Potentially Dangerous Space Rock Lost, and Found
Minor planet 6344 P-L (=2007 RR9) among the stars on October 3, 2007. This is currently still a faint object, recorded in 3-minute exposures tracked on the target object by Paulo Holvorcem of Brazil, remotely using the 0.35-m telescope at Tenagra Observatory in Western Australia (at Shenton Park, near Perth) operated by Paul Luckas.
Credit: SETI Institute

When I was an astronomy student at Leiden Observatory in the Netherlands, pioneer asteroid hunters Ingrid van Houten-Groeneveld and her husband Kees van Houten were about to retire, after very fruitful careers. Their legacy was the discovery of over 2000 asteroids, of which some 1800 yielded orbits, in collaboration with Prof. Tom Gehrels of the Lunar and Planetary Laboratory at Tucson, Arizona. In 1960, Gehrels made a sky survey using the large 48-inch Schmidt Telescope at the famed Palomar Observatory, long before modern asteroid reconnaisances, and shipped the photographic plates to the van Houtens at Leiden Observatory. They used a large optical bench, equipped with a binocular and flip mirror, to compare each set of two large photographic plates and find the faint dots of light moving among the stars, a technique called blinking. This task is now routinely done by fast computers, or by us watching avi movies of asteroids.

In the mid-eighties, this asteroid searching was pooh-poohed by some. Why find more, if you already have thousands? Yet, among all those asteroids, Ingrid and Kees discovered two objects in peculiarly elongated orbits, 6344 P-L and 6743 P-L, on plates taken the very same nights of Sept. 24 - 28, 1960. The original designation of P-L stands for the Palomar-Leiden Asteroid Survey. 

Most asteroids move in near-circular orbits in the main asteroid belt between Mars and Jupiter, but 6344 P-L and 6743 P-L's elongated orbits bring them occasionally to within 0.05 AU from Earth's orbit. These are Potentially Hazardous Asteroids (PHA), we now know, of the type that killed off the dinosaurs. When that role of asteroids became established in the late eighties and early nineties, the search for asteroids became suddenly important insurance for our very existence. Only when an asteroid's orbit is precisely determined can we be certain it will not impact Earth in the foreseeable future.

6743 P-L, was soon recovered and has since been renamed 5011 Ptah. On the other hand, 6344 P-L remained the oldest lost Potentially Hazardous Asteroid out there. We knew it existed, it had a roughly determined orbit, but we did not know where to point the telescope.  

On the afternoon of October 3, 2007, I scanned the list of newly discovered PHA's to search for potential parent bodies of our meteor showers. In October of 2003, I found that an object called 2003 EH1 moves among the meteoroids of our most intense annual shower, the Quadrantids, and many other asteroids have been unmasked as dormant comets since. From our meteor observations, we find that these objects appear to erupt on occasion by breaking off significant chunks of matter that fall into the small dust we see on Earth as meteor showers some centuries later. The Quadrantids are suspected to have formed as recently as 1490 AD, when Chinese astronomers noticed a comet that moved in the same plane as the meteoroids.

In 1819, another such dormant comet breakup resulted in a brief comet called "Blanpain." In 2004, I found a fragment of this breakup, now called 2003 WY25, which is still moving among the meteoroids, and responsible for the December Phoenicid shower. In collaboration with Finnish astronomer Esko Lyytinen, we were able to trace an outburst of Phoenicids in 1956 to this very 1819-breakup dust.

I am sure that there are many more dormant comets out there that have not yet been identified as parent bodies of our meteoroid streams, because many streams move in low inclined orbits and their parent bodies hide among the 896 (and counting) other known PHA's. In a paper that was just accepted for publication in Icarus, I have identified 42 such candidate parent bodies, many of which need confirmation from better observations of the object and of the associated meteor shower.

So, I was content to find that newly discovered 2007 RR9 was also a possible parent body, now for an obscure shower called the gamma Piscids (#236 of the IAU shower list), which is active in mid-October and early November. 2007 RR9 had been discovered by the Catalina Sky Survey four weeks earlier on September 10. Interestingly, I already had identified a possible parent body for this shower, namely 6344 P-L, the long-lost asteroid.

