Note: This article was written in part to help promote marketing Asteroid Day June 30th, a global effort to raise awareness of the dangers and scientific significance of asteroids. It is June 30 every year, the anniversary of the great Tunguska effect in 1908, and the B612 Foundation mentioned below is one of the founders. I was supposed to be in Luxembourg – Asteroid Day HQ – to moderate some panels and talk about asteroids, but a health problem (now solved!) Kept me from traveling. Still, I hope you all take a look at the cool events that are planned, including live broadcasts with scientists, astronauts and other experts. Learn things and have fun!
Finding terrestrial asteroids just took a huge leap thanks to THOR.
Yes, different THOR. This stands for Tracket-less Heliocentric Orbit Recovery, and it is a method to not only increase the speed of how quickly asteroids can be found, but also allows the search to be done using old, archived images regardless of when they were taken. It’s faster and can use the huge database of observations just lying around the web. So yes, this is a big deal.
Finding asteroids in general is not difficult, it is just time consuming. As they orbit the sun, they appear to move slowly across the sky. So you use a telescope to take a picture of a single place, wait a while – usually go to other places in the sky to observe them – and then observe the same place again. Do it again, and now you have three pictures of the same sky spot.
Stars do not move, so if you align the three images, all the stars will appear in the same place, but the asteroid will have moved, forming a line with three dots. This is the trace of its motion over that time, so this short line is called a tracklet. It may be enough to use the hundreds of years old equations of motion to make a predicted orbit for the object, and the equation describing that orbit can then be projected into the future or past to see where it will be or was in the sky; future observations or previously archived can be searched to see if it is there, and the path can be refined.
In practice, of course, it is much more complicated, but it is more or less the way it has been done. One problem is that this method is very intensive with data time and not very effective. Another is that asteroids do not always appear to move in straight lines; Earth’s motion around the sun – or the motion of an orbiting observatory around the earth – can cause these lines to wiggle, making it harder to detect asteroids. Also, as huge surveys come online over the next few years, they will find millions of asteroids (!!), and this method will get stuck by trying to track them all.
Enter THOR [link to paper], a project developed by the Asteroid Institute, a project of the B612 Foundation. The idea here is not to track the asteroids themselves, but to create theoretical test tracks for an asteroid, which is a bit backwards from the usual way of doing things. A test track is really just the equation for a composite track, say one that is circular with a distance of 300 million km from the Sun at a given slope and orientation. This generates a set of numbers called orbital parametersand they again define an equation that can be solved for where an asteroid is at a given time.
That test path is then projected forward or backward to the times of the other observations, which are then searched for objects close to that path. Algorithms for this type of search are common and tend to be quite fast.
There are several benefits to this method – the Asteroid Institute has a good FAQ to explain all of this – but one that is really striking is that it does not necessarily need observations taken close together in time and with a given cadence to work. The location of a potential asteroid on a test track can be calculated for the time of a given observation from any observatory. Since we know when an observation was made and also where in the sky it was taken, it is possible to see if the potential asteroid was in that observation at that time, even if it was taken weeks ago or more.
It is extremely powerful. There are many – a a lot – of astronomical observations stored in databases, and in fact the team that created the algorithm tested it on real data. They spent two weeks observing from the Zwicky Transient Facility, a huge sky survey, to search for potential asteroids, and were able to recover more than 97% of previously known asteroids that appeared in the data! Impressive.
They also used data from the NOIRLab Source catalog, a huge database of astronomical observations, and examined a month’s observations. They found 104 new asteroids in the data, which is confirmed by the Minor Planet Center. So it can find known asteroids as well as new ones. This is important because new observations can trigger thousands of warnings about potential asteroids; if these can be scrapped quickly for known asteroids, there is a huge time saving there.
THOR can hammer out asteroids quickly and across different observations, and can use old images to really nail down the orbits as well. When the new huge surveys come online, it seems that THOR will be incredibly useful in finding many of the asteroids that are expected to be discovered – something like 6 million in the next decade.
There are a lot of rocks. Knowing where they are and, more importantly, where they want to be, is obviously quite important, so I’m all for this.
Note: If you are a code nerd, THOR is on GitHub.
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