Priest Lake Night Sky by ©Craig Goodwin BUY PHOTO
Comets, Meteors, Meteorites
John D Sahr
I'd like to offer some comments about interesting things in the night sky that you can see at Priest Lake: Comets and Meteors. Although they are superficially different, they are in fact related.
Let's start with comets. Comets are muddy snowballs of rock and ice which orbit the Sun, typically in very elongated orbits. An example would be Halley's Comet, which is visible to the naked eye and appears every 76 years. It has been observed since at least 240 BCE, nearly 2500 years ago. Mark Twain was born during a year in which Halley's Comet passed, and he died 76 years later during its next pass. The next appearance of Halley's Comet will be in 2061.
Comets are not necessarily very big, but they can be bright enough to see when they get near the Sun, because the Sun heats them up and liberates some of the ice, and ionizes some of that water vapor. In fact, comets can have *two* tails which point in different directions --- a neutral gas tail blown by the solar wind, and an ionized plasma tail which is blown by the solar wind *and* the magnetic field in the solar wind.
Over millions of years, the dust and debris expelled from Halley's Comet has become stretched out along its orbit, and results in two meteor showers each year: the Eta Aquariids (May) -- Mother's Day weekend! -- and the Orionids (October; see
Other comets have left similar debris in their wake, and produce other meteor showers. For example, the Leonid Meteor Shower (November) is due to the debris of Comet Temple-Tuttle, and the Perseid Meteor Shower (July-August) is due to the debris in the wake of Comet Swift-Tuttle.
Although comets are interesting, they are less dramatic than meteors. There is some jargon here that is worth mentioning:
meteor: something that hits the Earth's atmosphere at pretty high speeds, sometimes burning brightly enough to be seen by humans.
meteorite: a meteor that that manages to at least partially survive travelling through the atmosphere, and make it to the ground. Most meteorites have high metal content, which makes them strong and heat-resistant. The most recent, largest, most famous meteorite was the one that fell in Chelyabinsk, Russia in 2013 (https://en.wikipedia.org/wiki/Chelyabinsk_meteor)
meteoroid: not particularly well defined, but basically a meteoroid is a bit of rock or ice that hasn't run into anything yet. You could think of a meteoroid as a "small asteroid."
fireball: an *extremely* bright meteor, bright enough that it can be seen during the day. I've seen one, and it was awesome. It is occasionally possible to *hear* meteors if they get down into the the stratosphere; they'll still be supersonic, and you can hear the sonic boom from them; I've also been fortunate to hear this once.
So, let's talk about meteors. Almost all meteors burn up at an altitude of 85 km -- about 55 miles. The reason that they burn up is that they are moving really fast and compress the atmosphere in front of them. The fastest bullet from an ordinary rifle travels at about Mach 1.5 (about 1500 mph). The Space Station is moving at about 5 miles per second (about Mach 25). The *slowest* meteor hits the atmosphere at about 8 miles per second; imagine being able to travel from Seattle to Priest River in under a minute, rather than in six hours.
But that's the slowest meteor speed. Because 8 miles per second is the escape velocity of Earth. Anything that hits the earth at a lower speed is in orbit around the Earth, not around the Sun.
So, there is an upper speed limit, too: about 70 km/s or about 50 miles per second. This is the escape velocity of the Sun --- if you travel this fast, you're going to leave the solar system no matter which direction you're headed.
But interestingly, there are a few meteors that travel faster than this -- and these meteors come from outside the Solar System. You might wonder, "how do we measure the speed of meteors?" There are a few radars on the planet that are capable of measuring the speed of meteors when they hit the atmosphere: Jicamarca in Peru, Millstone Hill near Boston, ALTAIR in the Kwagelein Atoll, the Sondre Stromfjord radar in Greenland, and several European instruments in Sweden, Norway, Finland, and Svalbard. 50 miles per second gets you from Seattle to Priest River in about 6 seconds, the time of one breath. Occasionally something big comes from far away, such as asteroid Oumuamua, which passed through our solar system in 2017.
Anyway, there are some interesting questions you can ask about meteors, such as, "how much meteor mass does the Earth collect?" Although the answer is bit fuzzy, it is somewhere between about 10 tons and 100 tons every single day. Keep in mind that the vast majority of meteors are the size of grains of sand --- but there are a lot of them. This is about 10 large dump truck loads of meteor debris, every single day.
As Priest Lake's glorious summer approaches, do keep in mind the Perseid Meteor Shower which lasts from 17 July -- 24 August, but the best viewing will be 12-13 August.
This is probably the best meteor shower of the year, so keep your eyes peeled.