For the purposes of planning Earth observing photography or remote sensing, there are four important points about the orbits of the ISS. Particulars of the orbits depend on the exact altitude of the station, and the exact altitude depends on the frequency that the station is reboosted to a higher orbit.
FACT 1
The station travels from west to east on an orbital inclination of 51.6 degrees. Each orbit takes 90-93 minutes, depending on the exact altitude of the ISS. During that time, part of the Earth is viewed under darkness and part under daylight. The ISS orbital altitude drops gradually over time due to the Earth's gravitational pull and atmospheric drag. Periodic reboosts adjust the ISS orbit. As the ISS orbital altitude decays, the orbit tracks on Earth change slightly.
FACT 2
With each orbit taking 90-93 minutes, there are approximately 16 orbits per day (24 hours). The exact number of orbits per day is usually less than 16 (generally 15.5 to 15.9 orbits/day) depending on the altitude of the ISS. Each orbit shifts to the west by approximately 22.9° of longitude (measured as the position where the orbit crosses the equator).
FACT 3
There is an approximate repeat of orbit tracks over the same area on the ground every 3 days. Again, the ISS altitude will determine how closely the tracks repeat.
FACT 4
The part of the Earth visible to ISS astronauts in daylight changes due to the interaction between the orbit patterns of the station and the rotation of the Earth. The daylight portion of the orbits shifts slightly eastward along track each day. This lighting procession follows an approximate 63-day cycle from a descending track covering the mid-latitudes, to southern hemisphere lighting, to ascending tracks, to northern hemisphere lighting. This cycle, plus seasonal changes in solar incidence, cause the sun illumination angles to vary every time the station passes over a given region.
Cynthia A. Evans and Julie A. Robinson, Earth Sciences and Image Analysis, NASA Johnson Space Center
As an enthusiast with a profound understanding of Earth observing photography and remote sensing, particularly in the context of the International Space Station (ISS) orbits, I aim to provide a comprehensive insight into the four crucial points outlined in the article by Cynthia A. Evans and Julie A. Robinson from NASA Johnson Space Center.
FACT 1: Orbital Characteristics
The ISS follows an orbital inclination of 51.6 degrees, traversing from west to east. The orbital duration ranges between 90-93 minutes, contingent upon the ISS's exact altitude. During each orbit, a portion of the Earth is observed under darkness, while another is bathed in daylight. The gradual decline in the ISS's orbital altitude due to Earth's gravitational pull and atmospheric drag necessitates periodic reboosts to maintain the desired orbit. This decay influences the tracks on Earth as the ISS orbits.
FACT 2: Orbital Frequency
With an orbital period of 90-93 minutes, there are approximately 16 orbits per day. However, the exact number varies slightly (generally between 15.5 to 15.9 orbits/day), contingent upon the ISS's altitude. Notably, each orbit shifts westward by around 22.9° of longitude as it crosses the equator.
FACT 3: Ground Track Repeat
The ISS exhibits an approximate repeat of its ground tracks over the same area every 3 days. The degree of repetition is influenced by the ISS's altitude, adding a layer of complexity to the planning of Earth observing photography.
FACT 4: Sun Illumination Variation
The visible portion of Earth to ISS astronauts in daylight undergoes changes due to the interplay between station orbit patterns and Earth's rotation. This results in a gradual eastward shift of the daylight portion of the orbits. Remarkably, a 63-day cycle characterizes this lighting procession, transitioning from a descending track covering mid-latitudes to southern hemisphere lighting, ascending tracks, and finally, northern hemisphere lighting. Additionally, seasonal changes in solar incidence contribute to varying sun illumination angles during each pass over a specific region.
In summary, understanding these four facts is crucial for planning Earth observing activities or remote sensing initiatives involving the ISS. The intricacies of the ISS orbits, influenced by altitude, decay, and Earth's rotation, highlight the need for precise planning to optimize observational outcomes.
