Note: This article was written in part to help promote Asteroid Day June 30, a global effort to raise awareness of the dangers and scientific importance of asteroids. It is June 30 every year, the anniversary of the Tunguska Great Impact of 1908, and the B612 Foundation mentioned below is one of the founding partners. In fact, I was supposed to be in Luxembourg — Asteroid Day HQ — to moderate some panels and talk about asteroids, but a health issue (now resolved!) prevented me from traveling. Still, I hope you all check out the cool events scheduled, including live streams with scientists, astronauts, and other experts. Learn stuff and have fun!
The discovery of near-Earth asteroids has just taken a very big leap forward thanks to THOR.
Yeah, different THOR. This one stands for Tracket-less Heliocentric Orbit Recovery, and it’s a method to not only dramatically speed up how quickly asteroids can be found, but also allows the search to be performed using old archived images, regardless of when they were taken. It’s faster and can use the extensive database of sightings lying around online. So yeah, that’s a big deal.
Finding asteroids in general is not difficult, it just takes time. Orbiting the Sun, they seem to move slowly across the sky. So you use a telescope to take an image of a single location, wait a bit – usually by going to other places in the sky to observe them – and then observe that same location again. Start over and you now have three images of the same piece of sky.
The stars don’t move, so if you line up the three images, the stars all appear in the same place, but the asteroid will have moved, forming a line of three points. It is the trace of its movement during this period, so this short line is called a tracklet. It may suffice to use the secular equations of motion to create a predicted orbit for the object, and the equation describing that orbit can then be projected into the future or the past to see where it will be or was in the sky; future observations or previously archived ones can be searched to see if they are there, and the orbit can be refined.
In practice, of course, it’s much more complicated, but that’s more or less how it was done. One problem is that this method is computationally intensive and not extremely efficient. Another is that asteroids don’t always seem to move in a straight line; the movement of the Earth around the Sun – or the movement of an observatory orbiting the Earth – can cause these lines to shift, making it more difficult to detect asteroids. Also, as huge surveys come online in the next few years, they will find millions of asteroids (!!), and this method will get bogged down 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 orbits for an asteroid, which is a bit upside down from the usual way of doing things. A test orbit is really just the equation of a made-up, say circular, orbit at a distance of 300 million km from the Sun at a given inclination and orientation. This generates a set of numbers called the orbital parametersand they in turn define an equation that can be solved to find out where an asteroid is at any given time.
This test orbit is then projected forward or backward at the times of other observations, which are then searched for objects near that path. Algorithms for this type of search are common and tend to be quite fast.
There are several advantages to this method – the Asteroid Institute has a good FAQ explaining all of this – but the really striking one is that it doesn’t necessarily need close observations in time and at a given rate to work. The location of a potential asteroid in a test orbit can be calculated for the time of a given observation from any observatory. Since we know when a sighting was made and also where in the sky it was taken, it is possible to see if the potential asteroid was in that sighting at the time, even if it was taken ages ago. weeks or more.
It is extremely powerful. There are many – one plot — of astronomical observations stored in databases, and in fact the team that created the algorithm tested it on real data. They used two weeks of observations from the Zwicky Transient Facility, a huge survey of the sky, to search for potential asteroids, and were able to recover over 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 reviewed a month’s worth of observations. They found 104 new asteroids in the data, which was confirmed by the Minor Planet Center. So it can find known asteroids as well as new ones. This is important because new sightings can trigger thousands of warnings about potential asteroids; if these can be eliminated quickly for known asteroids, that’s a huge time saver.
THOR can hammer asteroids quickly and across disparate observations, and can also use old images to really determine orbits. As these huge new surveys come online, it looks like THOR will be incredibly useful in finding many asteroids that should be discovered – something like 6 million in the next decade.
That’s a lot of rocks. Knowing where they are and, more importantly, where they will be, is obviously quite important, so I totally agree.
Note: If you’re a coding enthusiast, THOR is on GitHub.
It’s a fan thing
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