Wednesday, June 6, 2018

2018 Daytime Arietids Peak June 10 as Invisible Yet Audible Shower


Summary: The 2018 Daytime Arietids peak Sunday, June 10, as a mostly invisible yet radio-audible meteor shower.


Shooting stars from Daytime Arietids appear to radiate from Aries the Ram constellation: Bob King ‏@AstroBob_bk via Twitter June 6, 2017

The 2018 Daytime Arietids, which shower mostly invisible yet radio-audible meteors between May 22 and July 2 each year, peak Sunday, June 10, at 22:07 Coordinated Universal Time (6:07 p.m. Eastern Daylight Time).
The Arietids occur as the strongest annual daytime shower. Their radiant in Aries the Ram constellation, however, renders them largely invisible. The radiant is a meteor shower’s apparent point of radiant in the sky.
Bruce McClure, EarthSky Tonight’s lead writer, notes the sun’s apparent passage in front of Aries occurs annually from about April 19 to May 14. In June, Aries is only about 30 degrees from the sun and, therefore, still within the sun’s glare.
Brief Arietid visibility is possible for Earth-based observers, though, because the radiant precedes sunrise by about 45 minutes. Peter Jenniskens, senior research scientist at SETI (Search for Extraterrestrial Intelligence) Institute’s Carl Sagan Center and meteor showers expert, points to the possibility of early morning observations, “in the hour before dawn,” during June’s first three weeks. Tony Phillips, formerly Science @NASA production editor and now Spaceweather.com’s science writer, explains that pre-dawn visibility is available to observers in both the Northern Hemisphere and the Southern Hemisphere.
For mid-northern latitudes, Phillips places the Daytime Arietid radiant at about 15 degrees above the eastern horizon at 5 a.m. on the date of the shower’s expected maximum, known as the peak date. Robert Lunsford, the American Meteor Society’s weekly Meteor Activity Outlook columnist, expects approximately six sporadic meteors per hour during pre-dawn’s last hour at rural sites at 45 degrees north latitude. Pre-dawn rates of nearly nine meteors per hour are suggested for rural sites at 25 degrees south latitude.
The radiant’s pre-dawn low altitude discourages vividly flaring, steep-angle entries into Earth’s upper atmosphere by Arietid particles. Instead, the shower tends toward producing nearly horizontal atmospheric skimmers. Known as Earthgrazers, the meteoroidal skimmers move slower than atmospheric meteors but streak brightly and lengthily.
Lunsford hypothesizes that, if the radiant were higher in the sky, Arietid visibility would rival that of August’s famous Perseids. Yet, Phillips considers the value of pre-dawn Arietid watches.
“Watching for early morning Arietids is a no-lose proposition,” Phillips says, “because, even if you don’t glimpse any meteors, you’ll be able to see the two bright planets Jupiter and Saturn less than 2 degrees apart rising above the eastern horizon.”
NASA’s Meteoroid Environment Office at Huntsville, Alabama’s Marshall Space Flight Center forecasts a maximum zenithal hourly rate (ZHR) of 74 meteors per hour for the Daytime Arietid shower’s peak date. The Arietids’ maximum ZHR rates as fourth in NASA’s 2018 list of 30 meteor showers. First place goes to the Geminids, with a maximum ZHR of 127 meteors per hour. The Quadrantids occupy second place, with a maximum ZHR of 120 meteors per hour. The Perseids claim third place, with a maximum ZHR of 106 meteors per hour.
The Meteoroid Environment Office assigns second place to the Daytime Arietids in its forecasted ranking of 2018’s six most active meteor showers in terms of flux. First place goes to January’s Quadrantids. July’s Southern Delta Aquariids, August’s Perseids, December’s Geminids and December’s Ursids hold third, fourth, fifth and sixth places, respectively.
Visual observation is not the only method for detecting the Daytime Arietid meteor shower. Radio astronomy identifies the Arietids as blips, reflected by meteor trails, as the shower’s particles hit Earth’s atmosphere at a velocity of 39 kilometers per second (87,000 miles per hour).
Phillips explains that as Daytime Arietid meteoroids “. . . move rapidly through the air, these specks of space dust heat and ionize the gas around them. During major meteor showers like the Arietids . . ., radio signals from TV stations, RADAR facilities, and AM/FM transmitters are constantly bouncing off these short-lived meteor trails.”
Radio astronomy, not observational astronomy, actually is credited with the Daytime Arietids’ discovery. Radio astronomy pioneers James Stanley Hey (May 3, 1909-Feb. 27, 2000) and G.S. Stewart discovered the mostly invisible yet radio-audible meteor shower in 1947 at Jodrell Bank Experiment Station (now Jodrell Bank Observatory), established on the University of Manchester’s Cheshire site.
The parent body for the Daytime Arietids is unknown. Proposed candidates comprise eccentric, near-Earth asteroid 1566 Icarus and short-period sungrazing comet 96P/Machholz. German astronomer Wilhelm Heinrich Walter Baade (March 24, 1893-June 25, 1960) discovered Icarus June 27, 1949, at Palomar Observatory, San Diego County, southwestern California. Using 29x130 binoculars, amateur astronomer Donald Machholz (born Oct. 7, 1952) discovered 96P/Machholz May 12, 1986, on central California’s Loma Prieta peak.
Peter Jenniskens profiles the Daytime Arietid meteor shower as having a breakup type of formation rather than the gas drag formation of gradual ejection by water vapor drag at perihelion (closest approach to the sun). He places the breakup formation date as around 1059 C.E.
The takeaway for the 2018 Daytime Arietids, which peak Sunday, June 10, is that the placement of the shower’s radiant near the sun accounts for mostly invisible shooting stars but does not prevent radio-audibility as blips reflecting Daytime Arietid meteor trails in the sky.

