Hello, friends! Welcome to Sciency Words, a special series here on Planet Pailly where we talk about those wild and crazy words scientists use. Today’s Sciency Word is:
SAFETY ELLIPSE
I don’t know about you, but when I’m trying to dock my shuttle pod with another spaceship, I like to take a few long, leisurely loops around that other spaceship first. You know, like this:
Spaceships are pretty! Who wouldn’t want to get a good look at them from every conceivable angle before completing docking maneuvers? But it turns out that circling round and round a spaceship like this is not just for admiring the view. It’s also for safety! As explained in this paper:
A “safety ellipse” is an out-of-plane elliptical periodic relative motion trajectory around the primary spacecraft such that the trajectory never crosses the velocity of the primary.
That clear things up? No? Okay, how about a quote from this paper instead:
This paper defines a safe trajectory as an approach path that guarantees collision avoidance in the presence of a class of anomalous system behaviors.
Still confused? Here’s a short video demonstrating what a safety ellipse (a.k.a. a safe trajectory) looks like:
Basically, if your shuttle pod experiences engine failure or any other major malfunction, flying in a safety ellipse ensures that you will not collide with the ship you were trying to dock with. At least not for a good, long while.
I first heard about this term the other day while watching the livestream of the SpaceX Dragon capsule approaching and docking with the International Space Station. Several times, the livestream commentators mentioned that Dragon was utilizing a “24 hour safety ellipse” or “24 hour safe trajectory,” meaning that if anything went wrong, mission control would have at least 24 hours to fix it before Dragon and the I.S.S. collided.
So remember, friends: the next time you’re going to dock with another spacecraft, do that out-of-plane elliptical periodic relative motion thing. In other words, circle around the other ship a few times before making your final approach to dock. It’s for safety reasons.
P.S.: It’s also for enjoying the view. Spaceships are pretty!
Planet Pailly 
I got some terms from the Dragon docking to ISS too: I like ‘soft capture ring’
LikeLiked by 1 person
Listening to these sorts of things is a great way to pick up the lingo. Audio recordings from the Apollo missions are also really interesting.
LikeLiked by 2 people
I thought it was Michael Collins that talked about having out-of-plane errors on Gemini resulting in a ‘whifferdill’ movement that would allow docking only with a brute-force drive to the target (which the fighter pilots wanted to do anyway, but used lots of fuel and lost them lots of bets).
LikeLiked by 1 person
I hadn’t heard about that. I’m sure there were a lot of learning experiences on the Gemini missions, though.
LikeLike