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Astronomers have discovered a very rare asteroid from data from the Zwicky Transient Facility (ZTF) sky survey: an asteroid whose orbit is entirely in the interior Earth's orbit around the Sun, and therefore the asteroid with the shortest "year" found so far. It only takes 151 days to orbit the sun!
Rocks like this are called Atira asteroids and, as a class, they are very difficult to spot. As they are always inside our orbit, they tend to be near the sun in the sky. This means that the only time we can observe them from Earth is very soon after sunset or just before sunrise.
The newly discovered asteroid, called 2019 LF6, is even more difficult. Its orbit is very elliptical and extends from within the orbit of Mercury to the outside of that of Venus. This means that it never gets away from the sun and can only be seen in about half an hour after sunset or sunrise. Worse still, its orbit is inclined at about 30 ° to that of the Earth; it is therefore generally not due to the east or west of the Sun, where most asteroids hide.
This is certainly the reason why he has long escaped our prying eyes; At about 1 km wide, it would have been discovered long ago if it was not hiding near the Sun. Surprisingly, it was around 19 magnitude when it was found; the least star that you can see with the naked eye is 150,000 times brighter than that! Detecting this asteroid was therefore a feat, since it was also against a background of brilliant sky.
The short year and the inclined orbit immediately made me think that this little rock had had one or two encounters with Mercury or, more likely, with Venus; it is very close to Venus and this planet is large enough to affect the orbit more than Mercury could do at the same distance. We know very little about LF6 so far, but (speculating here), it could be an asteroid from the main belt dropped into the internal solar system by Jupiter and then of its orbit modified by Venus. Perhaps other observations, no matter how difficult they may be, could help to determine how this happened where it is.
The short orbital period of 151 days only beats the previous record holder of the shortest year: 2019 AQ3, discovered earlier this year and also by the ZTF! This asteroid has a period of 165 days and is larger than LF6 with about 2 kilometers in diameter.
Few Atira asteroids have been found because they are so hard to see. At the moment, only two dozen are known, but it is likely that ZTF will find more.
The ZTF is a 123 cm telescope with a 600 megapixel camera. The telescope and the camera have a wide field of view and each image covers an astounding 47 square degrees – more than 200 times the surface of the full moon in the sky. It allows to imagine the sky available in the northern hemisphere in three nights only and to see objects a little weaker than the 20th.
All this is designed to search transient, objects that change brightness or position. This includes variable stars, supernovae, eruptions of active galaxies, and solar system objects such as comets and asteroids.
We have already found 100 asteroids close to the Earth and more than 2,000 in the main belt between Mars and Jupiter. But we do not know much about the asteroids closer to the Sun than the Earth, so each one of them adds to our knowledge. Knowing that ZTF has found the AQ3 and LF6 2019, I even wonder if it could even spot a theoretical Vulcanoid, a class of asteroids that gravitate around the Sun inside Mercury's orbit. . None have yet been found, although much research has been done.
Nevertheless, it is important to learn more about Atiras. How did they get there in these unusual orbits? How many are there? Are they unique to rocks in the main belt or close to the Earth? How does incident solar light and high average temperature affect them?
It's funny; these asteroids are close enough to Earth, even closer than the asteroids of the main belt. The LF6 reaches about 100 million km from the Earth! Yet we know so little about them because the sun overwhelms them in the sky. Even better than ZTF, it would be a space-based infrared observatory, especially an observer placed in orbit inside the Earth to better locate these rocks. Such an observatory was, moreover, encouraged in a report of the national academies last month. You can read more about it here.
I am all for finding these things. Certainly for science; Asteroids are fascinating and are a key to understanding our solar system. But also for our own interest! Some of them may come close enough to Earth to pose a threat, and the idea of an impact is something we must take very seriously. Such an inquiry would be a big step in the right direction.
Until then, ZTF and other investigations always keep their eyes fixed on the sky and find these asteroids quite easily. We can not do anything against a potential threat until we find it. I'm glad we were looking for
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