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# It's ALIVE: Juno back online after reboot

NASA's Juno space probe has successfully rebooted after a recent Jupiter fly-by was disrupted by an unexpected “safe-mode” event. The data-gathering on the October 19 flyby didn't happen, because a software performance monitor rebooted its computer just beforehand. The probe then entered safe mode while NASA worked through its …

## COMMENTS

1. #### Safe Mode?

How do they reach the F8 key from that distance?

1. #### Re: Safe Mode?

It's easy, they have a wireless keyboard.

1. #### Re: Safe Mode?

Thanks, both gave me a laugh.

Just pictured an operator in NASA desperately spamming the F8 key...darn I missed it. Have to wait another 43.2 minutes* to try again.

*I borrowed that from Google. Not sure of accuracy.

2. #### Re: Safe Mode?

The R2 unit punches the F8 key for them. Duh.

2. #### Figures sound wrong

3.6 kg of fuel to change velocity by 5.8 mph? That sounds horrifically inefficient!

1. #### Re: Figures sound wrong

Agreed. Has someone got some units wrong somewhere?

2. #### Re: Figures sound wrong

Is it fuel, or is it thrust mass? i.e. cold gas thrusters rather than combustion fuel rockets - chuck 3.6 kg of stuff in *that* direction, change velocity in *this* direction?

1. This post has been deleted by its author

1. #### Re: Figures sound wrong

According to your calculations, Symon, the thrusters used for this maneuver have a specific impulse of 147 seconds. Before analyzing that, let's compare it to my homework.

Mass of Juno postburn: 1700.0kg

Mass of expended fuel: 3.6kg

deltaV: 2.59 m/s

The ideal rocket equation:

2.59m/s = (Isp)(9.8m/s^2)(ln(1703.6/1700)) gives an Isp of 125 seconds, differing from your estimate because I estimate Juno is 300kg closer to empty than you do. Still, our numbers match fairly closely.

What does this mean? Juno's RCS thrusters use hydrazine monopropellant, which has a theoretical maximum Isp of ~230 seconds. So this burn was in fact fairly inefficient in a thermal sense. Since hydrazine decomposition is exothermic, this inefficiency may be attributed largely to the small size of the engine, and the desire to run 'cold' as to avoid damage to the motors and payload. The engineers who figured this stuff out when planning the mission probably realized the savings on engine mass, thermal shielding, and radiators, would outweigh the fuel mass efficiency of larger, hotter RCS engines of the same reliability.

3. #### So they...

...tried turning it off and on again?

4. #### It's full of stars

It was rebooted when it got too near to the monolith.

5. #### Bingeing on Netflix

Juno was busy, watching Season 2 of The Flash.

6. #### Presumably another of those Microsoft security updates

"Please do not survey any remote planets while the update is in progress"

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