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The Lucy trajectory described in several excellent answers to this question involves a number of fairly large (in delta-V terms) deep space maneuvers, totalling (same source) about 1.5 km/s over almost ten years.
What form of propulsion will be used for these? I can't see any information about the spacecraft on any of the project web pages, just the destination and instrumentation. What fuels are efficient enough for such large delta-V but stable enough for such long term storage?
This article on the Psyche mission says Lucy's propulsion is to be chemical (in contrast to Psyche's electrical propulsion).
Since the fuel needs to be stored long-term, it's almost certainly a non-cryogenic hypergolic combination like MMH/NTO.
There are a number of proven thrusters using that combination which are used for primary propulsion in deep space missions; another question here is dedicated to figuring out exactly what engine Lucy intends to use, but let us say for sake of argument that it's the Aerojet R-4D. With a moderately large nozzle that thruster achieves 311 seconds of specific impulse. We can apply the rocket equation to work out the necessary mass ratio. I saw another source give 1.68km/s required for Lucy's maneuvers, so I'll use that figure.
$$\Delta v = v_\text{e} \ln \frac {m_0} {m_f}$$
$v_\text{e}$ is 311s x 9.801 m/s2 = 3048 m/s
Thus the log of the mass ratio is 0.5512, and so the initial-mass to final-mass ratio is 1.735.
This requires ~43% of the mass of the spacecraft be propellant at the beginning of these maneuvers. The QA that @uhoh referred to in comments shows that this is towards the high end of tankage ratios for deep space missions, but not unusually high.
R-4D might be overkill for a small spacecraft like Lucy, but most smaller thrusters will have poorer specific impulse, so they might cost more mass in propellant than they save in engine hardware.
A few Lucy team members were asked this at a Q&A event. Wilfredo Santiago, a thermal engineer at Lockheed Martin, answered:
...there's a few tanks onboard. One of them is carrying the fuel, which is hydrazine — highly energetic, toxic, so we have to be really safe around [it] once the spacecraft is fueled and working through it — and then there's an oxidizer.
The event was aimed at middle and high schoolers, so the team didn't go into much technical detail, but this is one of the only references to propulsion I could find from public Lucy Mission material.
