November 17, 2017 4:52 pm
Today’s guest blog post is by StarTalk intern Kirk Long. Kirk is majoring in astrophysics while minoring in applied mathematics and piano at Boise State University. He spends his summer weekends working at the largest public observatory in Idaho, the Bruneau Sand Dunes State Park Observatory, where he gives educational astronomy presentations and operates various large telescopes for the public. Kirk also helps run the StarTalk Snapchat (startalk-radio).
Editor’s note: This is an updated version to our post about the Leonids last year.
The Leonids – named because meteors appear to radiate from the constellation Leo (see EarthSky’s Leonid’s guide) – reached their peak in the early morning of the 17th, but they will still be near peak levels throughout the weekend. The Leonids are the result of Earth plowing through the cosmic debris left behind by comet Tempel-Tuttle, which visits the interior of the solar system every 33 years. Each time the comet approaches our Sun, the intense solar radiation causes fractures and fissures, causing the comet to spew pieces of dust and ice into space. This process has been occurring throughout the comet’s life, and since it has a relatively predictable orbit this debris ends up in what is perhaps best analogous to a cosmic chemtrail. When our orbit collides with the path of this comet, we run through some of that debris. Interestingly, the comet doesn’t lay down the same amount of debris each time—there are a lot of variables involved with how much debris a comet will emit, and this changes over their lifetimes. About every 30 years, Earth’s orbit hits a much more dense patch of debris left behind by Tempel-Tuttle from a period when it was much more active, and these years can produce spectacular meteor storms, historically with more than 1,000 meteors raining down per hour.
The Leonids are predicted this year to fall at only a rate of 10-20 an hour, but the new moon will make for a beautifully dark night and allow you to see even the faintest of meteors. The next outburst isn’t predicted until 2034, and the last Leonid outbursts occurred from 1997-2001, where around 3,000 meteors fell at the peak each year. That may sound impressive, but the earliest and most spectacular Leonid outburst recorded happened at the beginning of the 19th century. In the fall of 1833 there was an extraordinary showing—estimates place the meteor count between 100,000 and 240,000 meteors an hour—and for the first-time researchers began to take seriously the idea that “falling stars” were not mere atmospheric disturbances but the results of Earth plowing through cosmic debris. Studying meteor showers is a relatively imprecise science, but the Leonids were integral to fostering the discoveries that we have made. To this day we still have a lot of questions about meteor showers, and they are one of the most talked about astronomical events, especially to the general populace. During my summers off from school I volunteer at an observatory, and whenever there’s a meteor shower we see a spike in curious patrons who often have a lot of good questions to ask. Below I’ve outlined some of the most common questions I’ve heard over the years, and the answers I normally give to them. StarTalk fans are pretty smart, and I suspect you’ll already know the answers to a few of these, but it’s always good to brush up to impress the in-laws.
What gives meteors their color?
Short Answer: The composition of the meteor itself and its interaction with the atmosphere are what results in the different colors we see from Earth.
Long Answer: The color a meteor produces as it burns up in the atmosphere is mainly determined by two things – its speed and its chemical composition. Speed is important, because in general the faster a meteoroid is traveling the brighter it will be (if mass is taken out of the equation for said hypothetical meteoroid). Most of the light emitted by smaller meteors is a result of the ionization of the atmosphere caused by the meteor’s intense speed. When meteors impact the atmosphere they compress the gas in front of their path drastically from its equilibrium position—the faster the meteor is travelling, the hotter the surrounding air is heated and compressed, and the more pronounced this effect is. The air then glows, emitting radiation depending on its composition – which is dependent on how high up in the atmosphere the interaction takes place. This same effect is what gives the aurora borealis its beautiful colors, and these colors emitted by the atmosphere are mainly the results of nitrogen (which makes up around 78% of the atmosphere) and oxygen (which makes up around 21% of the atmosphere).
Highly ionized oxygen will produce a greenish-yellow color (probably the most familiar color to any aurora or meteor shower watcher) and a lower-energy ionization will produce a reddish color. Ionized nitrogen generally gives off a blue hue, and these emissions can mix and produce a wide variety of colors which change depending on what part of the atmosphere you are interacting with and thus the different percentages of oxygen and nitrogen at that point. A slower moving meteor might thus appear to be more reddish in color, and faster moving meteors can appear to leave greener trails of ionized gas in the atmosphere. The fastest moving meteors (like the Orionids last month) ionize the atmosphere so intensely that they often leave pockets of ionized atmospheric gas behind – persistent meteor trails that can be seen for a few seconds after the meteoroid itself has burned up.
