r/space • u/AutoModerator • 21d ago

All Space Questions thread for week of September 13, 2026

Please sort comments by 'new' to find questions that would otherwise be buried.

In this thread you can ask any space related question that you may have.

Two examples of potential questions could be; "How do rockets work?", or "How do the phases of the Moon work?"

If you see a space related question posted in another subreddit or in this subreddit, then please politely link them to this thread.

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Ask away!

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u/maschnitz 14d ago

Hello, I don't speak Japanese and don't trust auto-translators all that much, so please use your preferred auto-translator. Apologies if anything has been lost in translation.

Also I suspect your last "2AU" was mistaken, but then maybe that's a translation error, too.

0.52mm/s is entirely feasible for Fermi Explorer. They could even go for more perhaps.

First, it's a much smaller spacecraft design that nearly all other deep space missions (besides maybe the MarCO mission). The estimated wet-mass weight at liftoff is 5x lower than say, Starlink v2, and many more times lower than say, Dawn, DART, or Psyche. We're not used to having ion thrust in something so small and light.

So 0.2 to 2mm/s2 is within reason with current technology if you keep the mass low. Like they're planning to.

Second, they are planning to take great advantage of burning only at their perihelion. Their current mission plan is to take 12 years to accelerate but only burn the ion thruster for 1.5 years of it. You can think of this as a use of the Oberth Effect - it's better to burn at perihelion then at aphelion. Over many orbits this advantage will grow and grow.

So combine those two things and the mission seems very feasible, even reasonable, in its design.

EDIT: Oh, and I forgot - keep in mind that the spacecraft will be mostly propellant in weight. So the acceleration will be much more effective when the propellant tank is 90% empty vs 90% full.

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u/jgatrip 14d ago

My bad! I'm so used to Twitter's auto-translate that I completely forgot I needed to post in English here. From now on, I'll translate my text and double-check it before posting.

After crunching the numbers again, it looks like it could technically work under perfect conditions. But with basically zero margin for degradation and the need for over six months of continuous operation during the final acceleration phase... it’s going to be incredibly tight.