Where, O Where Has Comet Boethin Gone? : The Case of the Missing Comet

Nowadays when an astronomer discovers something new in the sky it is easy to get the word out. An email can inform the proper authorities about a new discovery in seconds. Often times, confirmation by other observers is made in hours or even minutes. But, it wasn’t too long ago that getting the word out was both difficult and slow. This was especially true if you lived in a more remote part of the world.

The Discovery of Comet Boethin

Reverend Leo Boethin was a missionary living near Bangued in the Philippines. During the 1960s and 1970s he regularly hunted for comets with his 20-cm (8″) reflector. Between 1965 and 1973, he reported the discovery of 3 new comets. But due to the slowness of communications between his home in the Phillipines and the Central Bureau of Astronomical Telegrams (CBAT) (the organization responsible for announcing comet discoveries) in Cambridge, Massachusetts as well as ambiguities in the discovery reports, none of these comets reported by Boethin were confirmed by others.

On 1975 January 4, Boethin reported yet another new comet discovery. After a few more nights observing his discovery, he sent a letter to the CBAT which arrived on January 17. Soon afterwards, a 2nd letter was sent with revisions to the comet positions reported in the 1st letter. Amazingly, the letters crossed paths in transit and the 2nd letter arrived in Cambridge before the 1st letter which announced the discovery. Considering the 3 previous unconfirmed comets and the uncertainty in the reported positions for this new comet, there were doubts as to the existence of this comet.

To Boethin’s credit, he remained persistent and was able to follow the comet for the remainder of January. After his additional astrometry (measurement of the comet’s position on the sky) were submitted to the CBAT, the comet was confirmed by other observers and Boethin was credited with the discovery. At 11th magnitude, the comet was very faint for visual observations but rather bright for astrophotograhy with large telescopes. Enough observations were made over the next few months to determine that Boethin was a short-period comet that took 11-12 years to orbit the Sun. This is the only comet to bear Boethin’s name.

[In 2003, Gary Kronk and Brian Marsden reported that one of Boethin’s unconfirmed comets (from 1973 January) was actually Comet 104P/Kowal 2 which wasn’t officially discovered until 1979. Based on Boethin’s report and the brightness behavior shown by Comet Kowal 2, it is apparent that the comet was observed by Boethin after a major outburst in brightness and rapidly faded.]

Return of Comet Boethin in 1985/86 and 1997

Comet Boethin was next predicted to reach perihelion (closest approach to the Sun) in 1986 January. On 1985 October 11, A.C. Gilmore and P.M. Kilmartin found the comet on a photograph taken with a 0.25-m (10″) astrograph telescope at the Mount John University Observatory in New Zealand. As the comet approached perihelion it became brighter than expected at 8th magnitude. As a result, the comet was easily observable with a typical backyard telescope.

The next perihelion was expected in April of 1997. Unfortunately when the comet was expected to be at its brightest it was located in a part of the sky that was too close to the Sun to be easily seen. Not surprisingly, the comet was never observed in 1997. Since comets can and do get missed at poor returns, the lack of observations of Comet Boethin wasn’t too surprising. No doubt the comet would be observed at its next, and much better placed, return in late 2008.

Comet Boethin as a Target of the Deep Impact extended mission

A few years back, NASA conducted two highly successful comet spacecraft missions. Stardust flew past Comet 81P/Wild 2 and returned a sample of comet dust back to Earth. Deep Impact flew past Comet 9P/Tempel 1, but not before releasing a 370-kg (816-lbs) impactor that crashed into the comet’s nucleus. After their primary missions, both spacecraft still had working instruments and enough fuel to travel to other targets. NASA decided to extend each mission to fly-by and study additional comets. In the case of Stardust (now called Stardust-NExT), it will fly past the Deep Impact target comet, Tempel 1, and attempt to observe the crater that was caused by the Deep Impact impactor. The Deep Impact spacecraft itself (now called EPOXI) was to fly-by Comet Boethin.

