Sciency Words: Safety Ellipse

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:


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!

SpaceX Boldly Goes Where No Private Company Has Gone Before

Last week, SpaceX successfully launched its Dragon Capsule and docked it at the International Space Station.  They are the first private company to ever send a spacecraft to the ISS.  It’s a historic moment, a definite cause for celebration.  This marks the beginning of a new era of corporate space exploration.  But is the corporate take over of space a good thing?

Space exploration should be motivated by our sense of wonder.  We should do this not because of greed but because we want to learn more about the universe we live in, want to see it and experience it in all its beauty.  As humanity spreads across the stars, I hope we retain the romantic ideals espoused by science fiction like Star Trek.

Elon Musk, the CEO and Chief Designer at SpaceX, reportedly wants his company to someday take people to Mars and other planets in the Solar System.  I’d love nothing more than to see him succeed.  But I remember a science fiction movie where the first spaceship to land on Mars was plastered with ads like something out of NASCAR.  American commercialism has a way of cheapening things, and I worry about that.

Money is what makes our society work.  Maybe by turning space exploration over to the private sector, we can return to the Moon, go to Mars, and eventually leave our Solar System a little sooner.  After all, Congress has cut NASA’s budget so much they can’t even afford their own Space Shuttle.  If NASA can’t do this, someone else has to take their place.

What do you think?  Are you glad the private sector is taking over space exploration?