Watching the preparations for the launch of the Crew Dragon to the International Space Station on Wednesday, I was also watching the skies. A tropical storm had already barreled into North Carolina that morning and the skies over the Florida Space Coast didn’t look too conducive to a rocket launch.
Sure enough, just a few minutes prior to launch, the mission was called off. And watching the wind down and removal of the astronauts from the capsule later, my neighbor, who had come over to watch with me, wondered why they couldn’t just wait an hour or so until the skies cleared up.
The two most critical parts of a space mission are the launch and landing, or in the case of the Crew Dragon, the docking with the ISS. SpaceX put the Crew Dragon capsule through many tests, including 27 drops of the parachute landing system. The space company also accomplished an emergency separation of the Dragon capsule from the rocket.
In the event of an emergency or malfunction of the rocket, the Dragon capsule’s eight engines would fire, lifting the capsule with the astronauts up and away from the rocket where parachutes would deploy, helping the capsule to make a safe landing. The Falcon 9 rocket has made 83 successful launches.
The science behind the “Launch Window”
First of all, the space station has an orbital velocity of 7.7 kilometers (4.8 miles) per second. That is very fast, in anyone’s book. So if the Crew Dragon wants to rendezvous with the ISS, the launch must be carefully timed so that the orbits overlap at some point in the future.
Launch periods and launch windows are very dependent on both the rocket’s capability and the orbit to which it is going. A launch window can be just a few seconds to as much as several hours, according to NASA.
Launch windows and days are usually calculated in UTC and then converted to the local time of where the rocket and spacecraft operators are located. Coordinated Universal Time (or UTC) is the primary time standard by which the world regulates clocks and time. It is within about 1 second of mean solar time at 0° longitude and is not adjusted for daylight saving time.
Here’s where it can get tricky. If the spacecraft intends to rendezvous with an object already in orbit, like the ISS, the launch must be carefully timed to occur around the times that the target vehicle’s orbital plane intersects the launch site.
The orbital plane
An orbital plane is an imaginary, gigantic flat plate containing an Earth satellite’s orbit. The orbital plane passes through the center of Earth. The orbital plane of a revolving body is the geometric plane in which its orbit lies. Three non-collinear points in space suffice to determine an orbital plane.
The ISS orbits around 400 Kilometers (248.5 miles) above Earth, on an orbit that is inclined to the Earth’s equator by 51.6 degrees. This is why the station can be seen from most locations across Earth.
So, the bottom line is that the capsule must match the station’s speed, altitude, and inclination, and it must do it at the correct time so that the two spacecraft find themselves in close proximity to each other.
Keep in mind that the difference in velocity between the ISS and the Dragon capsule must then be near to zero at the point where the orbits of the two spacecraft intersect.
And if all goes well, and it should, the next step will be the astronauts maneuvering the Dragon capsule to the ISS docking port, using a series of small control thrusters arranged around the spacecraft. We must not forget that the ability of the private sector to put astronauts on the ISS or even the Moon is a giant step in space exploration. This is a very exciting moment.
