Why does NASA sometimes schedule a rocket launch for the middle of the night, or aim for a lift-off time when weather is notoriously unlikely to cooperate? The best time to start a mission is based on a blend of factors: the flight’s target and goals, the needs of the spacecraft, the type of rocket, and the desired trajectory, which refers to the path the vehicle and spacecraft must take to successfully start the mission. The dynamics change from mission to mission, and determining the launch period and launch window is an important part of the overall flight design.
The most significant deciding factors in when to launch are where the spacecraft is headed, and what its solar needs are. Earth-observing spacecraft, for example, may be sent into Low Earth Orbit (LEO). Some payloads must arrive at a specific point at a precise time, perhaps to rendezvous with another object or join a constellation of satellites already in place. Missions to the Moon or a planet involve aiming for a moving object a long distance away.
Let’s recall that launches involve many elements: the payloads that give missions their names, the rockets that will carry them into outer space, launch dates and times, and where the launch will take place. In the context of spaceflight (ballistic flight into or through outer space), a launch period is the collection of days and a launch window, a term used to describe a time period in which a particular mission must be launched, is the time period on a given day during which a particular space vehicle must be launched in order to reach its intended target. If the rocket is not launched within a given window, it has to wait for the window on the next day of the period.
If a spacecraft intends to rendezvous with another spacecraft, a planet, or other point in outer space, the launch must be carefully timed so that the orbits overlap at some point in the future. If the weather is bad or a malfunction occurs during a launch window, the mission must be postponed until the next launch window appropriate for the flight. Why does the launch schedule change so frequently?
A launch depends on the success of a wide variety of considerations: payload, launch vehicle, communications, launch pad, weather and good timing. Problems with any of these can potentially result in an adjusted launch window. Another important factor is the whether the “range” is available to support launch. When a launch is postponed, or “scrubbed” during the countdown, the nature of the problem determines how long the delay will last. Schedule changes are a normal part of the space business and reflect a commitment to launch safety.
Kris Walsh, former Boeing Director of NASA Expendable Launch programs, describes the difference between launch period and launch window. The launch period is the number of days specified in which a mission can be launched to arrive at the proper orbit within a specific time period. The launch window occurs each day within the launch period:
“The launch period is how many days we can launch a mission to the proper orbit. For Low Earth Orbit (LEO), basically there’s no concern. That launch period is three hundred and sixty-five days a year. To go to Mars, or to go to L1 such as Genesis, or to an asteroid, you’re severely limited because you need a certain energy to be imparted to that satellite. They might use another planet, or the Moon to get into the proper orbit. So we work with the satellite provider. We work with the National Aeronautics and Space Administration (NASA) organisation that gives us targets; we get the satellite to that target with a certain energy and then the mission designers take over from there”.
“The launch window is on a day-to-day basis. And that can range from one second to over an hour. All our Mars missions were one-second windows and we got those all off in the first or second day. When we do have twelve minutes or more, if we have a problem with the launch countdown, we can save the vehicle, recycle, and attempt again on the same day. If we have a one-second window, if we have any problems in the last four minutes of the countdown, we are down for twenty-four hours”.
Launch periods and launch windows are very dependent on both the rocket’s capability and the orbit to which it is going. A launch period refers to the days that the rocket can launch to reach its intended orbit. A mission could have a period of three hundred and sixty-five days in a year, a few weeks each month, a few weeks every twenty-six months (Mars launch periods), or a short period time that won’t be repeated.
A launch window indicates the time frame on a given day in the launch period that the rocket can launch to reach its intended orbit. This can be as short as a second (referred to as an instantaneous window) or even the entire day. For operational reasons, the window almost always is limited to no more than a few hours. The launch window can stretch over two calendar days; launch windows are rarely exactly the same times each day.
As a good image, let’s look at ESA’s example: “Imagine the Solar System as an athletics race track. If you were watching the 400 metres race from the centre of the track and wanted to intercept one of the runners taking part, one way would be to simply chase the runner you wish to stop. If you were fast enough, you might eventually catch up but only after expending a lot of energy and travelling a long way. A much better way to intercept your athlete is simply to walk across the centre to the other side of the circular track. It is a much shorter distance and you use a lot less energy and time getting there. You calculate your walk so that you arrive at the other side of the track at the same time as they do. Too early and you are waiting around for them. Too late and you have missed them completely – you’d have to wait one lap until they came around again”.