Summertime Balls of Fire Posted on May 30th
I have identified
another minor planet that is likely responsible for one of our meteor showers. The
cometary breakup that created the Kappa Cygnid meteor shower 4000 to 6000 years
ago has a fragment remaining: minor planet 2008 ED69!
Meteor
showers, as they go, tend to become personal after a few unusual sightings.
Especially when they contain exploding fireballs with multiple flares. No, I’m
not talking about the phenomenal Leonid showers, but about a delightful treat
of summer nights: the Kappa
Cygnids.
I recall a
nice Kappa Cygnid fireball in 1993, the first summer after I had moved to California. I trained my binoculars at the position of the fading fireball, and saw the
glow of a persistent train: a thin line that changed shape in the upper
atmosphere winds like a twisting thread of silk on a damp day. That year saw
many Kappa Cygnid fireballs so many, in fact, that they outshone a widely
reported outburst of Perseids. Observers at one site shouted so frequently that
local canines became excited, and the event became known as “the night of the
howling dogs.”
The Kappa
Cygnids showed again in 2007. NASA and SETI Institute scientists videotaped
several of these meteors during a test flight for an airborne observing
campaign to study the predicted return of the rare
Aurigid shower on September 1, 2007. Among the
fast moving Perseids were several slow moving meteors that radiated from a
point between the bright stars of Vega and Deneb. Some were as bright as the
first quarter Moon and flashed in multiple colors.
The crazy
thing about this shower is its long duration, stretching through most of
August. Some showers endure because Earth travels through the meteoroid stream
at a grazing angle, but not so for the Kappa Cygnids. This stream is inclined
at a steep 28-38 degrees. This also means that its
parent body is not easily confused with that of the thousands of asteroids that
have so far been discovered, most of which move in orbits close to the plane of
the planets. So far, none matched the Kappa Cygnid orbit. We used to
think that the long duration of the shower was due to an advanced age with the
parent long gone.
Instead,
the periodic showers of fireballs suggested to me a younger age. For years
now, I have been on the lookout for a remaining fragment of the parent body
that created the Kappa Cygnid shower. I had found earlier that other well-known
showers, such as the Quadrantids and Geminids, have such remaining fragments.
No object was known for the Kappa Cygnids in 2006, when I published my book “Meteor
Showers and their Parent Comets.” In chapter 24, I write at length
about this stream, assuming it is one of many that were created in the
disruptive breakup of a mostly dormant comet in the recent past. The lack of a
reasonable candidate parent body, however, left this chapter unfinished, and
the Kappa Cygnid fragment became a very personal quest for me.
On March
11, 2008, at 11:54 UT, the Catalina Sky Survey detected an intrinsically bright
H = +16.7 magnitude minor planet, now named 2008 ED69. The discovery was
quickly confirmed by the Mt. Lemmon Survey. It moved in an unusual orbit,
passing close to the orbits of Jupiter and Venus in a 37-degree inclined orbit.
Briefly, it was listed as a Potential Hazardous Object, but the latest orbit
calculations keep it relatively far from Earth’s orbit. That is good for us,
because the object would cause a big impact if it was to hit Earth. Assuming,
as with other dormant comets, that only 4 percent of the sunlight striking 2008
ED69 is reflected, it could measure 2.9 km in size. This is the same size as
that of minor planet 2003 EH1, the remaining fragment in the
Quadrantid stream.
As soon as
the discovery was announced, I calculated the theoretical radiant of the then
very premature orbit and noticed that its meteors would radiate from Cygnus at
the time of the Kappa Cygnid shower. Over several days, more observations were
made and the orbit quickly became better known, much better than that of any
other Kappa Cygnid. I traced the orbital evolution of 2008 ED69 back in time
and found that it evolved with a large amplitude oscillation called a nutation
cycle, lasting about 1800 years, in a manner typical for Kappa Cygnids.
A paper was
submitted to the Astronomical Journal on March 22. Review of the paper
took a few weeks, during which the orbit of 2008 ED69 became better known.
Jérémie Vaubaillon of Caltech calculated the orbital evolution of 2008 ED69
farther back in time, and then projected forward in time the orbit of
meteoroids ejected one, two, or three cycles ago. He found a good match between
the calculated distribution and the observed dispersion of dust all over August
if the stream was created 2 or 3 cycles ago. The long duration of the shower
resulted from the nutation cycle grazing the Earths orbit, some meteoroids
evolving slower than others. We determined that the stream is more massive than
the remaining parent body, hence some form of fragmentation was implicated. The
revised version of the paper was accepted for publication on May 21.
We conclude
that the Kappa Cygnid stream was created in the breakup of a Jupiter-family
comet around 4000-1600 BC, 2-3 nutation cycles ago. The now mostly dormant
minor planet 2008 ED69 is a remnant. Most of the debris passes close to the
orbit of Venus, making the Kappa Cygnids a significant shower on Venus.
Fortunately, the cloud of debris stretches far enough from the comet position
to make the Kappa Cygnids also a joy for Earth-bound observers, including
myself, who will never look at this shower with the same eyes again.
2008 ED69
is closest to Earth on June 22, and brightest in the first week of June when
its apparent visual magnitude +18.45 makes it a point of light in the constellation
of Cassiopeia. I hope it will be studied in detail for signs of its cometary
origin. For more information on its association with the Kappa Cygnid shower,
see the article: “Minor planet 2008 ED69 and the Kappa Cygnid meteor
shower” by P. Jenniskens and J. Vaubaillon, which will appear in The
Astronomical Journal later this year, when 2008 ED69 has moved on.