by Galin Borisov and Apostolos Christou

This September, we had the first opportunity to make astronomical observations while physically present at the telescope since the start of the COVID19 pandemic. Our instrument of choice was the 2-m RCC telescope at Rozhen Observatory on the Rhodope mountains in SW Bulgaria. So here we are at 1700m above sea level with a forecast for good weather and ready for new discoveries!

One of our targets was the asteroid 2017 SL16. It belongs to the class of near-Earth objects, so-called because they approach the Earth and occasionally even hit it. In this case, the asteroid was about 2 million mi away or ten times the Earth-Moon distance. Its size is not precisely known but, based on its brightness is probably very small, less than 50 meters across and perhaps as small as the length of a school bus. What we expect for such an object is that it is either a single solid chunk of rock – we call this a monolith – or a conglomerate of smaller particles or “pebbles”. We want to find out which category SL16 belongs to by measuring the change of its brightness, the so-called light curve, as it rotates about its axis (the asteroid’s equivalent of a day-night cycle).

During the first two nights the air above the observatory turned out to be unsettled and not suitable for observing this faint asteroid, but the night of the 22nd September proved perfect, with a very calm atmosphere and low humidity which led to very sharp stellar images.

As SL16 is a very small and faint object, the only possibility to observe it with “only’’ a 2-meter class telescope is when it passes very close to the Earth. In turn, this means that it has a very high velocity relative to us and we need to tune the telescope to follow it, this is why the background stars appear as trails which you can see on the first figure. Following such a fast-moving object is technically not a problem for a modern telescope, yet it presents a challenge for the analysis of the data that follows the observations.  

An image of asteroid 2017 SL16 (object near center) from the 2-m RCC telescope. Star images were trailed as the telescope followed the fast-moving asteroid across the sky.

The plan was to follow the object for about 4 hours and measure the change of its brightness, the so-called light curve, which is due to its rotation and irregular shape. By the way, once you set up everything and start the sequence of hundreds of camera exposures the observations can be very monotonous. That’s why we decided to reduce some of the data in real-time straight from the telescope and … the result looks very messy, as can be seen from the top panel of the second figure. The asteroid brightness was jumping up and down without following any pattern. 

Obtaining the asteroid’s lightcurve from the observations. The top panel shows the raw brightness measurements as they were obtained at the telescope. The bottom panel shows the result of the Fourier analysis rearranging the measurements to bring out a pattern of brightening and fading as the asteroid rotates about its axis. The red line represents a theoretical model of the lightcurve.

So, in the beginning, we thought that there is some problem with the instrument but then we remembered that small asteroids like SL16 sometimes rotate very rapidly. We then analysed the data, using a “magical” tool from mathematics called Fourier analysis, to search for a pattern. Fourier successfully extracted such a pattern out of the seemingly random data, showing a very fast rotation of approximately 0.3 hours or 19 minutes. The light curve is presented on the bottom panel of the second figure.

Wow, we observed for the first time by ourselves such a fast-rotating asteroid!

The first thing we immediately can say for such a fast-rotating asteroid is that it is monolithic, because an object that is a conglomerate of pebbles cannot keep itself together if it is spinning this fast! We cannot say for certain how it formed but it could be either a piece of a larger conglomerated object or it was a fragment in a collision between larger asteroids.

These observations ultimately tell us about the first generation of solid matter in the solar system. Thousands of millions of years ago, objects just like SL16 and its larger siblings came together to form the Earth and the other rocky planets. These planetary building blocks have now been churned into the cauldron of planetary geologic evolution and are no longer available to study as individual objects. All that remains are the leftover bits, the asteroids. 


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