"Heliosynchronous orbit" is another name for a sun-synchronous orbit, from the Greek word helios for sun. It is one of the most important orbits in spaceflight, because most Earth observation satellites use it. This article explains the term, how the orbit works and what it means in practice.
Definition
According to ESA, a sun-synchronous orbit is a particular kind of polar orbit in which satellites are in sync with the Sun. They always appear in the same position relative to the Sun, which means they pass over the same spot on Earth at the same local time every day. ESA gives the example of a satellite passing over Paris daily at noon.
How it works
A heliosynchronous satellite flies roughly from pole to pole, while the Earth rotates underneath. The orbital plane slowly turns so that it keeps pace with the Earth's yearly motion around the Sun. NASA's Earth Observatory describes the effect: whenever and wherever the satellite crosses the equator, the local solar time on the ground is always the same.
An example: NASA's Terra satellite
NASA uses its Terra satellite to illustrate this. Terra always crosses the equator at about 10:30 in the morning local time, for example over Brazil. On its next pass about 99 minutes later, it crosses the equator over Ecuador or Colombia, again at about 10:30 local time. Terra launched in 1999 and completed its 100,000th orbit on 6 October 2018, as shown on the cover image.
Typical altitude and speed
According to ESA, a satellite in a sun-synchronous orbit usually flies at an altitude of between 600 and 800 kilometres. At 800 kilometres, it travels at approximately 7.5 kilometres per second.
Why heliosynchronous orbits matter
- Consistent lighting: NASA explains that the orbit keeps the angle of sunlight on the Earth's surface as consistent as possible, although it changes with the seasons. Scientists can compare images from the same season over several years.
- Global coverage: as a type of polar orbit, it lets a satellite observe almost the whole planet over time.
- Dawn dusk variant: ESA notes that some sun-synchronous satellites ride along the boundary between day and night, in constant dawn or dusk.
Launching into a heliosynchronous orbit
Because the orbit is near polar, launches head north or south rather than east. This is why high latitude spaceports such as Andøya in Norway focus on polar and sun-synchronous missions, see our overview of European launch sites. For booking details, read our guide to the sun-synchronous orbit.
Sources
Cover image: View recorded by the MISR instrument as NASA's Terra satellite passed its 100,000th orbit, October 2018. Credit: NASA/JPL-Caltech (PIA22836), public domain.