Explaining Meteor Shower Radiants [VIDEO UPDATE]

I made a video today to use in my Astro 3105 course, "Physics, Chemistry, and Geology of the Solar System."  There's no narration because I plan to explain it myself in-class, but I'll give a quick explanation in the text below.  The goal is to explain the dates and radiants of a few of the big meteor showers.

We begin by looking at the orbits of Halley's Comet and the Earth.  Although the orbits don't intersect directly due to the comet's orbital inclination, the closest approach between the two paths occurs in early May every year.  Now picture the Earth moving along the green orbit circle counterclockwise, the direction indicated by the arrows.  And visualize a long elliptical ribbon of comet-chunks left behind like breadcrumbs all along the comet's orbit, each chunk moving along the orbit clockwise (again according to the arrows).  This is a nearly head-on collision, so the chunks would appear to be coming at us from the bottom of the image...  And as we change perspective we see that they will appear to come from the constellation Aquarius.  We've just discovered the Eta Aquariids meteor shower, which peaks in early May.

Now we move to the other close-approach point between the two orbits, in late October.  This is again a near-head-on collision, and the meteors would appear to come at us from the upper-right.  Changing perspective, we see that this points to the Orionids shower in late October.

Now we switch to a different comet, 55P/Tempel-Tuttle, whose orbit appears nearly tangent to but slightly inclined to Earth's orbit.  Actually, the inclination is nearly 180 degrees; in other words, the comet and its debris-ribbon again move clockwise around the orbit, whereas the Earth moves counterclockwise.  The meteors should appear to come from directly in front of the Earth - which in mid-November points to radiant of the Leonids shower.

And finally, we switch to the numbered asteroid (3200) Phaethon, which orbits counterclockwise like any respectable asteroid should.  But we are discussing Phaethon because it's not a respectable asteroid - it must be some sort of dead comet or "rock comet" that has spread debris all along its orbit like a comet would.  Now since the Earth is moving mostly "upward" and the debris is moving mostly "leftward," we would expect the meteors to seem to come from the upper-right. And when we change perspective, we see the radiant of the Geminid meteor shower of mid-December.

I've always had a hard time visualizing how debris left behind by a comet could appear to hit the Earth's atmosphere from the same direction at the same time of the year, year after year.  Hopefully this animation has helped you as much as it's helped me.

Map of All Asteroid Observatories (MPC Obs Codes)

In my previous blog post, I made a very rough map of all MPC Observatory Codes (sites who are approved to submit asteroid measurements to the Minor Planet Center).  Not being one to waste effort figuring something out once and forgetting about it, I made a little extra effort.  What I ended up with was this:

Click here to view in Google Maps.

In the process, I remembered a little bit of trigonometry, learned a little bit about geocentric vs. geodetic latitudes, and discovered some cool new capabilities in Google Maps.  Speaking of which, I had to divide the world into 4 regions to upload it into Google without paying a monthly premium:  North America, Western Europe, Eastern Europe, and Everyone Else.  I made very basic latitude/longitude cuts to accomplish this, with only slight attention to common geographic divisions. In particular, I doubt most people would put Italy, Germany, and Turkey in "Eastern Europe," but it was quickest and easiest to do so.  Apparently, the eastern French border is roughly the center of mass of asteroid-observing sites in Europe.

Mystery Maps

Would anyone like to guess what data set I'm mapping in the images below?  Answer below the fold.......

Vizualizing Alternate Orbits for 2014 NZ64

Last month, this article was published on the website of the Express, a tabloid paper in the U.K.  A few days later, Phil Plait caught wind of it and debunked the article on his Bad Astronomy blog at Slate.com.  Essentially, the author of the Express article completely misinterpreted the real data on a newly discovered asteroid, turning this one object into an entire unknown asteroid belt that would wreak repeated havoc on the Earth over the next century. I don't want to repeat Phil's excellent analysis of the situation and how the original author got it so wrong. I just thought I'd bring some simple visualization to bear on the problem, and maybe do a little educating.

The asteroid is 2014 NZ64, and it was discovered on July 3, 2014. It was observed 4 times that night by the Pan-STARRS survey in Hawaii, and then 4 more times on July 5th by my friends at the Astronomical Research Observatory in Illinois. Eight observations over two days, that's it!  That's not very much to go on. In fact, the Minor Planet Center says that by July 9th, its predictions were so uncertain that you wouldn't be guaranteed to see the asteroid within a typical telescope's 10-arcminute field of view.  And by the article's publication on Sept. 5th, the uncertainty radius had increased to 75 degrees. That's half the sky!!  So it's pretty crazy to say we know much about this asteroid at all. 

On the other hand, we do know it was seen on those two nights, and we can do our best to extrapolate its motion from there. But we need to be honest about how good those extrapolations are!  So we won't just fit one orbit to the observations, but many orbits, and we'll use them to map out our uncertainties.  See my previous posts for more discussion about my methods. Let's jump straight to the results!  In the animation and simulation below, the best-fit orbit of the asteroid is shown in bright blues, with the alternate possible orbits shown in faded colors.  The portion of each orbit that is "above" the plane of the solar system is colored cyan, while the part "below" the plane is darker blue.  

Click here to control the simulation yourself!

The animation starts on July 5th, during the very short time period when the asteroid was observed.  See how the possible positions of the asteroid on this date all line up to point directly away from the Earth?  As I love to say, "Astronomers have horrible depth-perception!"  At that point, we knew how to point in the direction of the asteroid in the sky, but we had very little idea of how far away it was.  We had the best-fit position of the asteroid, but as you can see, the distance was quite uncertain.  And as time goes on, Kepler's 3rd Law causes the potential orbits that are on-average closer to the Sun to get ahead, and those on-average farther from the Sun to move more slowly.  So, rather quickly, that initially horrible depth perception turns into a huge uncertainty in the predicted position in the sky.

No further observations of this asteroid have been made after July 5th.  Today it is simply too faint to register on any but the largest and most powerful of telescopes on (or above) the Earth.  If (or when) another successful observation is made, all potential orbits that don't agree with the new observation will be rejected, and the best-fit orbit will be refined.  But at this point, we won't even be able to predict its next close approach of the Earth!  When we see it again, it will be because we got lucky.  We basically have to discover this space rock all over again.

So no, 2014 NZ64 is NOT a swarm of asteroids that will pummel us time and again over the next century - although this simulation may appear that way to the uneducated eye.  These are just the possible predicted positions of a single asteroid as it moves along its orbit into the future.  No wonder the JPL Impact Risk Table shows almost 400 possible collisions between the years 2017-2113!  We really just have no idea where this asteroid is and where it's headed at this point.