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The great value of a big eye in the night sky
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Asteroid 2023 TB2 recently provided us with an interesting example of how important it is to have access to optimal telescope resources when presented with the threat of some specific asteroid.

This asteroid was discovered by the Catalina Sky Survey's Mt. Lemmon station on 3 October 2023, as a fairly typical magnitude 21 NEO candidate. It is a moderately large asteroid, estimated to be between 50 and 100 metres in diameter, and its orbit brings it close to the Earth every few years.

Within a few days, an initial orbit determination by ESA's impact monitoring system showed that future collisions with this asteroid were possible, every few years starting in 2083. In total, this asteroid would have about a 1 in 6000 chance of hitting our planet over the subsequent four decades.

The object was then bright and easy to observe, and observations kept coming in, leading to almost daily orbit updates. By 9 October, the overall impact probability had risen to 1 in 1400, and for the first time the asteroid reached level 1 on the so-called "Torino Scale", bringing it to the attention of the community of NEO observers.

With new observations over the next 3 weeks, the estimated impact probability changed a bit, but by the end of October it was still roughly 1 chance in 2100. There was a problem though: the object was now moving away from the Earth, and at the same time it was getting closer to the position of the Sun in the sky. This meant that it was getting much fainter (almost magnitude 24), but also much more challenging to observe from the ground (with an elongation of 60°), because objects close to the Sun are only above the horizon at nighttime for a short time window each night.

In order to get additional data, we needed a telescope capable of detecting a magnitude 24 asteroid in just a few minutes of observing time. In addition, the object's northern declination required a telescope in the Northern hemisphere, thus precluding the use of ESO's VLT as we typically do for these faint targets.

Fortunately, there's a telescope that is just perfect for this observation: Spain's 10.4-m Gran Telescopio Canarias (GTC), the largest single-aperture optical telescope on the planet. Thanks to our collaboration with Javier Licandro and Julia de León, researchers at the Instituto de Astrofísica de Canarias, who have granted time at the GTC, we had access to this great facility.

On the evening of 3 November 2023, GTC observed 2023 TB2 for about half an hour. In the resulting dataset, the asteroid was extremely well detected, allowing us to extract very accurate astrometric measurements of its position. When included in the orbit computation process, these measurements single-handedly improved our knowledge of the object's orbit by almost a factor of four, and allowed us to revise our estimates of the future impact probability, which dropped from 1 in 2100 to just 1 in 40 000.

Furthermore, thanks to the GTC observation, we now know the future trajectory of this asteroid well enough to easily re-observe it the next time it becomes visible, in 2025. We expect that additional observations taken at that time will likely fully clarify whether the small remaining impact threat remains or not, still half a century before a possible impact date and with plenty of time to organise a DART-like mission, in the unlikely chance the threat becomes significant.

Caption: Trajectory of 2023 TB2 projected over the Ecliptic plane and in a rotating reference frame that follows the Earth in its motion around the Sun. The asteroid is located in the position where it was observed by the GTC, which also corresponds to a position in the Northern side of the Ecliptic. N.B.: the Earth, Sun and asteroid sizes are not to scale, while the orbits are. Credits: ESA / PDO.