The choice of Comet Boethin as the target of the Deep Impact extended mission was based on the ease of getting to the comet. Spacecraft don’t have enough fuel to go to any target so they are limited to those comets that are easiest to get to. Once Comet Boethin was selected as a mission target, an extensive effort was made to find the comet.

The comet needed to be found long before the planned 2008 December fly-by. For one, astronomers would like to know as much about the comet as they can. Knowledge of the size of the comet’s nucleus, how fast it rotates and what level of activity it will have at the time of the encounter are useful for planning the sequence of spacecraft observations. Not to mention, the spacecraft team needed to know the precise position of the comet in order to fly close enough to the comet for useful studies. This finally point was of prime concern since the Deep Impact spacecraft was going to fly close to the Earth and use the Earth’s gravity to “sling-shot” it to the comet. With the Earth fly-by (called a gravity assist in spacecraft lingo) scheduled for the last day of 2007, the comet needed to be found by no later than October of 2007.

It’s 2008 and Still No Sign of Comet Boethin

A team of the world’s best comet observers employed a world-wide network of telescopes and the NASA Spitzer infrared space telescope to hunt for Comet Boethin. But a funny thing happened, no one was able to find the comet. At the time of these observations, the comet was relatively far from the Sun so perhaps the comet was not active yet. If the nucleus of the comet was very small, it could have been too faint for detection. It is also possible that the comet was well off it’s predicted position and the hunters weren’t looking in the right spot though the search did cover all of the sky were the comet was expected to be. Or maybe the comet was just missed among the many millions of stars in their images. As a result, the Deep Impact spacecraft was re-routed to another comet which has proven much more dependable over the years, Comet 103P/Hartley 2. The Hartley 2 fly-by will happen on 2010 November 4.

Even after the extensive effort of many professionals, there still was a feeling among some (myself included) that Comet Boethin had simply eluded their searches and would show up at some point in the Fall of 2008. If it gets as bright as it did in 1975 and 1986, then it should currently be visible in amateur-sized backyard telescopes as a 9th to 11th magnitude comet. Although many amateurs have hunted for it, the comet has not shown itself. Even if much fainter, the professional Near-Earth asteroid (NEA) surveys should have had no problem finding it.

Comet Boethin is currently on an orbit that takes it from almost as far as the orbit of Saturn (aphelion at 9.1 AU) to as close as 1.15 AU (perihelion) from the Sun. It takes Boethin 11.5 years to orbit the Sun.

So What Happened to Comet Boethin?

There a number of possibilities that could explain what has happened to Comet Boethin:

1. The comet is well off of its predicted position and it still located too close to the Milky Way for easy observation. Since the Milky Way has a high density of stars, it is very difficult for the professional NEA surveys to find asteroids or comets. For this to be the case, the comet would have to much further off its expected position than expected. But with the comet moving away from the Milky Way, every day that goes by without a sighting makes this possibility less likely.
2. The comet is much less active than in 1975 or 1986. Perhaps the comet is completely inactive and will only appear as a very small dark asteroid. If this is the case, it would have to be very small (less than a few hundred meters in diameter) or the professional search in 2007 would have found it. If this small, it would be one of the smallest comets known. If this is the case, Boethin may come and go without anyone noticing because it would remain too faint for the NEA surveys. Perhaps the comet will once again become active enough for re-discovery at some future return.
3. The comet disintegrated sometime between 1986 and now. As a result, there is no longer any comet to find or the remaining pieces are too small to see. Now this possibility may sound unlikely, but many comets have been observed to disintegrate into nothing over the course of weeks to years.

While it’s still possible the Comet 85P/Boethin will be found in the next few weeks and put on a good show for backyard observers, it does appear that something has gone wrong with the comet. Comets can be very unpredictably and that is one of the main reason why I’ve always has an interest in them.

Other sources of info on Comet 85P/Boethin:

Deep Impact Press Release
Sechii Yoshida’s Comet Boethin page
Gary Kronk’s Cometography page on Boethin
Quarterly Journal of the Royal Astronomical Society article about the comets of 1975 by Drs. Elizabeth Roemer and Brian Marsden (the section describing the discovery and observations of Boethin in 1975 can be found starting on page 63)