origin of these objects appears to be the fragmentation of a larger, icy object within the main belt.
MBCs are not the only anomalies within the main belt. The Apollo class (and NEA and PHA) asteroid 3200 Phaethon (a: 1.27AU e: 0.89 P: 1.43years) has debris along the length of its orbit (as do most comets) and when the Earth passes through the object’s path the Geminids meteor shower is seen. Dynamically, it would appear that 3200 Phaethon (Tj: 4.511); and the Apollo class (NEA) asteroid 155140 2005 UD (a: 1.27AU e: 0.87 P: 1.44years Tj: 4.506) and the source of the Sextantids meteor stream, were part of a larger object which split circa 10,000 years ago.
Next month
We will look at hypotheses and theories as to the origin of the asteroids.
The ‘topic’ list for future monthly blogs is now included in our blog index (alongside previous monthly topics)
This asteroid/comet was the first of a new classification of very rare object: the MBC (Main Belt Comets) to be identified. In 2005 two further MBCs were detected (118401 1999 RE70 and 238/P Read) and up to today, just thirty-nine confirmed MBCs have been identified. All the objects thus far detected have dimensions of less than 5km. The most likely origin of these objects appears to be the fragmentation of a larger, icy object within the main belt.
MBCs are not the only anomalies within the main belt. The Apollo class (and NEA and PHA) asteroid 3200 Phaethon (a: 1.27AU e: 0.89 P: 1.43years) has debris along the length of its orbit (as do most comets) and when the Earth passes through the object’s path the Geminids meteor shower is seen. Dynamically, it would appear that 3200 Phaethon (Tj: 4.511); and the Apollo class (NEA) asteroid 155140 2005 UD (a: 1.27AU e: 0.87 P: 1.44years Tj: 4.506) and the source of the Sextantids meteor stream, were part of a larger object which split circa 10,000 years ago.
Next month
We will look at hypotheses and theories as to the origin of the asteroids.
The ‘topic’ list for future monthly blogs is now included in our blog index (alongside previous monthly topics)
