The American probe Lucy flies by its first asteroid

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Every weekend, an image that made the news or caught our attention. On November 1, the American probe Lucy, en route to the Trojan asteroids of Jupiter, encountered a small asteroid from the inner main belt, called Dinkinesh.

Attacking the Trojans of Jupiter…

Launched in October 2021Lucy is a space probe in NASA’s Discovery program offered by the incredible private research center Southwest Research Institute (SwRI), headquartered in San Antonio, Texas.

Its mission, very ambitious and lasting 12 years, aims to fly over no less than 10 asteroids, and to study their geophysics and astrophysics, in particular to validate or refute the Nice model: this ambition is already an audacity in itself!

But, among the asteroids targeted, Lucy will be the first space probe to study Jupiter Trojans.

These small solid bodies share the orbit of the giant planet around the Sun, around the Lagrange points L4 and L5 of the Sun-Jupiter system, and are more precisely located 60° before and after Jupiter.

…for large numbers of flights

The list of targeted asteroids is simply incredible.

Let us cite (52246) Donaldjohanson, whom Lucy will cross on April 20, 2025 at 922 km distance; the couple (3548) Eurybate and his companion Queta, August 12, 2027 at 1,000 km; or (15094) Polymelus and its recently discovered and not yet named companion, September 15, 2027 at 434 km.

We will have to wait until April 18, 2028 to fly over (11351) Leucos, 1,000 km away, then on November 11, 2028 (21900) Oros, also 1,000 km away, and finally on March 3, 2033 the craft will cross the binary system (617) Patroclus / Menetios more than 1,075 km away.

Last minute guest

In the meantime, serious things have already started on the 1stNovember 1, 2023 at 4:54 p.m. UTC, when Lucy flew by the asteroid (152830) Dinkinesh, at a speed of 4.5 km/s and 425 km away. This flyover was not initially planned in the program and was decided (to test the on-board instruments) in January 2023, i.e. after the launch of the probe.

(152830) Dinkinesh is an S-type asteroid with a size of approximately 790 m.

The few photos taken by the L’LORRI camera of the machine made it possible to discover that this small planetary body is in fact a binary system, and that it has a companion of approximately 220 m in diameter, which will remain to be named .

Like the asteroids (101955) Bénou and (162173) Ryugu, explored by NASA’s Osiris-REx probe (recovered on Earth on September 24), Dinkinesh and its orbital companion each have what looks like an equatorial rim, like a sort of compressive ridge, the precise nature of which is not yet determined.

In any case, the photos already reveal a general surface that is not very cratered, dotted with relatively small blocks.

Serendipity? No, there are actually no big surprises when reading these preliminary results.

Gilles Dawidowicz

Vice-president of the Astronomical Society of France

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