Orionids Peak This Weekend

Everyone know that the most famous comet is Comet Halley. Once every 76 years or so, Hally visits the inner Solar System. Sometimes is can be quite spectacular (such as in 1910), other times not so much (as in 1986). For those that missed Halley in ’86, it will be back again in 2061. If you don’t want to wait that long, there is a way to see pieces of Halley every year. Dust released by Halley over the past few thousand years produce meteor showers in early May (the η-Aquariids) and mid-October (the Orionids).

Computer simulations of the past movements of Halley and its dust suggest that many of this year’s Orionid meteors were released by Halley between 1265 BC and 910 BC (for some points of reference, the Trojan War took place around 1200 BC and King David ruled around 1000 BC).

This year the Orionids are forecast to peak tonight (Oct 20/21) though this shower usually produces high rates for a few days on either side of its peak time. According to the Live ZHR page on the International Meteor Organization’s page, rates last night reached a ZHR of 20-30. Tonight rates should be a little better, probably between 30-40. ZHR’s been as high as 70 per hour in the past but during the last 2 years ZHRs only reached 35-45 per hour. This year’s activity should be similar to the last few years.

The Orionids appear to come from an area in northern Orion. This area, called the radiant, rises around 10pm local time. It is best to wait till the radiant is high in the sky before looking for meteors (say 1am). The radiant is highest around 3:30am which is the best time to look. Meteors can appear anywhere in the sky so you don’t have to look at the radiant. The chart below is for around 3-4am local time and shows the radiant and directions of the Orionids.

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Hints for watching the Orionids:

  1. Orionid meteors are not visible before ~10-11 pm. Even then the radiant is too low to see many meteors. It is best to go out sometime between 2 am and the start of dawn.
  2. Even though the Zenithal Hourly Rate (ZHR) for the Orionids may be as high as 40 per hour this year.  Most people will see fewer meteors. The ZHR is calculated for perfect conditions (radiant overhead, dark skies, and no obstructions in your view). If you observe from rural areas where the Milky Way is bright and obvious you might see 40 per hour. Suburban skies were the Milky Way is just barely visible will probably only produce 10-20 per hour. City observers will see only a few per hour.
  3. It may take some time to see some meteors. Going out for a minute or two won’t cut it. Plan to spend at least 30 minutes of more outside. Also allow your eyes some time to get adapted to the dark. It will take at least 10-20 minutes after walking out of a well-lit house to start seeing faint enough to see most Orionids.
  4. Find a spot that is safe, free of as many obstructions (trees, buildings, etc) as possible and free of annoying lights shining in your face. This is not always possible these days. You don’t need to look directly at the radiant. In fact, it is better to place the radiant just outside your field of view. As long as you do this, it won’t matter what direction you look in.
  5. Be prepared to be cold. The early October mornings can get very chilly. Dress warm and bring a blanket. Also plan to be as comfortable as possible. A nice reclining chair works great. It will keep your neck from being strained and keep you off of the cold ground.
  6. Enjoy the show! The split second meteors are tiny dust grains from Comet Halley released thousands of years ago; are hitting the Earth’s atmosphere at speeds of ~40 miles per second (~66 km per second) and are burning up 60 miles (100 km) above your head.

Apr 26/27 to May 15/16 Meteors

April/May/June are usually clear months in Tucson and this year is setting up to be no different. Though bouts of cirrus are common, every night but one has been clear enough to allow the detection of a few meteors.

The past few weeks have seen a couple of showers come and go. Two minor showers, the Nu Cygnids and Northern May Ophiuchids, produced only 2 meteors apiece. These showers are recent discoveries and little is known about each. The Eta Lyrids produced 1-2 meteors per night at its best. This shower is produced by the long-period Comet IRAS-Araki-Alcock which passed closer to Earth than any other observed comet over the past 200 years (5 million km in May of 1983).

The highlight of May was the Eta Aquariids. Similar to the Orionids of October, the Eta Aquariids are produced by Comet Halley. This year’s display was hampered by a bright early morning Moon but still produced healthy numbers during the last hour or 2 of each early May night.

Obs  Date(UT)      Time    TOT SPO ANT PPU NCY ETA ELY NOP
TUS  2010-05-16   08h08m    10  8   1   -   -   1   -   -
TUS  2010-05-15   01h39m    3   3   0   -   -   0   -   -
TUS  2010-05-14   05h31m    8   5   1   -   -   1   1   -
TUS  2010-05-13   05h32m    12  8   1   -   -   1   1   0
TUS  2010-05-12   08h29m    15  8   1   -   -   4   2   0
TUS  2010-05-11   08h30m    22  13  3   -   -   4   2   0
TUS  2010-05-10   08h32m    9   3   1   -   -   2   2   1
TUS  2010-05-09   08h14m    12  6   1   -   -   5   0   0
TUS  2010-05-08   04h06m    13  8   1   -   -   3   0   1
TUS  2010-05-07   08h37m    18  10  0   -   -   8   0   0
TUS  2010-05-06   04h49m    15  9   0   -   -   6   0   -
TUS  2010-05-05   08h41m    11  4   1   -   0   6   -   -
TUS  2010-05-04   08h43m    7   1   0   -   1   5   -   -
TUS  2010-05-03   07h27m    14  10  0   -   0   4   -   -
TUS  2010-05-02   07h41m    5   2   2   -   0   1   -   -
TUS  2010-05-01   08h49m    12  9   2   -   0   1   -   -
TUS  2010-04-30   00h45m    3   2   0   -   0   1   -   - 
TUS  2010-04-29   08h52m    8   4   0   -   1   3   -   -
TUS  2010-04-28   00h00m    Bad Weather
TUS  2010-04-27   00h48m    2   2   0   0   0   -   -   -

