Catching up with a 17,500 mph Space Station

When the Antares rocket took off from NASA's Wallops Flight Facility in Virginia this past Thursday, its Cygnus payload didn't go straight to the International Space Station. The fact is that very few vehicles destined for the ISS go straight from launch to the station. A typical trip to the station is a multi-day voyage and it's only been in the past year that manned flights have begun taking an expedited path and timing that can get crews there within 6 hours of launch.

To understand why we don't usually go from ground to station in one quick trip there is a key concept to know first: in orbit, speed equals altitude. 

The basic concept of an orbit is that an object is moving parallel to the surface of the Earth. Gravity is still pulling on that object so it's in a constant state of "falling", but the object is moving so quickly that by the time it reaches the horizon the surface of the planet has curved down and away from the object. So if that object, such as a Cygnus capsule, keeps going fast enough it will "fall" indefinitely and remain in orbit around the planet!

Hopefully this terrible illustration above helps to visualize the concept. As a projectile (the black circle) moves faster it travels farther along the horizon before finally falling to the ground (the black line).

This second terrible illustration shows that with enough speed the projectile would move forward and downward at the same rate as the surface of the planet, creating an orbit. Because of this concept we have to operate differently in space when trying to go "up", or away from the planet. A spacecraft can't just point away from the planet and fire its engine because it would actually slow down. What it needs to do is fire its engine in the direction of travel! If it goes faster it won't just move parallel to the horizon but actually overshoot the path of the ground. Hm...this may seem confusing. Perhaps its time for yet another awful illustration?

On the left side we see an object orbiting a blue planet. To get that object farther from the planet it fires its engines, pushing it in the direction of the arrow. Eventually it moves so quickly that its path (the red curved line) gets wider than it was before and it moves outward to the second orbit seen on the right side of the diagram.

Now that you have this concept in mind I can tell you that the reason why a vehicle doesn't usually go directly to its destination orbit is because a higher orbit means a faster orbit and a faster orbit means a bigger rocket to give it the "get up and go." Bigger rockets are more complex, heavier, and more expensive so their use is a potential hindrance to regular and affordable launches. But wait! Science to the rescue! Once the vehicle is in orbit it has shed the weight of the rocket that carried it there and its in a vacuum so there's no air resistance. This means a rocket motor is MUCH more efficient in space than in our atmosphere, allowing the Cygnus, in this case, to use its own on-board rocket motor to fire several times and incrementally increase its orbit, which during the ongoing Orb-1 mission calls for 5 "Delta V" (change in velocity) burns that take the vehicle from its original orbit of 134 miles to 226 miles. This stops Cygnus four miles beneath the space station where it then makes a slow, controlled approach. That approach occurred this morning (Jan 12) around 6:00 am EST and by 8:00 am the astronauts aboard the station used the Canadarm 2 robotic arm to grapple and reposition the vehicle for final docking.