The 1 September 2007 Aurigid outburst.
THE 2007 AURIGIDS AT A GLANCE
An airborne and ground-based observing campaign is
being organized to monitor the unusual September 1, 2007, meteor shower
called the "Aurigids". Click here for the mission website of the Aurigid Multi-Instrument Aircraft Campaign.
Predictions for the shower are summarized in this paper in the August 7 issue of EOS, Transactions of the AGU.
Figure 1: Comet C/1911 N1 (Kiess)in an image by William Hoegner and Nikolaus B. Richter. At the peak of its return in 1911, the comet was a large diffuse nebula of +7 magnitude with a vague central condensation and a 0.2 - 0.5 degree long tail. [Click on picture for larger version]
THE 2007 AURIGIDS AT A GLANCE
What: A 2-hour outburst of bright (-2 to +3 magnitude) meteors radiating from the constellation Auriga (Radiant at RA = 92, Decl. = +39, speed Vinf = 67 km/s).
Why: The Earth will travel through the dust trail of long-period comet C/1911 N1 (Kiess), shown in Figure 1. Comet Kiess was discovered by Carl Kiess, a post doc at Lick Observatory, only days after Lick Observatory was hit by a damaging earthquake (an aftershock of the 1906 earthquake that destroyed San Francisco).
With the comet back on its way out, when was the dust ejected that makes up the comet dust trail: During the comet's prior return of around 83 BC, give or take a century, 2000 years ago at the beginning of our year count. The dust has since made one revolution, some particles making a wider orbit than others and returning later. There is a continuous stream of dust particles passing by Earth orbit, but only when the planets steer that dust trail in Earth's path do we see a meteor outburst.
- Only known case in our lifetime: crossing of the dust trail of a known long period comet.
- Long-period comets, such as were Hale-Bopp, Hyakutake, and McNaught, tend to sneak up on us. They are a potential impact danger. What can the dust trail tell us about their physical nature?
- Their dust is a danger to satellites in orbit and man working on the Moon.
- Dust stream could contain material from original cosmic-ray produced crust of comet.
When: 2007 September 1, about 11:36 +/- 20 minutes UT (= 04:36 a.m. Pacific Daylight Savings Time).
How long: Whole event will last about 2 hours, the time between the rate being half that of the peak equals about FWHM = 25 minutes.
Where: Visible from California, Oregon, Hawaii, Eastern Pacific.
Moon: 4 days past Full Moon high in the sky. Fortunately, the meteors will be bright enough for the display not to be dimmed much.
Best viewing direction: All over the sky, but keep the Moon behind a telephone pole or some other obstruction. Best viewing is towards the East and NorthEast. Best to start watching at least an hour before the predicted peak time, then see the rate of meteors increase and decrease while Earth travels through the shower.
The 2007 dust trail crossing was first recognized during an investigation of potential dust trail crossings related to long-period comets by Esko Lyytinen and Peter Jenniskens (Figure 2). The Aurigids were seen three times before, in 1935, 1986, and 1994. Now thy found another crossing in 2007. They put the peak time at 11:37 UT on September 1. This trail is not crossed again in the next 70 years. We know of no other known long-period comets with similar dust trail crossings. Full paper.
Figure 2: The position of the 1-revolution dust trail of comet C/1911 N1 (Kiess) in terms of distance to Earth orbit on September 1 in each year as a function of time, reproduced from Lyytinen and Jenniskens (2000).
That study followed from an earlier investigation by Dr. Jenniskens, where it was discovered that the dust trail of long-period comets is steered into Earth's path only once or twice every 60 years, in a pattern similar to the Sun's reflex motion. That concept was subsequently proven by the successful prediction of the return of the alpha-Monocerotids in 1995. Full paper.
The 1995 alpha-Monocerotid outburst is still the best observed dust trail crossing from a long-period comet, but the comet itself is unknown. The shower consisted of meteoroids that penetrated 5 km deeper in Earth's atmosphere than other meteors of similar speed and the meteors lacked the element sodium, which is contained in the most volatile minerals.
It was concluded that the Earth encountered meteoroids that originated from the pristine crust of a long-period comet. Only long-period comets still have such "pristine crusts" created by 4.5 billion years worth of cosmic ray exposure when the comets reside in the Oort cloud. The meteor shower provided the only direct evidence for the existence of these crusts.
In this context, it is interesting that Bob Lunsford, eye witness of the 1994 Aurigid outburst, described the outburst Aurigids as greenish or bluish, typical of meteors that have unusually intense metal atom lines compared to air plasma emissions.
