Earlier this week, I started seeing a lot of a buzz about the “Spooky” asteroid, 2015 TB145 set to pass close by to Earth on Halloween day. It was only discovered quite recently as it has an unusual orbit for an asteroid, well outsize of the ecliptic plane.
In a previous post, I discussed the likelihood of a cometary collision. Comets are relatively few and far between, so impact probability with these is very low – though they are large and fast moving when they come in and contact would be catastrophic. There are many, many thousands of known asteroids or minor planets much closer to home and these are somewhat more likely to strike.
Of course, the bulk of the asteroids are in the Main Belt and circling in a safe Indy 500 pattern out between Mars and Jupiter – there are likely millions out there large and small. The Trojan asteroids are in stable orbits of various kinds and are generally OK. Then there are the rogue NEO asteroids. While many of these are safely out of range, there are a number that we do have the potential to cross paths with.
The NEO asteroids are generally smaller in size and don’t have cometary tails and so are quite hard to spot. A number of very powerful telescopes around the world are watching as constantly as they can to find and catalog minor planets of every type. (And they find most of the comets these days as well). But a new one could pop up at any time that could pose a threat. 2015 TB145 was found just a few weeks ago by the Pan-STARRS survey and this is an excellent example of why these surveys are very important.
Still, there is a lot of room out there and luckily this pass of TB145 will be at a safe 1.25 times the lunar distance. The object is estimated to be 600m across. That’s not enough to be very bright normally, but at the closest approach the apparent magnitude was projected to increase to about 10 or so. This is not bright enough to see with the naked eye but pretty easy to spot in a modest telescope.
At least, when the Moon is not out! The asteroid was projected to reach closest approach on Halloween day around 17:00 UT or 2 PM my time in the US Eastern timezone. But estimates of the brightness showed it to be visible on the approach around 4-6 AM local time, with a track along the top of Orion – but passing fairly close to a nearly full moon.
I wasn’t sure if catching the flyby would be doable, but I set up my scope the night before and aligned it as described in the previous post.
There were a number of articles on line with finder charts and tables giving locations for the passage. But with an asteroid passing this close, the apparent position against the star background varies significantly in different places due to parallax. So it looked like using a start charting program that can track and display solar system small bodies would be the best way to go. I recently upgraded my ancient copy of The Sky and used that, but a number of other packages have this capability including Stellarium.
So I started up The Sky and worked out how to lookup and import the orbital elements for TB145 from the database at the venerable Minor Planet Center. Here is what I imported the day before:
This very handily brings in all the values for you as-is without typing which is a huge help! But I wasn’t sure whether the program would give you the position of the body relative to the center of the Earth or calculated for my actual location. So I went to the JPL site to calculate the local ephemeris and had an hourly chart for that as well.
So after taking a few shots of the Moon as described previously, I pointed to the rough location of the TB145 and scanned a bit by eye just to see what would be visible. I could see a number of stars but not very many, so it was unclear whether I’d be able to see the asteroid or not. (At 600 meters across, I’m calling it an asteroid!).
I had also recently bought cables allowing my CGEM mount to be accessed by a computer. Celestron includes a program to do this, but a telescope interface is also supported by my version of The Sky, and this can be done by other packages as well. The hand controller has a telephone-style plug providing an RS-232 serial interface. I haven’t had a computer having one of these in quite some time, so I also had to get an adapter to convert to USB. I set this up the night before, and after installing the driver and rebooting, was able to find the telescope at a virtual COM4 and connected.
With The Sky, when you connect the telescope, it’s position is displayed on the chart as a yellow circle so you can see where the telescope is pointing. Or at least where it THINKS it is pointing!You can also enter an object or coordinate and slew to that position from the program. I had never used a computerized mount before, so this was awesome!
Of course, this is only as good as the alignment of the mount, but this seemed work quite well in the neighborhood where I had aligned to. The CGEM has functions to calibrate further but I thought I would give it a shot with the initial polar alignment and 2 star alignment. The smaller sized scope and DSLR give about a 1 x 1.5 degree viewing area, so getting in the ballpark would be good enough.
Now things were set to point the scope to the expected location of TB145 and take some pictures. To start, I slewed to the expected position and took two 10 second exposures there at 5 minutes apart. Then I had the computer slew again to the current expected position. Since that was actually a fair ways away from the first location, I decided to take a series of three exposures 2 minutes apart to better find a moving object. (Also after learning early on in science experimentation that if you can take 2 measurements of something you may as well take 3!)
So I ended up taking 5 series of shots from about 05:15 local to 6 AM. I decided to use the position provided by The Sky since that was easiest, and that turned out very well! Here’s what it looked like at the time (rolling the clock back):
The display shows RA/Dec in Topocentric and epoch 2000.0 coordinates. I wasnt sure whether this compensates for the parallax or not, but it did seem to. At least, I did find the body in all of the shots given the position it provided!
So I took and saved the shots, naming they by the group and approximate time of capture. I glanced at the images and could see a number of stars. There were a fair number of them and they looked in good focus so I just saved them without looking further. I guess I was expecting the asteroid to have a pinpoint star-like appearance with only a 10 second exposure. At least that was the case when I imaged a few main belt asteroids a while back – but those are much farther away!
So I came in from the cold to have a look and tried looking at some images side-by-side. I could see bright pixel sized spots that appeared to move, but then noticed there were a lot of them and they would move or disappear at random! So these are hot spots and I guess you get more of these in an uncooled stock DSLR. Maxim DL comes with the blinking utility SN Search designed to look for supernova. That looks at pairs of images placed in 2 separate folders. So I took a few files and made copies to try it. After fiddling for a bit I worked out how to do the blink comparison and saw a faint streak that appeared to move in the first pair I took. I was able to find similar streaks in all 5 pairs, so it looked like I had it!
Here are sections of the third group I took around 05:30 local having the streak:
The stars around the streaks can be easily aligned across the 3 images, but the streaks clearly travel in a line across the stellar background, so this appears to be TB145 passing by at a good clip.
So this looks to be it, or at least some kind of fast moving object! I’ve yet to calculate the positions and confirm this is TB145, but I was able to see it in each group around where it was expected. Next, I’ll look into how to calculate the position based on nearby stars and compare against the expected locations.
This was very exciting to see and well worth getting up very early for. I was also watching reports on Twitter and saw a number of other people chasing TB145 early that morning and saw some great results. And I could not resist posting the above picture myself!
While this did turn out well, there were a couple of lessons learned for me. For one, if I had looked more at the info provided by The Sky or other sources, I could have seen that the rate of movement of TB145 at the time was an expected 2.1 arcsec/sec in RA and 2.6 arcsec/sec in DEC. That gives a total movement of over 3 arc-seconds per second! So a 10 second exposure would show a movement of 30 arc seconds or 20 pixels or so at the resolution used. So I should have expected that the body would streak at this exposure time. Taking images 2 min apart would show a travel of 200 pixels which was a pretty good guess, though using a 1 minute interval and taking 4 or more would have given a more interesting collage. And with this rate, it would have been worth taking some time to look over the first set while I was at the telescope, as the streaks would have been pretty noticeable.
But this turned out great all in all, and I’m very happy to be getting a handle on using the CGEM mount – with a small scope at least!