Science

Bad astronomy | The THOR program speeds up the search for near-Earth asteroids

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Note. This article is partly written to promote Asteroid Day June 30th., a global effort to raise awareness of the hazards and scientific significance of asteroids. It is June 30 of each year, the anniversary of the great Tunguska strike of 1908, and the B612 Foundation mentioned below is one of the founding partners. I was actually supposed to be in Luxembourg – the headquarters of Asteroid Day – to moderate a few panels and talk about asteroids, but a health problem (now resolved!) prevented me from going. However, I hope you all take a look at the cool events planned, including live broadcasts from scientists, astronauts and other experts. Learn things and have fun!


Near-Earth asteroid detection has just taken a big leap thanks to THOR.

Yes, another TOR. This means restoring a heliocentric orbit without a trace, and this method not only greatly speeds up the search for asteroids, but also allows you to search using old archival images, regardless of when they were taken. It’s faster as well as you can use the vast database of observations just lying on the Internet. So yes, it’s a big deal.

Finding asteroids is generally not difficult, it just takes a lot of time. As they revolve around the Sun, they appear to be slowly moving across the sky. So you use a telescope to take a picture of one point, wait for a while – usually head to other points in the sky to observe them – and then re-observe the same point. Do this again and you now have three images of the same area of ​​the sky.

The stars don’t move, so if you align the three images, all the stars will appear in the same place, but the asteroid will move, forming a line of three dots. This is a track of his movement during this time, so this short line is called tracklet. It may be sufficient to use centuries of equations of motion to create a predicted orbit for an object, and the equation describing that orbit can then be projected into the future or past to see where it will or has been in the sky; one can search for future observations or previously archived ones to see if there are any and refine the orbit.

In practice, of course, this is much more difficult, but this is how it is done. One problem is that this method requires a lot of computer time and is not very efficient. Another thing is that asteroids do not always appear to move in a straight line; the movement of the Earth around the Sun – or the movement of an orbiting observatory around the Earth – can cause these lines to wriggle, making asteroids harder to spot. Also, as online surveys come out in the next few years, they will find millions of asteroids (!!) and this method will bog down trying to track them all.

Enter TOR [link to paper], a project developed by the Asteroid Institute, Project B612 Foundation. The idea here is not to track the asteroids themselves, but to create theoretical test orbits for the asteroid, which is a little different from the usual way of doing things. The test orbit is just an equation for a fictitious orbit, say, circular at a distance of 300 million kilometers from the Sun at a given inclination and orientation. This generates a set of numbers called orbit parametersand these in turn define an equation that can be solved for where the asteroid is at a given time.

This test orbit is then projected forward or backward on the time of other observations, which then look for objects close to that path. Algorithms for this kind of search are common and usually quite fast.

This method has several advantages – the Asteroid Institute has a good FAQ that explains it all – but one of them is really amazing – it does not necessarily require observations made close together in time and at a given frequency to work. The location of a potential asteroid in a test orbit can be calculated during any given observation from any observatory. Since we know when the observation was made, as well as where it was made, it is possible to see if a potential asteroid was in that observation at the time, even if it was made a few weeks ago or more.

It’s 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 used two weeks of observations from the Zwicky Transient Facility, a huge sky survey, to search for potential asteroids and were able to detect over 97% of the previously known asteroids that were in the data! Impressive.

They also used data from the NOIRLab Source catalog, a huge database of astronomical observations, and studied monthly observations. They found 104 new asteroids in data that was confirmed by the Minor Planet Center. Thus, it can find both known asteroids and new ones. This is important because new sightings could trigger thousands of warnings about potential asteroids; if they can be quickly selected for known asteroids, it will save a lot of time.

THOR can quickly pick out asteroids from scattered observations, and can also use old images to pinpoint orbits. As these huge new studies go online, it looks like THOR will be incredibly useful in discovering the many asteroids that are expected to be discovered – something like 6 million in the next decade.

That’s a lot of stones. Knowing where they are and more importantly where they will be is obviously very important, so I’m all for it.


Note. If you’re a code connoisseur, THOR is on GitHub.

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