Summary

The distinctions between a comet and an asteroid are quite a bit less clear than many of us were taught in middle school. There's a peculiar object named 362P/2008 GO98 that's being observed right now, and its story is evolving and ongoing. It highlights this increasingly fuzzy overlap in definitions and just goes to show that nature doesn't always cooperate with our attempts to categorize things into simple, discrete classes. Updates will be provided as more information is made available.

(last updated 25 Oct 2017)

On 3 July 2017, the Minor Planet Center sent out an electronic alert telling professional and amateur astronomers that observers at Catalina Sky Survey were looking at an object that was experiencing very obvious cometary activity. The details of this electronic alert are below:

G. J. Leonard, using the MLS 1.5-m reflector, reported a comet candidate, described as having a coma of 7"-8" diameter and a broad ~15"-long tail in P.A. ~260 deg. This object was linked by MPC automated routines with the numbered Hilda minor planet (457175). Follow-up observations by D. C. Fuls with the MLS 1.0-m reflector confirm the cometary nature: very bright coma at least 8" diameter, with a very broad 12"-long tail in P.A. 265 deg. Additional follow-up observation by Leonard show as 10" coma and a broad, diffuse 15"-long tail in P.A. ~260 deg.

Further observations of this object are encouraged to clarify the

nature of this cometary activity.`

Since then, a dozen or more astronomers have posted photos of 2008 GO98 across the internet and confirmed this cometary activity. There's a good primer of this effort here.

Copyright J.F. Souller on 5 July 2017 http://spaceobs.com

2008 GO98 was supposed to be an asteroid. It didn't look like a comet when it was discovered, so why does it look like one now?

In a 2016 paper, 2008 GO98 was identified as a quasi-Hilda comet candidate. Hildas are a class of asteroids outside of the asteroid main belt, typically on elliptical (non-round) orbits, and defined by 3:2 Jupiter mean-motion resonance--they orbit the sun three times for every two times Jupiter orbits the sun.

A schematic of the orbit of 153 Hilda (green), with Jupiter (red). Courtesy of Wikipedia

Being a quasi-Hilda, 2008 GO98 is almost at 3:2 resonance with Jupiter...but not quite. This type of orbit is not stable, which means that it will migrate out of this configuration at some point in the future. The paper also suggests that the object may be a Centaur and only arrived in the inner solar system ~1700 BCE (3700 years ago). Since, 2008 GO98 might have only "recently" arrived to the inner solar system, this paper predicts that it will likely begin showing cometary activity in the near future. So far, this prediction seems to have been accurate.

Centaurs are objects beyond Jupiter, which typically have orbits which are unstable long-term due to gravitational interaction with the giant outer planets Jupiter, Saturn, Uranus, and Neptune. Since Centaurs formed in the outer solar system, they can be rich in volatile materials and are expected to become comets if they are pulled into the inner solar system.

Ricardo Gil-Hutton, one of the authors of the paper published in Astronomy & Astrophysics, has said that they observed activity approaching perihelion (closest that the object will come to the sun) on 23 Aug 2016. I will update this page when results are published from their observation campaign leading up to perihelion.

2008 GO98's orbit is currently 7.89 years, so previous perihelion passages were in Oct 2008 and Nov 2000. Even assuming it was a "recent" arrival (3700 years ago) to the inner solar system, it should consistently exhibit cometary activity on every orbit near its current perihelion. Looking back on all the instances that it has been observed, it should have showed cometary activity in the times it was seen in 2008-2009 and also maybe in 2001. Since this was not the case, there seem to be some missing gaps of information to really understand what's going on here.

This is where some great work by @cbellh47 and @lukedones comes in. By running orbit calculations, they've been able to determine that a gravitational interaction with Jupiter on 20 Sept 2011 pushed 2008 GO98's perihelion from around 3.2 AU to 2.85 AU--which is close enough to heat up its water ice to the point of sublimation and become active for the very first time.

1 AU is the average distance from the Earth to the sun--about 150 million km

These data are from Horizons. Seems less surprising that it's active, given that nominal orbit was a Centaur until ~300 years ago. pic.twitter.com/rzmtMYhBrn — Solstice 2 Solstice (@lukedones) July 6, 2017

Furthermore, calculating these interactions with Jupiter back to the year 1600 shows that this object was indeed a Centaur...having migrated inside Jupiter's orbit in Mar/Apr 1710. Geologists and astronomers are used to dealing with very long timescales. 300 years is FAST! Continuing to learn more about objects like this will help gain a grasp for how dynamic and intriguing our solar system really is.

(457175) 2008 GO98

2016-Aug-23 perihelion passage is first passage since 2011-Sep-20 encounter with Jupiter which shortened q to 2.85 AU pic.twitter.com/GFqoU3mUhA — Charles Bell (@cbellh47) July 5, 2017

Looking to the future, this object will interact again with Jupiter in 2035, pushing it closer towards ~2.6 AU during perihelion passage in 2040.

While these studies and calculations do explain some of the mystery, there's still a lot we don't know about 2008 GO98.

Composition

This can be determined by obtaining spectra and can help understand where the activity is driven by the sublimation of H2O...or some less likely scenario such as being hit by another asteroid. From what I understand (which is admittedly somewhat limited), the cometary activity should have begun around the time 2008 GO98 passed inside 3.0-3.2 AU from the sun, but "deactivation" doesn't happen all at once.

According to Delsemme and Miller (1971) and Prialnik (2004), computed gas production rates for H2O fell off significantly at 3.5-4.0 AU but still does not reach zero, even out to 5-6 AU. If this work is applied to 362P/2008 GO98, it reaches 3.5 AU on 5 Oct 2017 and 4.0 AU on 23 Aug 2018. If the outgassing start to drop off in the next year or so, this will also coincide with it becoming faint enough that less and less amateur astronomers will likely be able to view it.

This is the way the world ends

Not with a bang but a whimper.

The Hollow Men - T.S. Eliot

On 7 Oct 2017, it was officially recognized as a periodic comet and renamed 362P/2008 GO98

As of Oct 2017:

Nobody has yet taken spectra

It's beyond 3.5 AU

It's STILL active! (latest observation on 13 Oct 2017 by Gianluca Masi seen here)

It's likely that the activity may fade away over the next few months...but there's always the possibility that something else may be going on. As mentioned previously, this story is ongoing...

Size

This can be determined by two measurements: absolute magnitude (brightness) and albedo (reflectivity). Currently, nobody has taken measurements to determine albedo, and there's significant discrepancy in the reported absolute magnitiude.

Depending on the assumed albedo based on composition...0.02 on the extreme low end if D-type or P-type, up to 0.22 on the high end of albedos reported for small Centaurs...this gives an extreme diameter range (calculated using this simple online tool) anywhere from 5.5 to 24.7 km. And maybe a nominal range on the order of 5.7 to 17.4 km, assuming cometary nucleus albedo of 0.04-0.05. That's still a huge difference.

Perhaps what is most important is understanding exactly when/if this object de-activates and constraining the next activation by observing it as it approaches 2022 perihelion.

I will keep this up-to-date as this story unfolds, and I welcome any information or comments. Let's keep learning! Let's keep growing!

@asteroidanalyst or info@asteroidanalytics.com