Aphrodite And The Comet (Planetary Science)

On December 19, 2021, the planet Venus will pass approximately 50,000 km from the orbit of Comet Leonard (C / 2021 A1), in the region where meteoroids larger than a millimeter should be present. This could cause a meteor storm in the planet’s atmosphere which – if observed – would help astronomers determine the activity of the comet’s core when it was far away from the Sun, beyond 30 astronomical units.

2021 began with the discovery of comet C / 2021 A1 (Leonard) when it was about 5 astronomical units from the Sun (equal to 750 million km). It is a comet that travels in a heliocentric orbit with great eccentricity at the limit of the hyperbolic: the orbit is retrogradewith an inclination of 133 ° on the ecliptic plane. The Leonard will pass into perihelion on January 3, 2022 – exactly one year after its discovery – reaching up to 91 million km from the Sun: a not too short distance, so it is very likely that the core will survive the heat of our star and return towards the boundaries of the solar system. Instead, from the geocentric point of view, the Leonard will reach the minimum distance of about 34 million km from the Earth on December 12, 2021, when it should show itself with an apparent magnitude of +5.5 in the sky of sunrise and sunset: no respect. to the show offered by the brilliant comet Neowise (C / 2020 F3) in July 2020. As far as we are concerned, a comet like there are many, but the orbit that the Leonard travels through is such thaton December 18, 2021 at 02:09 UT will bring it only 4.3 million km from Venus , the planet that in these months is clearly visible in the west immediately after the sunset.

Venus is a planet with an orbit internal to that of the Earth, so it can be observed at dawn before the sun rises, or after sunset as happens in these months: from the sky of the Earth Venus can never be seen in the middle of the night , as it happens instead for the planets with the orbit external to ours, for example Jupiter and Saturn, clearly visible in these months in the night sky. On October 29, 2021, Venus will be at its maximum elongation from the Sun – i.e. it will be at its maximum angular distance from our star (about 47 °) – after which it will move towards the lower conjunction, which will take place at the beginning of January 2022. As Galileo discovered in 1610, Venus shows the phases like our Moon, and in the passage from the maximum west elongation to the lower conjunction it passes, in practice, from the first quarter phase to that of extremely illuminated sickle subtle. At the lower conjunction Venus lies roughly between the Sun and the Earth, and we can only see its hemisphere in shadow.

We know that comets, following the sublimation of the volatile materials of the nucleus, emit gas and dust into space. Leonard is no exception, on the contrary: having discovered it when it was 5 astronomical units from the Sun is an indication of a good activity of the nucleus even at great distances from our star. Telescopic observations show that the emission is dominated by dust, with diameters between 0.1 and 1 mm, while the gas is scarce. Once emitted from the nucleus with a very low relative speed, the dusts undergo the action of the pressure of solar radiation, which tends to move them away from the nucleus. Smaller dust grains have a higher surface / mass ratio, are more subject to radiation pressure and tend to be pushed towards the sun,

On 19 December 2021 at 21 UT, the Leonard will already be about 12.5 million km from Venus, which however will pass only 50 thousand km from the orbit of the comet that is behind the nucleus. With this very short distance, Aphrodite might encounter a cloud of dust grains larger than 1mm. The conditional is a must: Leonard’s activity is known only approximately, while the largest specks of dust – emitted when Leonard was over 30 astronomical units from the Sun – are the best candidates for intercepting Venus. In fact, it is these grains that should have moved far enough away from the core to be in the right position to collide with the planet. We do not know what the comet’s activity was at those great distances (more or less at the height of Neptune’s orbit) nor how quickly the macroscopic grains were ejected from the nucleus, but it is reasonable to assume that the sublimation of super ices – birds has made their contribution.

Map for the potential meteors observable on Venus on December 19, 2021 at 9:00 PM UT. The white lines indicate the curves of the same zenith angle of the radian and surround the point of the sub-radian (marked by +). The dark lines indicate the equator and the axis of rotation of Venus, while the white crescent represents the portion of the planet illuminated by the Sun and visible from the Earth. Meteors are potentially observable anywhere on the disk where the zenith angle is less than 90 °, but are more likely near the sub-radiant point. Credits: Zhang et al., 2021

When it passes through the Leonard dust trail, Venus will be about 28 ° from the Sun and subtend an apparent diameter of over 50 arc seconds. The planet will appear to the telescope as a large sickle being illuminated only 12 percent of the disk facing the Earth. The Leonard meteoroids will enter the atmosphere of Venus with a speed of 78 km / s and could give rise to brilliant fireballs , quite similar to those we see in our sky. A rough comparison can be made between the Venusian Leonard swarm and that of the Leonids. Leonid meteoroids strike the Earth’s atmosphere at speeds of about 71 km / s and often generate fireballs of magnitude -12. If the Leonard meteoroids behave in the same way, a -12 meteoroid on Venus becomes +16 when viewed from Earth, still within range of 0.5-0.6 meter diameter telescopes equipped with Ccd / Cmos cameras in capable of shooting burst images with exposure times on the order of one second. Fortunately for us, most of the shadowed hemisphere of Venus with the sub-radiant point will be observable from Earth, and to have a non-zero probability of capturing some Venusian fireballit will be necessary to film the dark side of the planet from the sunset, at 15:37 UT, until the sunset of Venus, at 17:50 UT. To decrease the contribution of the illuminated crescent of Venus and enhance the radiation of the meteors in its atmosphere, it can be observed using an interference filter centered on the emission lines of the neutral sodium doublet (589.0-589.6 nm), a light that meteors emit in abundance. Even the use of a coronograph , which screens the brilliant Venusian sickle, can be of help in the observation of this elusive phenomenon.

The close encounter that will take place between the orbit of Leonard and Venus is among the closest ever between a planet and a long-term comet and is surpassed, in recent history, only by the flyby of comet C / 2013 A1 (Siding Spring) with Mars, which on 18 October 2014 passed only 140 thousand km from the planetary center, while a few hours later the planet passed only 30 thousand km from the comet’s orbit. This close passage has produced a meteor shower that has been indirectly observed by several spacecraft orbiting the Red Planet, and the same thing is expected to happen for Venus. The observation of any fireballs in the atmosphere of Venus will allow to haveinformation on the activity of the comet’s nucleus when it was over 30 astronomical units from the Sun : get your telescopes ready!

Featured image: False color image of comet Leonard (indicated by the arrow) taken on 7 April 2021 with the “GD Cassini” telescope of the Loiano Astronomical Station. Credits: A. Carbognani / Inaf Oas Bologna


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