Monday, July 07, 2003

Minor planet mad science:

Asked by Sam, a student in grades 10-12:

In which constellation could you find the minor planet 719 Albert?

Well, since Albert is really zipping along, it depends when you look. This summer it will actually be in a pretty good position for observers in the Northern hemisphere ? but even though it will pass pretty close to Earth, it?s pretty small (between 3 and 6 km radius) so it will be very dim and only observable through a large telescope.

Today, June 13 2001, Albert is in Delphinus, but by tomorrow it will have moved into Equuleus. Albert moves on into Pegasus by June 18, where on July 31 it will reach its highest point in the sky. Albert will move into Andromeda by August 8, where it then starts to go into retrograde motion as the Earth overtakes it (where it looks like it?s moving backwards, like a car you are overtaking, even though both of you are still moving forwards). By August 18 it will cross into Pisces, and by September 1 it is in Aries, where on the 5th it will reach its closest approach to Earth, about 43 million kilometers.

OK. Tell me more. Albert has an interesting history. The number (719) refers to the fact that it was number 719 in a list of asteroids that astronomers started making over two hundred years ago. It was discovered in 1911 by an Austrian, Johann Palisa, who was working in Vienna, and it was also seen by a Danish observer within a few days after. But then they lost it!

How can you lose an asteroid? Aren?t these things big? Umm, well, yes, but space is even bigger, and there?s a lot of it out there. Even something as big as 6 km across looks pretty small when it is several tens of million of kilometers away! Also, we need to observe objects several times (and be sure we are actually seeing the same one) to be able to figure out their orbit ? and the two times Albert was seen in 1911 were not enough to give us a good estimate of the real orbit. In fact, the next time we looked for it, we were off by several degrees, which, when you look though a big telescope, is a HUGE error.

So, how did we find it again? An astronomer in Arizona called Jeffrey Larsen observed an object on May 1, 2000 that he called 2000 JW8, which was tracked by Larsen and several others for several days to better determine its orbit. By May 9, Gareth Williams of the Minor Planet Center was able to determine that 2000 JW8 was in fact the long-lost Albert. (You can run the 2000 JW8 orbit ?backwards? and tell that it was in the right place in 1911).

So how do you know where it is now? To tell where any orbiting object is, you need seven pieces of information. The best way to think of it is as follows: you need three to tell you where it is (x, y, z) and three more to tell you how fast it is moving (Vx, Vy, Vz), and one more to tell you what time it was when you took these measurements (t). In real life, measuring orbits using x, y and z is REALLY HARD, so astronomers use another set of seven measurements that sound really strange (see below), but they are essentially giving us the same information. Then you use the laws of physics to tell you how the object will move either in the future or the past. It turns out that Albert orbits between the Earth and Jupiter, and will come ?close? to both planets on this time round the Sun. Albert takes about 4.28 years to orbit once, and comes close to Earth almost exactly every 30 years (...1911, 1941, 1971, 2001...).

I?m game ? gimme the full set of numbers in all their gory detail OK, here goes. These are also called the ?Orbital Elements.? You can use these in a lot of astronomy software for computers to see pretty much where Albert will be anytime. That?s how I figured out where it was over this summer ? I just plugged these elements into a pretty simple program on my Mac and presto!

Semi-major axis (a): 2.636667 a.u.
Eccentricity (e): 0.548273
Inclination (i):11.3095
Longitude of Ascending Node: 184.9305 (Epoch 2000.0)
Longitude of Perihelion:154.2885
Epoch (T): 2001 August 21.2582
Mean Motion (n): 0.2302084

Here are two good articles on 719 Albert that cover its history and rediscovery:

And here is the technical bulletin that gives the gory details:

An interesting website is the Spacewatch project of the Lunar and Planetary Observatory of the University of Arizona, who watch for asteroids all the way out to the orbit of Neptune: