It’s been a while since we did this.
Aside from it simply being great to see the ISS pass over, with the launch of a SpaceX Cargo Draon less than four hours ago I was hoping that I might see the Dragon following the ISS.
The reason that Dragons, Cargo and Crew, have an “instantaneous” launch window is because they launch into the ISS’s orbital plane as the instant that the orbital plane crosses over the launch pad. Getting high and getting fast are critical to rendezvous with an orbital target, but you’re maneuvering in three dimensions. Going “sideways” once in orbit, switching from one orbital plane to another, can be expensive in terms of fuel. But that orbital plane will cross right over you twice a day, so if you launch right then, you just have to go high and fast, not sideways. Restricting your chase to two dimensions simplifies the rendezvous considerably in terms of both complexity and fuel costs.
Because of that, when two objects in the same orbital plane pass overhead, you’ll see them playing follow-the-leader. If you see a set of Starlink satellites within a day or two of launch they’ll look like a string of pearls sailing across the sky. Similarly, if you see the ISS just after a Dragon, or Soyuz, or Cygnus has just left, or just before it arrives, you’ll see the bright ISS with the dimmer, smaller spacecraft following the same path.
It’s math. Physics. Orbital Mechanics!
Tonight, alas, the Dragon probably isn’t close enough since it just launched and won’t catch up to ISS until Saturday morning. Dragon will be in that orbital plane and will be cruising along that same path, but by the time it happens it won’t be in sunlight above Los Angeles. There’s another pass tomorrow night at 22:01, but it will be low to the horizon and might not even get above the level of those trees. But we’ll see what we can see. For now, enjoy tonight’s pass!