Semi-satellite (noun, “qwa-see-sat-lite”)
Semi-satellites are celestial bodies, like asteroids, that seem to orbit a planet akin to genuine satellites. However, these bodies are situated beyond the reach of much of the planet’s gravitational influence.
The prefix “quasi” denotes something akin to. A semi-satellite is similar to a true satellite, but it encompasses more than just one type.
The Moon is an example of a true satellite. Its orbit is primarily governed by Earth’s gravity, despite the Sun exerting a stronger gravitational pull on it. The Moon’s proximity to Earth allows the latter to exert significant control over its orbit, which occurs within what is known as the Earth’s ‘hill sphere’ — the zone around a planet dominated by its gravity. In contrast, the Earth remains within the Sun’s gravitational influence, also referred to as a ‘hill sphere.’
Semi-satellites orbit outside the planetary hill sphere. This does not imply that a planet’s gravity has no effect on these objects; however, the influence of other gravitational forces is more pronounced. For instance, the gravitational pull from the Sun is what governs a semi-satellite’s orbit around Earth, leading to changes in their orbital trajectories over time. Ultimately, they may drift from their orbits around their respective planets.
A semi-satellite will orbit the Sun in a path that may involve circling around its planet at the same time. As seen from space, both the semi-satellite and the planet move around the Sun, resulting in the semi-satellite appearing to loop around Earth. These loops are generally elliptical rather than circular, which may allow the objects to drift far from their planets.
There are several known semi-satellites near Earth, one of which is an asteroid named Chaldea, measuring less than 300 meters (985 feet). Astronomers predict that Chaldea might be ejected from its orbit in approximately 600 years. Another example is Kamooalewa, which scientists believe could be a fragment of the Moon. In May 2025, the China National Space Agency initiated a mission to collect samples from such objects.
In a sentence
A small semi-satellite circles the Earth as both travel around the Sun.
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Source: www.snexplores.org