Meteor showers like the Perseids get all the press. But have you ever wondered where all the random meteors come from? We explore their origins.
We've all seen them. The sporadics. Those random meteors that flash across the sky on any old clear night. If you were to make a lifelong tally of meteors, the sporadics would easily outnumber those from meteor showers. But where do they come from? Are they truly random, or were they once members of long-lost meteor showers witnessed by our distant ancestors?
No question about it, some sporadic meteors originate from ancient meteor showers that have long since dispersed. Over time, meteoroids that are dribbled out by vaporizing comets and colliding asteroids can spread into streams so broad we no longer know from whence they came.
Jupiter can bounce meteoroids right out of the Solar System; others get pulled in by the Sun's gravity or blown out by its wind. A couple thousand years from now, an amateur astronomer, looking up to see a former Perseid meteoroid ionize its signature across the sky, will dismiss it as an unknown.
The number of sporadic meteors visible varies over the course of the night, rising from a minimum of 4–6 per hour around 6 p.m. to twice that at the start of dawn. At dusk, the direction of our orbital motion is opposite that at sunset, forcing meteoroids to catch up to Earth from behind. Around dawn, the planet plows straight into whatever dribs and drabs of comet lie ahead of us, so meteor rates increase.
Superimposed on the daily rise and fall in the number of random meteors are additional dips and enhancements. If you live in the northern hemisphere, sporadic rates drop to their lowest in spring and peak in November. In the southern hemisphere, rates peak in late spring and bottom out in September and October. It used to be thought that the angle of the ecliptic to the horizon played a role in this cyclic variation. Now it's believed that there's a true lack of material north and south of the ecliptic plane (Earth's orbital plane) headed toward our planet at those times of year.
For something as seemingly random as sporadic meteors, you might be surprised to learn that they arise from a variety of radiants, many based upon Earth's motion around the Sun. There are six regions or portholes in the sky serving as key entry points for these orphaned crumbles of interplanetary debris.
Antihelion / Helion Meteors
Antihelion meteoroids orbit the Sun in the same direction as the planets in low-orbital inclinations centered on the ecliptic. They're located directly opposite the Sun in the sky. Like planets at opposition, the Antihelion radiant rises in the east when the Sun sets and reaches peak elevation around 1 a.m. local standard time (2 a.m. DST). Why not midnight? Earth's motion through space causes the radiant to appear 15° east of its true position, placing it highest at 1 a.m. instead. Patience is required to see this strain of space dust as the Antihelion rate is only around 3 per hour.
Because the Antihelions are centered on the ecliptic, which bisects the zodiac, the tiny trickle of meteors produced by used to have individual shower names like the Chi Orionids of December or the Virginids of February and early spring. Now they're grouped under one heading and represent meteoroid material on its way inbound to the Sun. Antihelions are best seen in November from the northern hemisphere when the radiant (at +23° declination) lies high in the southern sky around midnight to 1 a.m. They provide an extra boost to the Taurid meteor showers, which are also active at this time.
Helion meteors are similar to the Anthelions, but their radiant lies in the same direction as the Sun, making them nearly impossible to see. They strike the sunlit portion of Earth and are comprised of meteoroids outbound from the Sun. Both types are thought to originate from comets and asteroids under Jupiter's influence.
Apex / Antiapex Meteors
Material left by long-period comets moving opposite (or retrograde) to the planets and steeply tilted to the ecliptic plane make up this next group. Since they're moving opposite Earth's motion, we smack them head-on at a tremendous speed. Bright fireballs with long-lasting trains result. The Apex radiants form two diffuse regions: 15° north and 15° south of the ecliptic, 90° west of the Sun, placing them highest in the sky around 6 a.m. local time. It's thought the gap between the two regions results from the Earth clearing out the plane of its orbit or ecliptic.
If you thought finding Antihelion meteors would prove a challenge, the Apex variety are sparser yet. Both professional and amateur astronomers track these and other sporadic sources using low-light video cameras, radar, and visual observation.
The sisters to the Apex radiants are the Antiapex (Antapex) radiants. Like the former, they're a pair diffuse radiants located 15° north and 15° south of the ecliptic, 90° east of the Sun. That places them highest at the end of evening twilight. Because they play catch-up with Earth like other evening meteors, the Antapex clan appear to move slowly across the sky. Studies have shown that when the Antapex radiants lie highest in the sky — from mid-February to mid-April for the northern hemisphere — fireballs are more frequent. Meteors from the Antapex regions come from material shed by long-period comets streaming toward the Sun on their way to perihelion.
Toroidals represent a scanty group of meteoroids that approach Earth from steeply inclined angles and probably originate with the short-period Jupiter family of comets. The radiants are centered about 90° west of the Sun and 50° north and south of the ecliptic. Since they strike the atmosphere at almost a perpendicular angle, they produce medium-speed meteors. Based on the International Meteor Organization's video surveillance of the radiant, Toroidals are most common in late December around the 20th.
If it all sounds like a busy street intersection with no one directing traffic, I couldn't agree more. But the comings and goings of diffuse meteor streams, coupled with Earth's motion through space, guarantee we'll see a few meteors every hour of every night of the year.