The time has come again for the annual Perseid meteor shower. At its best, my previous experiences with the Perseids have caused me to exclaim, “Chicken Little was right!”
During the last hour before dawn’s twilight, I have seen as many as a dozen streaks of light within just a few seconds. It did indeed look like the sky was falling, that the stars were dropping from the sky.
This year’s shower looks to be a good one if the weather cooperates, but only if you observe it under precisely the right conditions.
For starters, start observing under a dark, rural sky around 11:30 p.m. on Aug. 11 and continue until morning twilight on Aug. 12. The number of meteors should increase each hour during those five hours, culminating in the best show around 4:30 a.m. as morning twilight begins to spoil the view.
You can try during the couple of nights preceding and after the peak night, but you won’t see nearly as many meteors.
Meteor showers happen because Earth crosses the orbit of a passing comet.
Our planet runs into the trail of rocky debris left by the comet. Because the debris travels at high speed relative to Earth, the rocky fragments burn up and leave a brief, glowing trail, which astronomers call a meteor.
For a meteor shower to occur, a comet and Earth must have intersecting orbits. Our planet intersects the trail of debris of a given comet once a year. Since there are 30 or so showers every year, Earth crosses the orbit of at least 30 comets.
For some, the intersecting orbits are a concern. Fear not. Comets rarely fall to Earth. Some comets make a single pass toward the sun and Earth and never return.
Returning comets mostly have orbits of many thousands of years. They spend most of their time far from Earth in the outer darkness of the solar system.
However, some comets have short orbital periods of 200 years or less. About 100 short-period comets come close enough to Earth to be considered potential hazards. They are called near-Earth comets, or NECs for short.
Unfortunately, NECs are not the only objects that could cataclysmically collide with Earth. Statistically speaking, near-Earth comets are far less of a risk than near-Earth asteroids (NEAs).
Asteroids are large hunks of space debris orbiting the sun. Most of them are corralled between the orbits of Mars and Jupiter.
The largest, called Ceres, is about the size of Texas. The rest vary in size, but most are a mile or two wide. They do not constitute an immediate threat to Earth because their orbits don’t cross Earth’s path around the sun.
However, over 25,000 NEAs cross Earth’s orbit. Their long, stretched-out orbits start near the asteroid belt and plunge close to the sun, passing relatively near Earth.
NEAs probably originated in the asteroid belt. Over millions of years, asteroid-belt objects occasionally come close enough to each other or to Jupiter to alter their orbits.
A close approach sometimes slingshots an asteroid into a new orbit that plunges toward the sun and crosses Earth’s orbit.
Thus, new Earth-crossing asteroids occasionally appear out of the asteroid belt. Even if we discover all the current ones, we can never be sure that a new one won’t appear or that gravitational influences on the old ones won’t change their orbits into collision courses.
About 100 known NEAs are larger than one kilometer (5/8’s of a mile) wide, i.e., large enough to cause severe problems for us if they hit our planet.
Taken together, all the comets, asteroids, and other, smaller Earth-orbit-crossing objects are called near-Earth objects, or NEOs.
At some point, a large NEO will strike Earth and disrupt its ecological system. That is a statistical certainty. However, the odds of a species-destroying impact happening in, say, the next hundred years are astronomically small.
Still, a killer NEO will certainly head our way sometime, and there’s absolutely no way to tell when it will show up.
As a result, The United States and the European Union have been tracking and discovering NEOs since 1998 in a program called Spaceguard. Currently, they are monitoring over 25,000 potentially hazardous objects.
Still, a few are bound to slip through the cracks. A notable example is the Chelyabinsk meteor, which exploded in Earth’s atmosphere on February 15, 2013. Although it was only 66 feet wide, its explosion 18.5 miles above the ground generated a shockwave equivalent to 30 times the explosion of the Hiroshima atomic bomb.
The shockwave damaged 7,200 buildings in six cities. About 1,500 people sought medical attention, mostly from flying glass.
Should you be worried? A little, perhaps.
In 1909, a small asteroid exploded in the atmosphere around 3-6 miles above the remote Tunguska region of Siberia. The Tunguska object was only 160-200 feet wide, yet it exploded with an estimated force of 3-15 megatons
The explosion flattened trees over 830 square miles. If the blast had happened over a city, the city and all its people would have been goners.
We simply don’t have that technology to prevent a NEO impact now. Our bombs aren’t big enough to put much of a dent in a giant asteroid.
To deflect it from its impact path, we would need a large spacecraft capable of carrying a crew of astronauts with a heavy load of bombs and other equipment far from Earth. Our now-defunct space truck, the Space Shuttle, barely made it into low-Earth orbit. We’d need at least a few decades to develop the technology to divert a killer asteroid.
However, all that doesn’t necessarily mean there’s anything to panic about. Most likely, we will discover an asteroid that might hit our planet only after it has orbited the sun three, 10, or 100 more times. That time lag would leave us with decades or even centuries to develop the necessary technology.
One way of determining the odds is to look at the geological record on Earth. Using Earth-orbiting satellites, astro-geologists have discovered the remnants of giant craters hundreds of millions of years old.
During Earth’s relatively recent history (say in the past billion years), our planet has been hit by many hunks of space debris, large and small.
Based on those studies, astronomers can calculate the statistical probabilities of various-sized impacts. For example, a hunk of debris one meter wide strikes Earth’s atmosphere about once a year. (One meter is a bit larger than a yard.) Such a tiny particle does little or no damage.
About every 10,000 years, a 100-meter asteroid hits Earth. Again, minimal damage results.
About every million years, a 1,000-meter asteroid (one kilometer, or 5/8 of a mile wide) strikes Earth. The resulting cloud of dust and debris might wipe out as many as 5% of Earth’s species.
Every 100 million years or so, a 10,000-meter asteroid (10 kilometers or over 6 miles wide) destroys over 60% of the species on Earth. That impact would be the one that does in human beings if it happened any time soon. The resulting 100-million-megaton explosion would send up enough debris to block the sun for years.
Will any of this happen? Some astro-geologists say yes. They point to the Chicxulub event, which 65 million years ago is the classic case of the “dust and debris” scenario mentioned above.
The resulting explosion sent an enormous cloud of dust and debris into our upper atmosphere, and it stayed there for years. Plants died from lack of light, and the animals that ate the plants died. The animals that ate the animals that ate the plants died.
Only those species that ate the festering, decaying remnants of life survived until the dust settled and the sun returned. When it happens again (and it eventually will), cockroaches will rule the Earth for a while.
The gap between killer asteroids has been a long one, thank goodness. We have developed as a species because a giant asteroid has not hit the planet while we’ve been on it.
Indeed, one might argue that we developed as a species partly because the Chicxulub impact wiped out the dominant species of dinosaurs 66 million years ago. The result was an environmental niche, which mammals were happy to fill. One of those mammalian species eventually developed into humans.
Giant impacts can be a GOOD thing if you’re not too high up on the food chain.
Given the statistical probabilities, an asteroid impact is certainly nothing to get panicky about. However, the few million dollars of public money spent on NEO watches may ensure that we discover a killer asteroid early enough to do something about it.
The “big one” may never come, of course. However, given the disastrous consequences if it happens, an occasional sidelong glance upward doesn’t do us any harm.
More on the Perseids next week.
Tom Burns is the former director of the Perkins Observatory in Delaware.