The Asteroid of the Vernal Equinox (Planetary Science)

Near-Earth asteroid 2001 FO32 will pass about 2 million km from Earth on March 21, the day after the vernal equinox, which this year occurs on March 20 at 10:37 am Italian time. This is the flyby with the largest asteroid of all 2021. Unfortunately, the asteroid will be scarcely visible from Italy due to the heliocentric orbit that favors observers from the southern hemisphere. In any case, it will be a great opportunity for astronomers who deal with the physical characterization of near-Earth asteroids, to try to determine their size, rotation period and surface composition.

On March 21, 2021 at 5:03 pm Italian time, the near-Earth asteroid (231937) 2001 FO32  will flyby with the Earth, passing at a  minimum distance of about two million km from our planet, more than five times the distance Earth- Moon. It is an asteroid – discovered on 23 March 2001 by the Socorro telescope of the Linear project – which follows a rather eccentric “Apollo” type orbit: at perihelion it passes to 0.29 astronomical units (Au) from the Sun, while at aphelion goes to 3.1 Au, ie in full main asteroid belt. The minimum distance between the orbit of the asteroid and that of the Earth ( minimum orbit intersection distance, Moid), is about 500 thousand km, a value that protects us from possible collisions, at least for the next centuries. In any case 2001 FO32, being able to reach less than 7.5 million km from the Earth and being larger than about 150 m, is counted in the list of Pha , that is the Potentially Hazardous Asteroids which – currently – are about 2000.

Estimates of the diameter of 2001 OF32 are of the order of a km or so: it is the largest known near-Earth asteroid that will flyby with Earth in 2021. It is unlikely that this record will be surpassed by some Nea still unknown to the today’s date: 95 percent of Neos with a diameter of one kilometer or more have already been discovered.

While it does not represent a danger to the Earth, it is always interesting to consider the effects of the impact of an asteroid of this size. The most important parameter to consider in these cases is the relative velocity with the Earth, which contributes quadratically to the kinetic energy possessed by the asteroid. In the case of 2001 FO32, the relative speed is 34 km / s . Assuming a diameter of 1 km and a typical asteroid density of 2.4 g / cm 3 , 2001 FO32 would cross the Earth’s atmosphere in a matter of seconds without being significantly slowed down . In contact with the ground, the asteroid would instantly release a total energy of about173 million kt , equivalent to the simultaneous explosion of about 11 million atomic bombs like that of Hiroshima . The crater excavated on the mainland would have a  diameter of about 9.5 km  with a  depth of 2.4 km  and the  larger ejecta would reach up to 20 km away . Still 100 km away from the epicenter, the heat wave would be able to burn trees and cause third degree burns, while the shock waveit could still tear down the upper floors of taller buildings. Overall, more than 30 thousand square kilometers of the earth’s surface would be wiped out. The climatic effects of the collision would be on the verge of global catastrophe. Fortunately, none of this will happen next March 21st .

The flyby of March 21 will be a great opportunity for scientific research on minor bodies because it will provide astronomers with the opportunity to obtain more precise data on the size, the albedo of the asteroid (i.e. on how reflective the surface is) and a approximate idea of its surface composition. These data will be obtained, in part, with the use of NASA’s Infrared Telescope Facility ( Irtf ), a 3.2-meter telescope atop Mauna Kea in Hawaii that will observe the asteroid in the days leading up to the closest approach using its infrared spectrograph. Furthermore, given the short distance it will reach from Earth, radar observations will also be carried out by the Deep Space Network ( DSN ).

The orbit of 2001 FO32 as seen from the north pole of the Ecliptic with the position of the asteroid on March 16, 2021 (Credits: JPL).

The DSN includes three stations: one in California (Goldstone), one in Spain (Madrid) and one in Australia (Canberra). The large satellite dishes of these stations can be used to send radio signals to 2001 FO32 so that other antennas can receive the return echo. Radar observations can offer additional insight into the asteroid’s orbit, provide a better estimate of its size and rotational speed, and help map surface features, such as large boulders or craters. In some cases, radar data has allowed the detection of small satellites, as in the case of the near-Earth (153591) 2001 SN263 .

In the flyby of 21 March 2001 FO32 will increase its apparent brightness until it reaches magnitude +11.7 : a remarkable value that puts it within the reach of small telescopes with a diameter of 15-20 cm . Unfortunately, even during the approach phase, the declination will always be strongly negative (from -34 ° to – 40 °) and from the Italian mid-latitudes the asteroid will be scarcely visible because it will always be very low on the horizon, even in the best conditions. of visibility. This poor observability is due to the inclination of the asteroid’s orbit, well 39 ° below the plane of the Ecliptic, which makes it much better observable from the southern hemisphere. After the flyby the asteroid will move towards the Sun and will no longer be observable.

Too bad, because it would have been a good opportunity to visually observe , through a small telescope, a near-Earth asteroid as it whizzes near the Earth: an experience capable of giving many emotions, even if the asteroid is always visible as a dot of slowly moving light in the eyepiece field of view.

Featured image: NASA’s infrared telescope on Mauna Kea which will be used for the 2001 FO32 infrared spectrum (Credits: UH / lfA)

Provided by INAF

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