10-26-2013, 06:45 PM
I believe you're right about Comet Halley although suspect that sadly many of us won't be around for the next appearance in 2061.
So, I know this is certainly getting as far away from Puna as it's almost possible to do, but just wanted to address something Paul mentions and it's a very brief astronomy tutorial. Those who are uninterested can leave now!
There are essentially two types of comets and the classification is based on their orbits; short and long period. Astronomers think there is a region of space about one light year from the sun and earth which contains the left over remnants of the solar system's formation. It's called the Oort cloud. These remnants are small icy bodies that never coalesced to form planets. Everything in our Galaxy is moving and since the time the solar system formed, 4.5 billion years ago, stars have passed quite close to us (i.e., a few light years away). Those encounters can disturb the orbit of objects in the Oort cloud and send them towards the sun. You have to remember we are talking about unimaginable distances so the timescale is enormous. Some believe we may also get comets from a region called the Kuiper Belt which is much closer, around the orbit of Pluto and a little further out. Not going to address that here just so I don't get into a debate about whether Pluto is a planet or not!
Once one of these objects is sent our way it is likely to encounter the gravity of Jupiter and that often determines what it becomes - a short or long period comet (or like Shoemaker-Levy 9 an ex-comet). Short period comets are captured by the sun's gravity and will orbit for potentially millions of years returning every few years (Halley is an example). Long period comets get shifted into an orbit that might mean they only return once every million years or so. Some comets arrive with an orbit that ends up with them being ejected into interstellar space, but that is rare.
Comet ISON is interesting because its orbit shows it is neither of these two, it is newly arrived from the Oort cloud although it took a hell of a long time to get here. This is also partly the reason why it's difficult to predict how bright it will be or how its close encounter with the sun will end up, it hasn't done it before. Periodic comets give you a clue, they've survived coming close to the sun many times and so provide clues as to how they will behave next time. ISON is an unknown. Is it only weakly held together and break up easily, or is it large and dense and survive getting heated to 5000 degrees and the sun's gravity? It'll be interesting to find out, but although it's not well understood, just in case anyone is worrying, there is no threat to our planet no matter what happens to it!
So, Paul, this was prompted by you saying it's amazing we can still be surprised by comets popping up. In a way, yes, you're right, we know when the comets we've seen before will return, but if they haven't been here in the last few hundred years or, like ISON, are new arrivals, they will definitely be a surprise!
So, I know this is certainly getting as far away from Puna as it's almost possible to do, but just wanted to address something Paul mentions and it's a very brief astronomy tutorial. Those who are uninterested can leave now!
There are essentially two types of comets and the classification is based on their orbits; short and long period. Astronomers think there is a region of space about one light year from the sun and earth which contains the left over remnants of the solar system's formation. It's called the Oort cloud. These remnants are small icy bodies that never coalesced to form planets. Everything in our Galaxy is moving and since the time the solar system formed, 4.5 billion years ago, stars have passed quite close to us (i.e., a few light years away). Those encounters can disturb the orbit of objects in the Oort cloud and send them towards the sun. You have to remember we are talking about unimaginable distances so the timescale is enormous. Some believe we may also get comets from a region called the Kuiper Belt which is much closer, around the orbit of Pluto and a little further out. Not going to address that here just so I don't get into a debate about whether Pluto is a planet or not!
Once one of these objects is sent our way it is likely to encounter the gravity of Jupiter and that often determines what it becomes - a short or long period comet (or like Shoemaker-Levy 9 an ex-comet). Short period comets are captured by the sun's gravity and will orbit for potentially millions of years returning every few years (Halley is an example). Long period comets get shifted into an orbit that might mean they only return once every million years or so. Some comets arrive with an orbit that ends up with them being ejected into interstellar space, but that is rare.
Comet ISON is interesting because its orbit shows it is neither of these two, it is newly arrived from the Oort cloud although it took a hell of a long time to get here. This is also partly the reason why it's difficult to predict how bright it will be or how its close encounter with the sun will end up, it hasn't done it before. Periodic comets give you a clue, they've survived coming close to the sun many times and so provide clues as to how they will behave next time. ISON is an unknown. Is it only weakly held together and break up easily, or is it large and dense and survive getting heated to 5000 degrees and the sun's gravity? It'll be interesting to find out, but although it's not well understood, just in case anyone is worrying, there is no threat to our planet no matter what happens to it!
So, Paul, this was prompted by you saying it's amazing we can still be surprised by comets popping up. In a way, yes, you're right, we know when the comets we've seen before will return, but if they haven't been here in the last few hundred years or, like ISON, are new arrivals, they will definitely be a surprise!