When I compared the orbits of both objects, I noticed that they were very similar. So much so that they could well be the same object. I wrote Brian Marsden of the Minor Planet Center of the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, the clearing house for such identifications: "If warranted, you can probably check if this could be the same object."

The next morning, I learned the good news via an electronic circular (MPEC 2007-T13), issued by the Minor Planet Center on October 4. It was titled "6344 P-L = 2007 RR9." Instantly, the orbital accuracy for 6344 P-L improved from lousy to phenomenal, now having been observed for over 47 years, instead of only four days. Later that day, 6344 P-L was swiftly removed from the list of asteroids that could pose an immediate impact danger.

A designation as Potentially Hazardous Asteroid means that 6344 P-L is a known asteroid bigger than 150 m (500 ft) in diameter that comes to within 0.05 astronomical units of Earth?s orbit (roughly 7,480,000 km or 4,650,000 miles). The size is estimated on the basis of the object?s observed brightness and an assumed reflectance of 13 percent.

This object may not, in fact, be an asteroid. 6344 P-L moves in a 4.70-year orbit nearly all the way out to the distance of Jupiter. The elongated orbit translates into a Tisserand parameter of T = 2.94 (was originally T ~ 3.05), a measure of the orbital energy and angular momentum of the orbit, which defines it dynamically as a Jupiter Family Comet (T = 2.0 - 3.0), not an asteroid (T > 3.0). Therefore, in my opinion, this object is a now-dormant comet nucleus. A fragment of a bigger object, perhaps, that after breaking up in the not-so-distant past may have caused the gamma Piscid stream of meteoroids. If so, it should be very dark, reflecting only 4 percent of light, and therefore be much bigger, about 530 m across. If this comet would hit Earth, it would strike at about 1 billion tons of TNT, enough to destroy a small country.

Interestingly enough, 6344 P-L is coming closer to the Earth as we speak. The object is approaching Earth from behind and will brighten to magnitude +18.5 in mid October, only to pass Earth on the inside of our orbit on November 7 at a distance of 28 lunar distances (0.072 AU). On November 17.8, it will arrive at the closest point to the Sun.

Somewhere along this path, the object could become weakly active and turn from an "asteroid" into an official comet. We have a rare opportunity to follow this process and see if the heat of the Sun can still nudge some gas and dust off this small planet.

All of this made for a great story, but it even got better. Just over a week later, another one of the van Houtens' discoveries, 5025 P-L, was recovered by the automated Spacewatch II asteroid survey program. 5052 P-L is a Near Earth Object, coming to within 0.2 AU from Earth's orbit, not quite as dangerously close. The news was announced on MPEC 2007-T58.

Brian Marsden wrote to me about the circumstances of this recovery on October 11: "An object reported to us by Spacewatch on Tuesday and placed on the NEO Confirmation Page and followed up yesterday was found here also to have been observed in NEO surveys on Sept. 10 and 30. This was good enough to reveal that 5025 P-L fitted, as well as a Spacewatch single-nighter in 1999.  This kind of thing is what should have happened last month with 2007 RR9 leading to 6344 P-L, but you came to the rescue instead." He added: "All the same, it's a curious coincidence that the two old P-L NEOs should be recognized just over a week apart."

Indeed, after 6344 P-L, this was the remaining P-L object waiting recovery. I also knew it, because 5025 P-L was linked in the past tentatively to the Taurid shower. Its initial orbit was based on only three nights of observations and was extremely uncertain. The original solution had it move far beyond Jupiter. Now, with T = 2.988, it falls among a whole group of other similar potential dormant comets, reaching to just inside the perihelion distance of Jupiter. Whether or not some of our Taurids originate from this object can now be investigated.

Brian further wrote: "The oldest lost NEO is now 1972 RB. Actually, with a 49-day arc the orbit is quite good. It's just that there hasn't been a chance to look at it since discovery. It comes to a distance of 0.1 AU in 2013 and should then be mag 16-17 or so."

In honor of the work by the van Houtens and Gehrels, the objects 6344 P-L and 5025 P-L will keep their designations, waiting to be numbered and receive an official name. Now the orbits of these objects are precisely determined, we can be certain that they will not impact Earth in the foreseeable future.

Dr. Peter Jenniskens is author of "Meteor Showers and their Parent Comets," published by Cambridge University Press in 2006, which provides many more background stories on other potential parent bodies and our meteor showers.

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