Sporadics

Last week I witnessed one of the brightest and most colorful meteors (aka "shooting stars") I’ve ever seen. It was the sort that makes you–and everyone around you–whoop like a rowdy fan at a sporting event.

A meteor or shooting star is the streak of light caused by a bit of space dust or debris (meteoroid) burning up as it encounters Earth’s atmosphere. The pyrotechnical display owes less to the size of the meteor than it does to the velocity at which it slams into our atmosphere. In fact, most meteors are merely the size of grains of sand or rice. The real whoppers–known as bolides–that explode and light up the night environment like flashbulbs, might be only the size of peas or pebbles. The occasional big chunk of rock that survives its screaming descent through our atmosphere to impact the ground (or an unfortunate vehicle) is called a meteorite.

I’m accustomed to seeing color in meteors. It’s not untypical to see yellow-colored or green-hued shooting stars, along with run-of-the-mill white ones. Some meteor showers are even known for reliably exhibiting a particular color. The Perseid meteor shower in August, for example, tends to produce meteors with a gold-colored trail.

My recent meteor of note, however, was big, bright, and unexpectedly blue–neon blue. It took my breath away. Color in meteors is caused by both the composition of the meteoroid and the composition of the air it encounters. The blue may have indicated a high magnesium content in the meteoroid.

The blue meteor was most likely a sporadic, a meteor that isn’t associated with a particular meteor shower. A meteor shower is a display that occurs annually around the same time, when the Earth encounters a specific cloud of debris left behind by a comet whose orbit brought it close to the Sun.

To determine if a meteor you spot is a sporadic, first check to see if a meteor shower is occurring. I use the calendar on the American Meteor Society’s website and check the column marked "Activity Period." If there’s no current meteor shower, then chances are any meteors sighted are sporadics.

If, however, you are within a meteor shower’s period of activity, observed meteors may be either part of the shower or sporadics. To determine which, ask yourself two questions:

1) Is the radiant–the spot in the sky from which the meteors of that shower appear to emanate–visible?
The radiant is typically located within the constellation for which the meteor shower is named (e.g., the Lyrids for Lyra and the Perseids for Perseus), and it moves slightly over the course of the shower. A number of websites, such as this one, offer star maps with the radiant location plotted for the duration of the shower.

The radiant may not, in fact, be visible at the time you spot the sporadic because the constellation where the radiant’s located may not yet have risen over the eastern horizon. It’s typical for a meteor shower radiant to rise after midnight, making midnight to morning twilight the best time to view showers. If you see a shooting star before midnight, there’s a good chance it’s a sporadic.

2) If the radiant is above the horizon, can you trace the trajectory of the meteor back to the radiant?
Observe the starting point, ending point, and direction of the meteor observed. Trace the streak of light from the ending point back through the starting point and beyond to see if that path crosses the meteor shower’s constellation of origin. If so, it’s probably associated with the shower. If not, it’s most likely a sporadic.

There are anywhere from two to sixteen sporadic meteors per hour that strike our atmosphere nightly. The farther away from city lights you get, the more shooting stars you’ll see.

As my memorable blue meteor proved, sometimes one random bit of wayward space dust is just as visually exciting as the peak of a major meteor shower.






Astronomy Essential: A comet is a dirty snowball that orbits the Sun.

A comet is a solar system body composed primarily of ice, gas, and dust trapped in the ice, in other words, a dirty snowball. The snowball begins to melt a bit if the comet’s orbit takes it close enough to the Sun. This melting of the ice releases some of the dust, which creates a "tail" of dust streaming out in the comet’s wake. Looking at the dust tail shows us the comet’s path.

A comet has a second tail, which is usually not as easily visible as the dust tail. Electrically charged gas particles streaming from the comet are blown back by the solar wind, a continuous stream of particles given off by the Sun. Therefore, this ion tail, as it is called, always points away from the Sun.

The Pursuit of Perseids

The Perseids (pronounced PURR-see-yidds) are a major, reliable meteor shower and one of my personal favorites. It simply wouldn’t be August if I didn’t try to spot some of those long, yellow-tailed beauties streaking across the sky.

Sometimes a big Moon will thwart or at least hamper my attempt. That’s the case this year. The peak of the meteor shower--when the number of meteors per hour is greatest--is predicted to occur in the early morning hours of Tuesday, August 12. As this is less than one week before Full Moon, the waxing (growing) Moon will be in a gibbous phase. Gibbous means the Moon is greater than 50% illuminated without being fully illuminated. In other words, a big Moon.

A big Moon tends to wash out the sky, reducing the contrast needed to see faint streaks of light. No Moon is optimum. Since the Moon doesn’t set until two to three hours after midnight (depending on location) on the morning of the 12th, you’ll have a narrow window of opportunity to spot peaking Perseids before morning twilight lightens the sky. But the reward will be great: the predicted peak rate is 100 meteors per hour!

By all means, don’t get too hung up on peak predictions. I’ve seen some awesome meteor shower displays on nights prior to and after the so-called peak. In some cases, I was pleasantly surprised to discover they bested the peak display. The Perseids are active this year through August 24. Pick any night from now until then, go out after midnight when the Moon’s not around, and you’re bound to see some dreamy Perseids.

There’s a reason why I specified “go out after midnight” above. Perseids emanate from, and are named for, the constellation Perseus (pronounced PURR-see-yuss). The show can’t begin until the radiant in Perseus rises above the northern horizon. The radiant is the point in the constellation where the Perseids appear to originate. The higher the radiant rises in the sky, the more likely you'll see meteors. The radiant is well above the horizon--and rising--from midnight through dawn.

The radiant is marked on the map below. Despite their origination point, you can see Perseids in any part of the sky, in any direction you happen to be looking. However, knowing the radiant will help you identify which “shooting stars” are Perseids and which are sporadics, meteors not associated with a particular meteor shower. If you can trace a meteor’s trajectory back to Perseus, it’s most likely a Perseid.

Chart created with Your Sky

1) Go to as dark of a site as you can, with no line-of-sight outdoor lights. Take extra layers of clothing or blankets with you. Even though it’s summer, you won’t be moving around much and could get chilled.

2) Take a lounge chair or a blanket for the ground. Reclining is really the best way to watch a meteor shower, because it allows you to see the most sky at one time. Although the meteors originate from the radiant, you can see them fizzling anywhere in the sky. I normally lie with the top of my head pointed toward the rising radiant (which will be in the northeast) so that I can be scanning the rest of the sky.

3) A meteor is the streak of light caused when a bit of interplanetary dust, comet debris, or manmade space junk slams into Earth’s atmosphere at high velocity. Typically, the meteoroid--the bit of debris--vaporizes upon impact with the atmosphere. But occasionally, a large chunk may survive its screaming 100,000-plus mph entry and hit the ground, at which point we refer to it as a meteorite.

As you watch the fireworks, keep in mind that meteors are typically no bigger than a grain of sand. The really bright ones that go off like Roman candles may be no bigger than a grain of rice. It is their high entry velocity that causes the wonderful light show.

4) A meteor shower occurs when Earth encounters a stream of debris left behind by a comet’s close approach to the Sun. Comet Swift-Tuttle takes 135 years to complete its orbit of the Sun. Each time it approaches, the Sun's heat releases dust trapped in the comet's icy nucleus or frozen center. This periodic renewal of the discarded dust stream ensures us cosmic fireworks every August when we move through it.

Meteor showers are one of the supreme naked-eye treats for lounge-chair astronomers everywhere, beginners to veterans. Do make the pursuit of Perseids part of your summer recreation. Do make a memorable evening with a friend or family member. Don’t forget to make a wish.