Beta angle

[1] The beta angle determines the percentage of time that a satellite in low Earth orbit (LEO) spends in direct sunlight, absorbing solar radiation.

An imaginary observer standing on the Sun defines a beta angle as positive if the satellite in question orbits in a counterclockwise direction and negative if it revolves clockwise.

[4] The maximum amount of time that a satellite in a normal LEO mission can spend in Earth's shadow occurs at a beta angle of 0°.

This allows the object to spend the maximum possible amount of its orbital period in the Earth's shadow, and results in extremely reduced absorption of solar energy.

[1]) On the other extreme, an object launched into an orbit parallel to the terminator results in a beta angle of 90 degrees (

Beta angle can be controlled to keep a satellite as cool as possible (for instruments that require low temperatures, such as infrared cameras) by keeping the beta angle as close to zero as possible, or, conversely, to keep a satellite in sunlight as much as possible (for conversion of sunlight by its solar panels, for solar stability of sensors, or to study the Sun) by maintaining a beta angle as close to +90 or -90 as possible.

The value of a solar beta angle for a satellite in Earth orbit can be found using the equation

The RAAN and inclination are properties of the satellite's orbit, and the solar longitude is a function of Earth's position in orbit around the Sun (approximately linearly proportional to day of year relative to the vernal equinox).

That same satellite also will have a beta angle with respect to the Sun, and in fact it has a beta angle for any celestial object one might wish to calculate one for: any satellite orbiting a body (i.e. the Earth) will be in that body's shadow with respect to a given celestial object (like a star) some of the time, and in its line-of-sight the rest of the time.

When the orbiter was in-flight (not docked to ISS) and it flew to a beta angle greater than 60 degrees, the orbiter went into "rotisserie" mode, and slowly rotated around its X-axis (nose to tail axis), for thermal regulation reasons.

[6] Therefore, the mission duration affected launch timing when the beta cutout dates were approaching.