Figure 3: Position of the node of the 1-revolution alpha-Aurigid stream particles that are near Earth's orbit at the time of past Aurigid outbursts. The graph shows the ecliptic plane and location of the particle orbit nodes. The density of points corresponds to the expected intensity of the shower.
Calculations by Jeremie Vaubaillon (Caltech) and Peter Jenniskens (SETI Institute) show the distribution of dust in the trail at Earth orbit (Figure 3, Table I). The 2007 dust trail is at the same position relative to Earth's orbit as in past Aurigid outbursts. A meteor outburst is expected. Preliminary results were announced at the 2006 IAU General Assembly in Prague. Full paper. Latest results were published in a paper in the August 7, 2007, issue of EOS, Transactions of the AGU.
year trail Nrev dE-dR Delta_a f_M Sol. Long. day time
2007 83 BC 1 -0.0003863 6.97261 0.005810 158.56148 UT Sep. 01 11:36
1994 83 BC 1 -0.0008137 6.02787 0.004612 158.73800 UT Sep. 01 08:01
1986 83 BC 1 -0.0003673 5.44971 0.016433 158.53043 UT Sep. 01 01:38
1935 83 BC 1 -0.0005241 1.74593 0.031045 158.65638 UT Sep. 01 03:05
Table I. Circumstances for the encounter with the 1-revolution (83 BC) trail of C/1911 N1 (Kiess) at the time of Aurigid outbursts. Nrev = number of revolutions since ejection. dE-dR = heliocentric distance of Earth's orbit at the node - that of node of meteoroid orbit. Delta_a = difference in semi-major axis of orbit compared to that of the comet(in astronomical units). f_M = dillution factor due to spreading of the grains. Sol. Long. = solar longitude (J2000) of peak of shower. Day/time in Universal Time (hours:minutes).
In February 2007, Danielle Moser of the NASA Marshall Space Flight Center's Meteoroid Environment Office presented preliminary results of her Kiess dust trail model, in collaborative work with Bill Cooke, during a talk at the Meteoroids Workshop in Huntsville Alabama. She confirmed that particles will be in Earth's path on September 1, 2007.
Jenniskens and Vaubaillon put the most likely peak rate at around ZHR = 200/hr, or 3 Aurigids per minute, based on the handfull of Aurigids that were seen in past outbursts.
Observing circumstances were bad in prior sightings. In 1935, the shower rate was still increasing when twilight arrived in Germany and the Czech Republic, where the shower was seen by two groups of experienced observers. In 1986, few sporadic meteors were seen by the one eye witness, Istvan Tepliczky of Hungary, suggesting hazy sky conditions. And in 1994, the shower was seen by only two observers, Bob Lunsford and George Zay near San Diego, CA, who had the radiant very low near the horizon.
After correcting for radiant elevation (geometric dillution), observer perception, and sky limiting magnitude, past peak rates (in a ten minute interval) were ZHR > 100 /hr in 1935, ZHR = 200 +/- 25 /hr (1986) and ZHR = 200 +/- 25 /hr (1994), about three times the rate of the Perseids in summer. The models show the dust density in 2007 being very similar to those in 1986 and 1994.
The abundance of bright (-2 to +3 magnitude) meteors and the short duration of the shower should make for an impressive display. Of course, the Moon light will take some of that away, especially under hazy observing conditions and when the Moon is not blocked in the field of view. Best to put the Moon behind an obstruction, such as a telephone pole, and observe with your head in the Moon's shadow of that obstruction. In Hawaii, apparent rates will also be lower because of a lower radiant elevation.
Figure 5: The Earth as seen from the shower. The terminator shows the moment of civil twilight. Meteors will start to become hard to see a little before that time. Best viewing locations are marked.
Figure 6: Twilight conditions near beginning and end of observing interval.
The outburst is best seen from the western United States, including California, Oregon, Alaska, and Hawaii. The figures above show the moment of civil twilight for different times, at the onset of the shower, at the peak, and towards the end. In San Francisco, the peak out the outburst is at 4:36 a.m. Pacific Daylight Time, while the beginning of civil twilight is at 6:13 a.m. PDT.
Figure 6. Artist impression of view from San Francisco, CA.
The Moon is waning gibbous, 4 days past Full Moon, with 76% of the Moon's visible disk illuminated. It will be high in the sky. Best viewing is in high altitude locations, where scattered moonlight does not brighten the night sky. Watch towards the East and NorthEast.