Final answer:
The shadow on the ISS can be caused by spacecraft above it, an astronaut, or other objects between the ISS and the Sun. Clouds from Earth cannot cast shadows on the ISS. The ISS remains in orbit due to its high horizontal velocity which matches the curvature of the Earth. the correct option is d.
Step-by-step explanation:
The shadow cast upon the International Space Station (ISS) can be due to various objects obstructing the sunlight. When considering what could create a shadow, we must identify which objects are in a direct line between the ISS and the Sun.
Clouds from Earth, though they can cast shadows on Earth's surface, are not responsible for casting shadows on the ISS because the space station orbits above the Earth's atmosphere. The spacecraft hovered above it could indeed cast a shadow if it were between the ISS and the Sun. An astronaut could also cast a shadow while on a spacewalk if they pass directly between the Sun and the ISS.
Given these considerations, the correct option in the final answer would be D. All of the above. However, when discussing what keeps the ISS in orbit despite the presence of gravity, we must understand that the ISS, like all satellites, is in continuous free fall towards Earth. It has a high horizontal velocity that keeps it in orbit because the curve of its fall matches the curvature of the Earth, thus it never collides with the Earth's surface.