Juno has been given several mission extensions, the last (and most exciting) of which involves close passes to three of the Galilean satellites, also allowing them to trim its orbit every time, and approach closer to ever higher latitudes of the Jovian atmosphere, until it is intentionally crashed in 2025.

It is my understanding (correct me if I'm wrong) that if nothing is done, Juno's orbit will keep drifting, then have a perijove directly over the north pole, and then come out to the other side, and allow a new set of close passes over Io, Europa, and Ganymede, that alone would be excellent news for science. If I recall correctly, the failure on the main valve and the decision to keep it on a longer orbit, means that most of the propellant is still there. It was not vented.

Unrelated, many years ago I saw a report (and a Wikipedia page) showing that Cassini had many exciting options to end its mission, one of them was to turn it into a Titan orbiter.

Some engineers believe it may even be possible to go into orbit around Titan. "Aerobraking”, or using a body's atmosphere to slow a spacecraft down, has been tested with other spacecraft and is planned for at least one JPL’s mission in the future. Aerobraking and/or spacecraft maneuvers could be sufficient to place Cassini in orbit around Titan, allowing the spacecraft to study Titan very closely over a long period of time.

Now, I know Titan has a huge atmosphere, and none of the Galileans do. Setting aside the likehood that such decision is taken: Does Juno have enough propellant to attempt orbital insertion around a Galilean moon?

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    $\begingroup$ thank you for the suggestions @uhoh , I don't know the site that well. The questions are both to do with fuel availability and delta v, so they are very related even though they are different missions, I'm keeping it like this at the moment if that's ok, but I'll keep in mind for future questions. $\endgroup$
    – we'll see
    Commented Nov 22, 2021 at 7:40
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    $\begingroup$ I'm going to remove the Lucy bit, as almost nothing is actually about Lucy in the question. It would make a great question in and of itself, feel free to ask it in a separate question. I'm also going to remove the Hayabusa 2 bits. $\endgroup$
    – PearsonArtPhoto
    Commented Dec 1, 2021 at 15:57

1 Answer 1


Juno used a lot of propellant to get to (and capture at) Jupiter$^{1,2}$ so it does not have most of its propellant remaining. The main engine has been effectively decommissioned since orbit insertion, leaving oxidizer for the bipropellant engine as trapped dead weight. Luckily, the hydrazine fuel is common to the main and RCS engines and, I suspect, physically shared between the two so the fuel 'budgeted' for the main engine is now exclusively for the RCS engines, an effective increase in their supply.

I estimated the mass history of the spacecraft (until pre Ganymede flyby) using the Juno Launch Press Kit to get the spacecraft's mass and fuel levels at launch along with reconstructed$^{1,2}$ and predicted (twice per orbit, albeit small)$^3$ maneuver $\Delta V$'s. Main engine efficiency is given as $318.6$ $s$$^1$. RCS engines, used for TCM's and all maneuvering post JOI, effective efficiency was fitted to $175$ $s$ using data from a long duration burn in 2019 (Orbit 22):

Juno Mass History

This leaves about 389 kg of hydrazine (at a S/C wet mass of 2079 kg) for $~350$ $m/s$ of $\Delta V$ remaining. This gets whittled away at a rate of a couple of meters per second each orbit$^3$.

The $\Delta V$ required to capture at each moon is enormous:

Moon Capture (C3=0) $\Delta V$
Io $27.0$ $km/s$
Europa $21.1$ $km/s$
Ganymede $15.4$ $km/s$
Callisto $10.7$ $km/s$

Math assumes:

  • moon & Juno velocity 90° apart (moons are ~equatorial, Juno is polar -> $V_\infty = \sqrt{V_{moon}^2 + V_{Juno}^2} $)
  • impulse (instantaneous) $\Delta V$


  1. Pavlak et al., "Maneuver design for the Juno mission: inner cruise," AIAA Space 2014, San Diego, California, August 5-7, 2014

  2. Stephens, Stuart K., "Juno at Jupiter: The Mission and Its Path to Unveiling Secrets of the History of the Solar System," 2018 IEEE Aerospace Conference, Big Sky, Montana, March 3 - March 10, 2018

  3. Pavlak et al., "Juno Trajectory Redesign Following PRM Cancellation," 2017 AAS/AIAA Astrodynamics Specialist Conference, Stevenson, Washington, August 20-24, 2017

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    $\begingroup$ Thank you very much, it is very detailed and really helps to put things in perspective. I made wrong assumptions. I'll tick it "solved" since the Lucy's part is probably just as unlikely. $\endgroup$
    – we'll see
    Commented Nov 30, 2021 at 21:10

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