TUS - Camera in Tucson operated by Carl Hergenrother
SDG - Camera in San Diego operated by Bob Lunsford
TotTime - Total amount of time each camera looked for meteors
TOT - Total number of meteors detected
SPO - Sporadics (meteors not affiliated with any particular meteor shower)
ANT - Antihelions
PPU - Pi Puppids
NCY - Nu Cygnids
ETA - Eta Aquariids
ELY - Eta Lyrids
NOP - Northern May Ophiuchids

The Orionid Meteor Shower in 2009

Back in 1986, Halley’s Comet returned after a 76 year journey through the outer solar system. Not predicted to return again until 2061, you might think you missed your chance to see Halley. Well, yes and no. It will be a long time before anyone sees the comet again, but twice a year small pieces of Halley are visible as they burn up in the Earth’s atmosphere. Both the Eta Aquariid meteor shower in early May and the Orionids of mid-October were created by small dust particles released over thousands of years by Comet Halley.

Comet Halley is one of the most active and brightest known comets. Though there are brighter comets than Halley, it is the brightest comet which returns after a reasonable length of time (~76 years). As a result, it has been observed at 30 returns dating back to 239 BC. It was likely observed even earlier but those records have not survived. Halley travels in an orbit that takes it from as far as 35 AU (just beyond the orbit of Neptune) from the Sun to as close as 0.59 AU (just inside the orbit of Venus) from the Sun. It’s orbit is inclined 162 degrees from the Earth’s orbit. This means the comet orbits the Sun in almost the exact opposite direction than the Earth and the other planets which is common for comets  with orbital periods greater than ~30 years. Two plots of Halley’s orbit are posted below.

//neo.jpl.nasa.gov/orbits/Outer Solar System view of the orbit of Comet Halley. Image created at http://neo.jpl.nasa.gov/orbits/
//neo.jpl.nasa.gov/orbits/Inner Solar System view of the orbit of Comet Halley. Image created at http://neo.jpl.nasa.gov/orbits/

What exactly are the Orionids?

Comets are mountain sized rocks which contain a large amount of ices (water, carbon monoxide, methane, and many others). Every time a comet passes close to the Sun, these ices sublimate (go directly from a solid to a gas). As the gases build up inside the comet, they eventually erupt into space much like geysers on Earth. These geysers of gas (on comets they are called “jets”) carry lots of dust with them. It is all of this escaping gas and dust that gives comets their “fuzzy” appearance. Over time the dust particles spread out and line the entire orbit of the comet. As a result, even though the comet may be far away (right now Halley is beyond the orbit of Neptune) dust can be found at any point in Halley’s orbit. When the Earth passes close to the orbit of Halley we can see some of this dust as it burns up in the atmosphere as meteors.

Over time the orbit of Halley changes. Computer simulations of the past movements of Halley and its dust suggest that most of this year’s Orionid meteors were released by Halley over 2000-3000 years ago. Enhanced activity may be possible due to an excess of particles released by Halley during its returns in 1400 BC and 11 BC.

So what can we expect this year and when should you look?

This Orionid meteor shower is usually active from Oct 3 to Nov 11 with a broad peak between Oct 18 and 24. During their peak, rates can be as high as 20-70 meteors per hour. In 2007 rates of 70 meteors per hour were observed but “only” 39 per hour were seen last year. How active this year’s Orionids will be is unknown though the 1400 BC and 11 BC streams should keep active at a level no less than last year’s.

The Orionids appear to come from an area in northern Orion. This area, called the radiant, rises around 10pm local time. It is best to wait till the radiant is high in the sky before looking for meteors (say 1am). The radiant is highest around 3:30am which is the best time to look. Meteors can appear anywhere in the sky so you don’t have to look at the radiant.

Orionids

Map of the sky for 3:30 am on Oct 21. The Orionid radiant is the yellow "star" just north of Orion. Chart created with Stellarium.

If you live in a major city with only a few bright stars visible, you will not see too many, if any, meteors. So it is always best to observe from a dark site. The International Meteor Organization has a near real-time graph of the activity level of the Orionids at this page. Note that the rates shown have been compensated for bright skies (meaning this is the rate you would have seem if you were observing from a dark rural location). The actual rate of meteors will be lower (perhaps much lower) if you are at a site with a bright sky.

Additional information on the Orionids can be found at the American Meteor Society.

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