It’s not every day one spots a long-lost relation. After some tracking duration (called “observing arc” in our field), it appears we’ve found a new (…to us) Mars trojan asteroid. Object 2023 FW14 (provisional designation, sure to change ~2025-26) is bound by Mars’ gravity into a 1:1 orbit.
Announced by the IAC (Instituto Astrofisica Canarias) and Universidad Complutense de Madrid (UCM), 2023 FW14 orbits 60 degrees ahead (nominally) of Mars, in that stable Lagrange point (named “L4”). Due to a quirk of gravity, bodies at 60 degrees ahead or behind are trapped in a reasonably-stable orbit with the same “year” (period around the Sun) as the larger body. In reality, objects rock forward and backward a bit about the true 60-deg spot, and may also have some inclination (above-below rocking). On longer timescales, disturbances may destabilize the object, and Trojans are not thought to be primordial objects kept since the start of the Solar System. In the case of Mars specifically, it is between Earth and Jupiter, both larger bodies. Far larger. Mars Trojans, then, might be knocked out of the Lagrange points by the cumulative perturbation of these gravities.
Trojan asteroids would be a mission ‘to’ Mars. Many asteroid missions have, so far, been flybys, with their low propulsion requirements. Orbiters need more propellant, and propellant means mass and cost. A spacecraft must fire to be ‘caught’ by Mars, Jupiter, whatever. But because a trojan bodys’ orbit is already heliocentric, the catching turns out to be quite slow- more ‘merging’ (to use a mundane metaphor) or stationkeeping with its target, than decelerating and being captured. Thus the marginal propellant requirement (for a low-thrust, electric mission) is low.
In the case of Mars Trojans specifically, the majority are A-type, spectrally. A-type objects show the features of olivine in our telescopes, and olivine is a key mantle mineral of Earth, Mars, etc. Therefore, it is widely speculated (make of that what you will) that the majority are fragments, knocked from Mars by impact, and now the trapped percentage of fragments that made it to a Lagrange point and settled there. In other words, a mission to (most) Mars Trojans may very well be a mission to Mars’ interior, indirectly.
In the case of 2023 FW14 specifically specifically, this new discovery is not A-type. More likely, the object was a passing asteroid that, it just happens, approached the L4 point slowly enough to get captured, instead of ‘rebounding’ back to free space. Still, let’s take the lesson and the big picture. The number of small bodies is big- inhumanly big. The population includes multiple compositional categories, and multiple dynamical (trajectory) categories. Take your pick: some percentage of some percentage can answer (partly or mostly) a question: pretty much any Solar System question that’s being asked. And for less propellant cost (and thus mission demand) than other missions. Captivating!
de la Fuente Marcos, R. de León, J. de la Fuente Marcos, C. et al. 21 March 2024 Dynamics of 2023 FW14, the second L4 Mars trojan, and a physical characterization using the 10.4 m Gran Telescopio Canarias – Astronomy & Astrophysics vol. 683 L14 202449688
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