Design of an in-situ science mission to a Jupiter Trojan

Michael Khan, Olivier Witasse, Waldemar Martens, Arnaud Boutonnet


Planetary and Space Science
Available online 4 December 2022,

“The NASA mission Lucy, launched in 2021, will perform flybys at several Jupiter Trojan asteroids. The logical next step is a close orbiter and/or lander for in situ science at specific Trojans. The present paper reports results for a feasible design of such a mission to the largest Jupiter Trojan, 624 Hektor. The suggested principle, which involves a powered Jupiter gravity assist followed by an extended drift arc towards the target body and an insertion maneuver of around 1 km/s, has been found to lead to comparable mission options for a number of other target Trojans.”


The Jupiter Trojans, remnants of the early solar system, constitute a diverse population of minor bodies in two groups that head and trail planet Jupiter on its orbit. The largest of these is 624 Hektor, a possible contact binary with an approximate size of 400 × 200 km, orbited by a 12 km large moon.

The NASA mission Lucy (Englander et al., 2019), (Levison et al., 2021), launched in 2021, will encounter seven members of the Jupiter Trojan population in 2027, 2028 and 2033, performing flybys at speeds of 5.1–8.7 km/s 624 Hektor will not be visited by Lucy.

In addition to Lucy, other concepts for missions to the Jupiter Trojans have been addressed in previous work:

An idea called OKEANOS for an in-situ mission to the Jupiter Trojans was studied by JAXA (Okada et al., 2018). That mission was to use a hybrid system of solar sailing and ion propulsion for a spacecraft of 1400 kg launch mass and a transfer duration of over 11 years. The propulsion choice constrains the spacecraft design, as a solar sail of realistically feasible size requires the total spacecraft mass to be small.

A French-led proposal to ESA from 2012 (Lamy et al., 2012) called for a reconnaissance mission that included flybys of five Trojans in the Greek camp. The orbital strategy consisted in a direct trajectory to the first designated target. The high encounter velocities preclude going into orbit around any of the bodies.

Conversely, the present paper presents the design of an in-situ science mission to 624 Hektor. The mission design comprises a transfer to Jupiter followed by a Jupiter gravity assist with an impulsive maneuver at perijove to redirect the trajectory to the Trojan. Windows for the JGA to 624 Hektor exist every full Jupiter orbital period (12 years). If a delta-v penalty is accepted, windows for the JGA can be found every 6 years. The JGA is followed by another 7–9 years of flight until the Trojan is reached. Adding around 6 years for the Earth-Jupiter transfer (the detailed design of which is not within the scope of the present analysis, for reasons that will be explained), a total transfer duration of 13–15 years is required, followed by the in-situ science phase.

The fact that all Jupiter Trojans orbit the sun in essentially similar orbits (see Fig. 1) led us to try the obtained strategy also for the design of sample missions to other asteroids of this group. The next ten largest known Jupiter Trojans (617 Patroclus, 911 Agamemnon, 588 Achilles, 3451 Mentor, 3317 Paris, 1867 Deiphobus, 1172 Aeneas, 1437 Diomedes, 1143 Odysseus and 884 Priamus) were chosen as targets. These come from both the Greek and the Trojan camps; the inclinations of their orbit planes range from 3.1 to 26.9 deg. Using the strategy described above, transfers to each of these were obtained and the characteristics of the transfers were documented.

The mission concept assumed here is conventional in that it relies on chemical propulsion and gravity assists for all orbital operations at high solar distance, which makes it readily scalable from mid-sized to large, depending on the mission scope and budget. None of the regarded cases requires a close Jupiter encounter, so no large radiation loads will be incurred during such missions. The delta-v budget for the Jupiter→Trojan phase was found to be of the same order as the values found for missions to a Galilean moon.

This is the extent of the work performed so far. The outcome is documented in the present paper. In future work, the authors plan to automate the trajectory design task using the SOURCE software tool (Boutonnet et al., 2013), (Martens et al., 2013) and to extend the design work to a much wider range of samples, which will be representative of the entire Jupiter Trojan population. When this work will have been concluded, statistically meaningful data will have been obtained, allowing a general statement on the extent of the applicability of the proposed strategy.

Fig. 1 shows a snapshot of the ecliptic inclination over the osculating eccentricity and the absolute magnitude range for the more than 8000 Jupiter Trojans known to date. The population consists of two groups: the “Greek camp”, leading Jupiter around the SJL4-point, and the “Trojan camp”, following Jupiter around the SJL5-point. Inclinations of up to 40 deg are observed.

Only two Jupiter Trojans have an absolute magnitude brighter than +8 mag: 624 Hektor and 911 Agamemnon, with sizes of 225 km and 140 km), both in the Greek camp. Other large members of that group are 588 Achilles (ca. + 8.5 mag), 617 Patroclus (a binary body of +8.2 mag) and 3451 Mentor (ca. +8.5-+8.8 mag). These five are currently thought to be the largest members of the Jupiter Trojan population.

The Jupiter Trojan asteroids are primitive bodies located around the L4 and L5 Lagrange points of Jupiter at ∼5.2 AU. They have not yet been visited by any planetary missions; so scientific knowledge is limited to the information provided by ground-based measurements. The Trojan population is of high interest to science. Their origin is a matter of debate. They could have been formed near the Jupiter orbit distance or at much larger heliocentric distances, and then migrated to their current place. The study of these primitive objects can thus shed new light on the formation of the Jovian system and of the solar system in general, on planetary evolution and migration, on the trans-Neptunian objects, and the delivery of water and organics to the inner solar system (Levison et al., 2021), (Lamy et al., 2012).

The main science objectives of a mission to Jupiter Trojan asteroids are the characterization of their global shape, volume, mass, density, surface composition (ices, minerals, organics, dust) and geology, surface physical properties, possible activity, interior structure, search for satellites, asteroid environment and solar wind-surface interaction (Levison et al., 2021), (Liu and Schmidt, 2018), (Lamy et al., 2012).”