ACTIVE ASTEROID (6478) GAULT

CBET 4594 (issued on 2019, January 08) announces the serendipitous discovery by the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) team of unusual activity associated with the inner main-belt minor planet (6478) which appears to have developed a lengthening tail or trail. 

"A median combination of seven 30-s exposures on 2019 Jan. 5 shows a tail or trail 135" long in p.a. 290 degrees. The tail/trail first appears in individual ATLAS exposures on 2018 Dec. 8, as identified by Denneau, with a median combined 120-s exposure showing a tail 30" long in p.a. 290 degrees.  There is no evidence of a tail in previous ATLAS imaging in January 2018."

Credit: ATLAS - discovery image

On January 08.7, I performed follow-up measurements of this object. Stacking of 10 unfiltered exposures, 120 seconds each, obtained remotely from Q62 (iTelescope network) through a 0.43-m f/6.8 astrograph + CCD, shows a tail about 2' and 30" long in PA 292.

My follow-up image (click on it for a bigger version)

Asteroid (6478) GAULT was discovered on 1988, May 12 by astronomers Carolyn & Eugene Shoemaker at the Palomar Observatory in California. It was named for American planetary geologist Donald Gault:

   Credit: Dictionary of Minor Planet Names - Schmadel

The active asteroids represent a newly recognized class of small body in the solar system: their peculiarity lies in having both the orbital characteristics of asteroids, residing in orbits interior to Jupiter’s and having the Tisserand parameter substantially larger than 3, and the physical properties of comets, in the form of comae or tails. 

Credit: D. Jewitt

The active asteroids have been referred to as ‘main-belt comets’ and ‘disrupted asteroids’ previously: the new designation is preferred both because some of the known cases are not located in the main belt and because the nature of the activity. Actually the causes of the activity are thought to be many and varied. As regards (6478) GAULT at this stage we can't rule out sublimation (or any other of the suggested mechanisms) in favor of an impact or viceversa. Hopefully more data will help sort out the cause of its activity. See the excellent David Jewitt's Active Asteroids webpage for more info about these fascinating objects and his paper "The Active asteroids" (The Astronomical Journal, 2012 March). 

Below you can see active asteroid (6478) GAULT imaged on 2019, January 11 at @INGLaPalma.

Credit: INGLaPalma

by Ernesto Guido

The Multi-Tailed Main-Belt Comet P/2013 P5

Cbet nr. 3639, issued on 2013, August 27, announced the discovery of a new comet (discovery magnitude ~20.9) in CCD exposures obtained on August 15 by the Pan-STARRS1 telescope on Haleakala. The new comet has been designated P/2013 P5 (PANSTARRS). According to the description in the CBET, the object has a nuclear condensation with a stellar appearance and a thin tail that extends for approximately 30" towards position angle approximately 255 degrees.

Orbiting at the inner edge of the main asteroid belt, with semimajor axis, eccentricity and inclination of 2.189 AU, 0.115 and 5.0° , respectively, the Tisserand parameter relative to Jupiter is TJ = 3.66. So this object belongs to the newly recognized class of body in the solar system known as active asteroids or  main-belt comet - MBC. These objects are remarkable for having both the orbital characteristics of asteroids and the physical characteristics of comets: they look like comets because they show comae and tails but they have orbits interior to Jupiter's and Tisserand parameters substantially larger than 3, like asteroids. While some are suspected to contain water ice whose sublimation is responsible for the expulsion of dust, others are impact-produced while, for a majority, the origin is unknown. (see also our previous post about P/2012 T1 (PANSTARRS) & 596 Scheila).

Astronomers using NASA's Hubble Space Telescope have imaged P/2013 P5 on UT 2013 Sep 10 and 23, obtaining a total of 12 images with the WFC3 camera.  It has no less than six dust tails that seem to be forming sequentially. 

As an inner-belt asteroid and probable Flora family member, the object is likely to be highly metamorphosed and unlikely to contain ice. The protracted period of dust release appears inconsistent with an impact origin, but may be compatible with a body that is losing mass through a rotational instability. We suggest that P/2013 P5 has been accelerated to breakup speed by radiation torques.

You can read the full Hubble release here or the paper based on these observations here on Arxiv. Below you can see the Hubble images (click on it for a bigger version).

Credits: NASA, ESA, and D. Jewitt (UCLA)

Credits: NASA, ESA, and D. Jewitt (UCLA)

While below there is a summary of the main properties of active asteroid P/2013 P5 as outlined in the paper by Jewitt et al.

We performed follow-up measurements of this object on November 07. Stacking of 16 R-filtered exposures, 120-sec each, obtained remotely from MPC code E10 (Faulkes Telescope South) on 2013, November 07.4 through a 2.0-m f/10.0 Ritchey-Chretien + CCD (operated by Las Cumbres Observatory Global Telescope Network), shows a thin tail 25" long in PA 75 (magnitude ~21).

Below our follow-up image of P/2013 P5. Click on it for a bigger version.

by Ernesto Guido, Nick Howes & Martino Nicolini

(3200) Phaethon

Phaethon is an Apollo type asteroid with many peculiarities. It has been discovered in 1983 by Green and Davies while searching images of IRAS satellite. So it is the first asteroid to be discovered by a spacecraft. Moreover the orbit is unusually short (P 1.59 yr) with a small perihelion distance of q= 0.141 AU.

In 1983, Fred Whipple identified Phaethon like the parent of Geminid meteor stream. But it has never displayed unambiguous cometary activity. Some people hold on to the opinion that Phaethon is a comet (or extinct comet), while other thinks that it is more an asteroid or the likely product of a breakup.

In a recent article has been suggested that planet-crossing object 2005 UD is dynamically similar to Phaethon and so that:

"3200 Phaethon, 2005 UD, and the Geminids are pieces of a main-belt comet precursor that disrupted, perhaps due to unsustainable internal gas"

Where main-belt comets are a new class of objects introduced by Jewitt& Hsieh in 2006. To say it with the words of Jewitt & Hsieh:

"Phaethon's true nature must still be considered an open question"

Below you can see our recent image (November 04) of this object (or click here )

by Ernesto Guido and Giovanni Sostero

References

- Meteor Showers and their Parent Comets by P. Jenniskens

- Search for activity in 3200 Phaethon by Hsieh & Jewitt (2005)

- Physical Observations of 2005 UD: a mini-Phaethon by Jewitt & Hsieh (2006)