TABLE 4.1 Major Nighttime Meteor Showers Visible from Earth
|Shower||Max Date||ZHR||Θ (×10–6)||RA||DEC||Speed (km/s)||Parent|
|Quadrantids||Jan. 3||120||8.4||15 20||49||43||2003 EH1|
|Lyrids||Apr. 22||20||4.6||18 10||34||48||C/1861 G1 (Thatcher)|
|η-Aquariids||May. 6||60||6.4||22 30||–2||66||1P/Halley|
|S. δ-Aquariids||Jul. 29||20||6.2||22 44||–16||43|
|Perseids||Aug. 13||90||6||3 08||58||60||109P/Swift-Tuttle|
|Draconids||Oct. 8||var||var||17 28||54||23||9P/Giacobini-Zinner|
|Orionids||Oct. 21||20||2.2||6 20||16||67||1P/Halley|
|S. Taurids||Nov. 5||10||1||3 34||14||31||2P/Encke|
|N. Taurids||Nov. 12||15||1.4||4 00||22||30||2P/Encke|
|Leonids||Nov. 17||15||1.9||10 12||22||71||55P/Tempel-Tuttle|
|Geminids||Dec. 14||100||2.3||7 28||33||36||3200 Phaethon|
|Ursids||Dec. 22||10||2.2||14 36||75||35||8P/Tuttle|
NOTE: ZHR, zenithal hourly rate, the approximate number of shower-related meteors an observer would see under ideal conditions; Max Date, the date at which the highest flux of meteors is normally expected each year; Θ (×10–6), the flux of meteors brighter than astronomical absolute magnitude +6.5 per km–2s–1 at the time of the maximum; RA, the right ascension of the radiant in equatorial coordinates at the time of the shower maximum; DEC, the declination of the radiant in equatorial coordinates at the time of the shower maximum; Speed (km/s), the speed of a meteor at the top of the atmosphere; Parent, the parent body (comet or asteroid) from which the meteoroid stream is believed to originate. SOURCE: Adapted, courtesy of the Royal Astronomical Society of Canada, from the Observers Handbook (2011).
atmosphere (having identical orbits about the Sun), they appear to emanate from a particular point in the sky (termed a “shower radiant”); the meteor shower’s name is derived from the constellation where this radiant is found. The comet or asteroid “parent” for many streams is known. Table 4.1 lists a selection of the strongest nighttime meteor showers visible at Earth, along with their parent bodies. Because meteor showers consist of many meteoroids traveling on similar orbits in a stream, which intersect Earth at a fixed point in its orbit during a short interval of time (typically on the order of days), the showers occur at about the same time each year. Several showers show strong variations in activity related to the details of how dust is produced and subsequently evolves in the stream. Such showers can produce strong flux enhancements for short periods in certain years. Examples include the Leonids in 1999 and the October Draconids in 1933 and 1946. Box 4.1 discusses meteor storms and spacecraft safety.
Meteoroids not found in streams are termed “sporadic meteors.” While they do not have a clear common origin, the sporadic background meteoroid population as detected at Earth shows strong directionality, reflecting the general orbital properties of the meteors’ parent body population. At Earth, in particular, several major sporadic meteor sources are noticeable with radiant diameters on the order of 20 degrees, as shown in Figure 4.1.4 Meteor showers from stream meteoroids are well known, whereas the sporadic meteoroid population is less well understood.
Unlike knowledge of orbital debris, knowledge of meteoroids results in part from ground-based observations of the interaction of meteoroids with the atmosphere (used as a detector for this purpose) to form a meteor (the plasma) at an altitude of between approximately 70 and 140 km. Broadly speaking, ground-based optical and radar instruments can detect both the plasma formed around a meteoroid particle traveling at its velocity—called the “head” or “radar head echo”—and the quasi-stationary plasma that can extend for kilometers behind the meteoroid, called the “trail” or “wake.” Direct measurement of the meteoroid is not possible using ground-based observational techniques, and significant modeling effort is required to translate measured meteor parameters to meteoroid
4 J. Jones and P. Brown, Sporadic meteor radiant distributions—Orbital survey results, Royal Astronomical Society, Monthly Notices 265:524-532, 1993, available at http://articles.adsabs.harvard.edu//full/1993MNRAS.265..524J/0000524.000.html, accessed July 13, 2011.