The second important factor in what color a meteor might appear is due to the chemical composition of the meteoroid itself. As the cosmic debris is burned up in the atmosphere, its distinctive elemental components emit light as electrons become excited and start to jump around. Each element has a distinctive spectral signature (this analysis is called spectroscopy, and it is integral to our understanding of everything from the chemical makeup of distant stars to measuring the expansion of the universe) and thus emits a distinct color of light as it burns up. You may remember doing flame tests in high school or college chemistry courses where you were tasked to identify a particular compound simply on the color of flame it produced when burned, and the concept with identifying meteoroid composition is similar (although instead of a Bunsen burner these meteoroids are being burned up by the frictional force of impacting our atmosphere, which is pretty awesome). Below I’ve included a table showing the different colors you might observe a meteor to be, and what elements those colors correlate to.
Why do many meteor showers happen annually? Will they ever stop?
Short Answer: Comets shed layers of gas, dust, and rock when they approach the sun. It turns out they leave a lot of stuff for us to hit!
Long Answer: Comets are icy, cold bodies normally in deep space – some of the leftovers from the early days of our solar system. They follow highly eccentric orbits, which means that although the majority of their lives are spent astronomically far away from the sun, occasionally they swing incredibly close to our star. As they approach the sun, they are rapidly heated and their ice turns to gas. As this water vaporizes and leaves the comet, it takes bits of dust and rock with it, almost like little geysers. Each time a comet swings by the sun, it leaves a trail of ejected particulate, and over time many of these filaments are laid down, waiting for the Earth to plow through them. The other planets (especially Jupiter) influence these filaments and put them into predictable paths, so that each year we plow through the remains of the same comets. Fine adjustments in the filaments positions and other factors such as radiation pressure (discussed more below) ensure that each year there are still particles for us to hit. Eventually, the comets laying down filaments might disintegrate and eventually we might plow through all of the matter left behind by their slow destruction, but these processes operate on a cosmic timescale and thus you probably don’t have to worry too much about your favorite meteor shower disappearing anytime soon. The Leonid meteor shower filaments are the results of material shed by the comet 55P/Tempel-Tuttle, which is a comet with a relatively short orbit of only 33 years.
Why are meteor showers so hard to predict?
Short Answer: Most meteoroids are really small, they can be difficult to observe, and orbital mechanics is really complicated.
Long Answer: One of the most common sentiments I get at the observatory from meteor watchers is disappointment. They travel all the way out to visit us where the skies are dark and the seeing is good, only to find that the meteor “shower” they have come to watch isn’t quite the once in a lifetime show they were sold on by the media. A common question I get asked is “if we can predict the orbit/position of the planets/satellites/spacecraft so accurately, why can’t we do the same for meteor showers?” Calculating the precise orbits of the particulate filaments left behind by comets (discussed above) is trickier than calculating the orbits of the planets for a few reasons. First, they are composed almost entirely of pieces of dust often smaller than a grain of sand, making them difficult to observe. Second, and more importantly, even when they are observed it is much more difficult to precisely predict their orbital motion due to their tiny mass. Planets are huge, and are largely impervious to the effects of radiation or the other planets. Kepler’s equations of orbital motion work very nicely on the planets because there are many fewer variables to worry about. We don’t have to worry a whole lot about the Earth being pushed out of its orbit by sunlight, but with tiny meteors the effect of simple radiation emitted by the sun can have a significant impact on their orbits. Likewise, because of their smaller masses they are more susceptible to being influenced by large gravitational masses, in particular Jupiter, and without incredibly detailed and precise measurements of the filament itself these orbital variations are impossible to predict precisely. We can get a good estimate, but sometimes our models and predictions fail. That’s not always a bad thing though – just as frequently as shows that disappoint are showers that turn out to be spectacular when they were originally predicted to be duds.
Do you have other questions on meteors that weren’t covered here? Let us know in the comments.
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