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I was surprised to see, with unaided eyes, a rocket like object in the skies on Dec 25, 2021 (Indian time) around the same time as JWST launch. My coordinates were approximately 10°N,80°E.

What surprised me more was I was able to see exhaust gases venting out and a clearly visible plume. There are some twitter post too enquiring about the same.

A recent APOD post has this beautiful picture taken from Thailand indicating Ariane's upper stage is visible quite vividly post engine cutoff. Ariane and comet

What is the reason for the venting out of the exhaust plume from Ariane's upper stage post engine cut off?

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  • $\begingroup$ Vented excess propellant makes a pretty flower in the sky. Unvented excess propellant makes a shrapnel-spewing KaBoom. $\endgroup$
    – CuteKItty_pleaseStopBArking
    Commented Dec 29, 2021 at 13:22
  • $\begingroup$ But why carry excess propellant?! 🤔Is it something like those in the fuel lines? And is it specific to Ariane or any Cryo upper stage? $\endgroup$
    – karthikeyan
    Commented Dec 29, 2021 at 13:26
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    $\begingroup$ +1. I was not aware that the Ariane upper stage is visible so far east! $\endgroup$
    – AJN
    Commented Dec 29, 2021 at 13:30
  • $\begingroup$ @cutekitty - why will unvented fuel explorlde? Is it a potential hazard? What could be the source of ignition, if the engine is cut off? $\endgroup$
    – karthikeyan
    Commented Dec 29, 2021 at 16:09
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    $\begingroup$ Rockets always carry extra fuel. Too much fuel is just a bit of cost and mass wasted. Too little fuel is mission failure. Unburnt liquid cryogenic fuel in tank+sunlight + unvented = tank full of high pressure gaseous fuel = KaBoom. The ariane5's upper stage is cryogenic hydrogen and oxygen. Venting is an absolute requirement, even 1% of tank capacity , turned to gas, will overpressure the tank if not vented, and the liquid H2 boils at -252,8 °C (20 Kelvin). $\endgroup$
    – CuteKItty_pleaseStopBArking
    Commented Dec 29, 2021 at 16:16

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ESA's space debris mitigation guidelines are aimed towards reducing the generation of space debris by following two approach

The most effective short­term means of reducing the space debris growth rate is through the prevention of in­orbit explosions (via passivation of space objects at the end of their operational life) or collisions (via collision avoidance manoeuvres while the objects are still active).

... Such passivation measures may include depletion burns, fuel and/or pressurant venting, the discharging of batteries and the inhibiting of pyro devices

Hence to ensure that future collision induced explosion or accidental explosion does not occur, upper stage passivation is undertaken.

Since 1997, Ariane orbital stages have performed a controlled fuel/pressurant venting and battery discharging

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