# Are there payload lights turning on and off in orbit on SpaceX STP-2?

In the video of the SpaceX STP-2 Mission during satellite deployment after around T+01:21:35 there are some areas that abruptly blink brightly. It's too fast and percussive to be reflections of Earth, it feels more like lights blinking on and off but that doesn't seem likely either.

What causes these blinks?

I've made a GIF to indicate the events I'm talking about, but you should watch the video to get a better feeling for how abrupt these things are.

Slow GIF:

This is meant to be cued at T+01:21:35 let's keep an eye on this in case they edit the video again.

## 2 Answers

The Falcon 2nd stage has 4 Draco (hypergolic) thrusters. In addition to the reflections you point out, their firing also illuminates the 2nd stage engine bell (and puffs dents in the engine cover blanket). I can't find a source that shows their actual location however -- can someone help?

• Okay that makes sense, but can you expand on that a bit? Why do Drago (hypergolic) thrusters make light? Does the exhaust luminesce in some way, or is that reflected sunlight from droplets of unburned liquid or condensed reaction products, or something else?
– uhoh
Jun 26 '19 at 5:38
• youtu.be/OcXouT8ggfI -- sorry, it's BW video and not in vacuum (basically, it's hot!)
– amI
Jun 26 '19 at 6:00
• nice, that video looks very convincing! Details of the light may warrant a separate question actually, rather than make you answer that here. I also haven't figured out why F9 Merlin engines make so much light from glowing soot in air but the exhaust from the 2nd stage Merlin becomes essentially invisible once in space. I guess that also needs a separate question.
– uhoh
Jun 26 '19 at 6:05
• youtu.be/fPmPUrE5IYI?t=363 -- here (at 6:00 min) are Draco thrusters firing as a Dragon capsule prepares to dock to the ISS (I don't know if any hypergolic combustion takes place in free space; ie, wasted energy)
– amI
Jun 26 '19 at 6:25
• okay, that certainly looks like reflected sunlight on droplets of something, thanks!
– uhoh
Jun 26 '19 at 6:35

For what I observe, these blinks seems to be the reflections of the RCS thrusters nitrogen exhausts of the second stage. Falcon 9 Block 5 uses nitrogen RCS thrusters for attitude control, unlike now-retired Block 2, it used hypergolic thrusters. Nitrogen thrusters, when operating, looks like white poofs of gas, similar to a deodorant spray being used. The fact that the Sun is illuminating from the back of the camera enhance the brightness of the nitrogen exhaust. And finally, you can see any SpaceX first stage landing attempt, between stage separation and reentry burn, the first stage perform a series of these nitrogen-thruster blastings, very short in duration, just as the blinks seen reflected in the sats above. (Here's an example, at Falcon Heavy / Arabsat-6A mission, T+00:04:11, all 3 cores performing thruster firings.)

• Thanks! Any idea why nitrogen thrusters look like white poofs of gas? I mean, gasses like nitrogen are transparent; the visible poofs from aerosol cans are visible because they are exactly that; aerosols; clouds of liquid droplets, and not gas.
– uhoh
Aug 14 '19 at 5:46
• The white poof is for the same reason a CO2 extinguisher makes CO2 fog & snow: the expansion cools the gas past condensation. These are “cold gas” thrusters, not heated by combustion, and in vacuum the gas is expanded a lot. Aug 15 '19 at 5:12