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Updated February 14, 2004

Like many sources, the Observer’s Handbook 2003 lists a dozen meteor showers in its annual table. Such lists are confined to the major showers, generally defined as having a maximum Zenith Hourly Rate of at least ten per hour. There are, however, many more minor showers, with several dozen to over a hundred recognized by various authorities. These showers generally exhibit the same general characteristics as major showers, but with much lower rates. 

Indeed, it has proven difficult to delineate the boundary between recognized minor showers and truly sporadic meteors, in part depending on who is doing the recognizing. According to Robert Lunsford, Secretary General of the American Meteor Society (AMS), ‘sporadic meteors are usually considered as random occurrences not associated with any particular meteor shower.’ However, most meteors are generated from cometary debris, and depending on the age, density, and orbital characteristics of the debris trail, events of related particles are simply too rare to connect the dots. 

However, a clear majority of the brightest fireballs and bolides are considered to be truly sporadic, one-of-a-kind events, including all known events which have resulted in a meteorite fall. Recovered meteorites have been determined to originate primarily from fractured asteroids, and some have been identified as chunks ejected (in the aftermath of collisions) by known sources such as larger asteroids, or even the Moon and Mars. 

Other meteors which the casual or intermediate observer might consider to be sporadic, are in fact associated with several known sources which move in lockstep with Earth. Meteors from three of these sources can be observed on a nightly basis, namely the Antihelion radiant, and the Northern and Southern Apex. These radiants are enormous compared to those associated with showers, as large as 20 by 20°, but in well-defined directions.

The Antihelion radiant is, as the name implies, at a point approximately opposite the Sun, where Earth intercepts low inclination particles (centred along the ecliptic) in prograde motion around the Sun. Again citing Bob Lunsford of AMS:

"Unlike most of the annual showers the antihelion source is produced by debris from unknown objects orbiting in a direct motion like the Earth. These objects are most likely asteroids, which produce stony and metallic debris whose density is much greater than material produced by comets. This material collides with the Earth on the inbound portion of its orbit, before its closest approach to the Sun. Therefore we best see them just after midnight when we are facing the direction from which this activity appears."

There is a time lag of about an hour from true (local) midnight caused by Earth’s own motion, which causes the radiant to be offset slightly from 180° opposite the Sun. The prograde nature of their orbit means collisions with Earth are not head-on, so antihelion meteors are of slow to intermediate speed, ~20 - 35 km/s.

The Apex meteors radiate from an area 90° west of the Sun, which like a third quarter moon can be seen only in the hours after midnight, peaking near dawn. These particles are in retrograde orbit which therefore collide with Earth at a much higher speed, 50 - 70 km/s. It seems that most of the lower inclination material from this source has been swept clean, leaving a double radiant of higher inclination particles located some 15° north and south of the ecliptic. The AMS’ Lunsford explains: 

"Unlike the antihelion debris, these particles orbit the Sun in a retrograde motion opposite that of the Earth and are most likely produced by unknown comets. They strike the Earth after their closest approach to the Sun. Since they are moving in opposite directions these particles strike the Earth at tremendous velocities often creating bright meteors with persistent trains. These particles strike the Earth on the morning side of Earth and are best seen just before morning twilight while the sky is still perfectly dark. This is not really a ‘shower’ per se, but an artificial radiant created by the Earth's motion through space… This source should provide at least 2 meteors per hour no matter your location."

Sporadic meteors can be observed any night of the year, and will increase in number throughout the night by a factor of two or three. The serious meteor observer is encouraged to try to identify meteors with the Antihelion and Apex sources. It is also advisable to be aware of minor showers that may be active. While a count identifying, say, ‘Geminids’ and ‘All Others’ is not without value, it does not require much additional effort to identify these others as Monocerids, sigma Hydrids, and Leo Minorids, all of which peak in mid-December around the time of the Geminids. Further identification of Antihelion and Northern and Southern Apex meteors should leave relatively few which are true sporadics, resulting in both more complete data and a more rewarding observing experience.

Most amateur techniques for detecting radio meteors are non-directional, and do not permit positive identification of individual spikes with any particular shower. In general, sporadics provide a fairly consistent level of ‘background’ activity which displays a diurnal rise and fall, peaking around 6 a.m. There also tends to be somewhat more sporadic activity in the autumn months. The rates of sporadic activity can be generalized as a few per hour, depending to a great degree on the sensitivity of a given radio observatory, but can be estimated by analyzing data over a number of nights when no major showers are active. During a major shower rates will rise significantly, even dramatically against this background level, however the minor showers are much more difficult to differentiate.

More on the above topics is available at the following websites:

http://comets.amsmeteors.org/meteors/calendar.html 

http://www.amsmeteors.org/imo-mirror/calendar/cal03.html#Table5

http://csep10.phys.utk.edu/astr161/lect/meteors/shower_list.html 

http://www.spaceweather.com/meteoroutlook/sporadics.html 

In particular, observers are encouraged to visit and bookmark Robert Lunsford’s weekly update on the AMS website which summarizes all current activity: