Issue 80

13 Aug 2023: This is a two pager astro lesson using the International Space Station (ISS) disposal of a big trash bag back in July 2022. I prepare these “astro lessons from Jack” now and then for the great folks serving in the National Space Defense Center. They truly “enjoy” my blurbs and sharpen their astrodynamics understanding! This is a relative motion related problem for the ISS and mission control team. First step is to watch two on-line videos (very short and pretty cool videos). I recommend you send it home and gather your family and friends as you take part in this short astro lesson. Let’s “take out the trash.”

Here are the links to 2 short videos of the deployment of a 172 lbs big bag (5 ft diameter, 4 ft tall) of trash from ISS.

https://www.digitaltrends.com/space/iss-trash-disposal-nanoracks/

https://www.youtube.com/watch?v=pyldo8pngV8

The first video (38 sec) is focused on the jettison of the trash bag and the second video (1:42) is multiple camera views of the trash bag jettison. Pretty cool!

Perhaps you could tell which direction the trash bag was jettisoned. It was loaded into the Nanoracks Airlock (NRAL) system. Nanoracks in Houston built the airlock/deployment system for ISS. This jettison took place 2 July 2022. They aimed the trash bag backward and down. Why? Let’s look at the relative motion astrodynamics of this “back and down” jettison.

The trash bag was sent away from ISS via the NRAL at 1.34 m/s aimed away from the direction of spaceflight and tilted down 45 degrees. Let’s look at my drawing of this jettison direction in the Radial-InTrack-Crosstrack (RIC) coordinate frame you all are familiar with.

I calculated “how much backward” and “how much down” Delta V was imparted on the trash bag. So the next question might be “so where does the trash bag go?” Hopefully NOT coming back to whack the station somewhere. Let’s look at the resultant relative motion of the trash bag relative to the ISS. Here’s a drawing to help understand “where does it go?” The RIC coordinate frame is fixed on ISS.

What you see is while the bag was tossed back and down, the resultant Delta V lowered the perigee of the trash bag’s orbit relative to the ISS by about 3.7 Km. That jettison took some energy out of the orbit that is now the trash bag orbit. That means it will advance ahead. The apogee or high point of the trash bag orbit it still at the ISS altitude, but after one orbit (about 94 minutes, but a wee bit less than ISS) it crosses about 18 Km ahead. For each orbit it “scallops” it’s way 18 km more distance. The bag is departing ahead and below. Yay! Let’s look at a bit of astro good news regarding where the NRAL is relative to ISS’s center of gravity. See next paragraph.

The NRAL deployment took place 22 meters below the center of gravity of the ISS (see NASA illustration below). This is good news since by that difference, even if the astronauts just “let it go” (you might say they just “put it on the curb”), it would be at a different altitude of ISS and would scoot ahead 800 meters per orbit (94 minutes). It’s lower, the trash bag would advance ahead in a linear drift manner.

There’s even more good news I wish to share. The trash bag will be affected by the atmosphere way up there and that force will make it start to descend and eventually decay into the Earth atmosphere and burn up. ISS periodically maneuvers to maintain it’s orbit of about 425 Km. The trash bag decayed 22 December 2022, 172 days since it was heaved overboard in this “back and down” manner. Hooray for Space Force, especially the 18th astro experts and the sensor sites for keeping track of this trash bag.

What would have happened if the trash bag was jettisoned “UP” only?

What would happen if the trash bag was jettisoned forward and up?

THANK YOU’S: Special thanks to Mr Ulisses Hernandez who is a NASA/JSC ISS Flight Dynamics expert for sharing his analysis that enabled me to create this fun filled lesson. I appreciate his great boss Lark Howorth and Bill Foster who are always so helpful vectoring my curiosity to the right person on their team. They tolerate me nicely and I am very grateful. 😊