Canadian Meteor Orbit Radar (CMOR), a multi-frequency HF/ VHF radar, detects ablating meteoroids' ionized trails used to detect the ionized trails associated with ablating meteoroids; ARI=Daytime Arietids; DLT=Daytime lambda Taurids; JMC=June mu Cassiopeiids; ZPE=Daytime zeta Perseids: EarthSky ‏@earthskyscience via Twitter June 6, 2017

Acknowledgment
My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:
Shooting stars from Daytime Arietids appear to radiate from Aries the Ram constellation: Bob King ‏@AstroBob_bk via Twitter June 6, 2017, @ https://twitter.com/AstroBob_bk/status/872191946630062085
Canadian Meteor Orbit Radar (CMOR), a multi-frequency HF/ VHF radar, detects ablating meteoroids' ionized trails used to detect the ionized trails associated with ablating meteoroids; ARI=Daytime Arietids; DLT=Daytime lambda Taurids; JMC=June mu Cassiopeiids; ZPE=Daytime zeta Perseids: EarthSky ‏@earthskyscience via Twitter June 6, 2017, @ https://twitter.com/earthskyscience/status/872083253175386112

For further information:
Astro Bob (Bob King). “Arietid Meteor Shower Peaks -- Watch for Earthgrazers.” Area Voices. June 6, 2017.
Available @ http://astrobob.areavoices.com/2017/06/06/arietid-meteor-shower-peaks-watch-for-earthgrazers/
Bob King ‏@AstroBob_bk. “Arietid Meteor Shower Peaks -- Watch for Earthgrazers.” Twitter. June 6, 2017.
Available @ https://twitter.com/AstroBob_bk/status/872191946630062085
Byrd, Deborah. “This Week, a Daytime Meteor Shower.” EarthSky > Astronomy Essentials. June 6, 2017.
Available @ http://earthsky.org/astronomy-essentials/arietid-meteor-shower-daytime-june-peak
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Available @ https://patch.com/michigan/detroit/arietids-daytime-meteor-shower-how-see-june-7-peak
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Available @ https://twitter.com/earthskyscience/status/872083253175386112
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Available @ https://twitter.com/HumanoidHistory/status/850800264764424192
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Available via Google Books @ https://books.google.com/books/about/Meteor_Showers_and_Their_Parent_Comets.html?id=QpajMuyXG8AC
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Available @ http://cams.seti.org/jenniskens-ari_offprint.pdf
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Available @ https://archive.org/details/MeteorAstronomy
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