🛰️ Satellites & Spacecraft

Scientific Overview & Physics

A satellite is any object placed in orbit around a larger body. Today more than 10,000 active spacecraft orbit Earth for communications, navigation, weather forecasting, science, and defense. The ISS is the largest and most-visited crewed spacecraft ever built.

How It Works

A satellite stays in orbit through a continuous balance between two forces: gravity constantly pulling it toward Earth, and its own forward velocity constantly carrying it sideways. Rather than falling straight down or flying off into space, a satellite moving at the right speed for its altitude perpetually "falls" around the curve of the Earth, tracing a stable orbital path. It's useful to distinguish between different kinds of spacecraft: satellites remain in orbit and typically operate without a crew, performing tasks like imaging, communication relay, or navigation; probes travel beyond Earth orbit to study other planets, moons, or the wider Solar System, usually without ever returning; and crewed spacecraft, like the International Space Station or the vehicles that carry astronauts to and from it, are specifically designed to support human life in space. Orbital altitude determines a satellite's job: Low Earth Orbit, a few hundred kilometers up, is used for Earth-imaging and the ISS, and requires an orbital period of only about 90 minutes; Geostationary Orbit, at nearly 36,000 kilometers, lets a satellite orbit at exactly the same rate Earth rotates, so it appears to hover permanently over one spot on the ground, ideal for communications and weather satellites. Every satellite in Low Earth Orbit experiences a small amount of atmospheric drag from the extremely thin outer edges of Earth's atmosphere, which very gradually pulls its orbit lower over time unless it periodically boosts itself back up.

What You Can See or Experience

The International Space Station is bright enough to see with the naked eye as it passes overhead just after sunset or before sunrise, appearing as a fast-moving, steady point of light — brighter than any star — that crosses the sky in a few minutes without blinking, distinguishing it from an aircraft. Many other satellites, including bright Starlink chains freshly launched into a temporary "train" formation, are also visible under the right conditions. Because visibility depends on precise orbital geometry and the satellite passing through sunlight while the ground below is dark, checking a real-time pass predictor for your specific location is the only reliable way to know exactly when and where to look on a given night.

Key facts

• •Low Earth Orbit (LEO): 160–2,000 km altitude
• •Geostationary Orbit (GEO): 35,786 km altitude
• •Active satellites (2024): >10,000
• •ISS altitude: ~420 km, orbits Earth every ~90 min
• •Starlink constellation: >6,000 satellites and growing

Why it matters

Satellites are directly exposed to space weather. Solar storms heat and expand the upper atmosphere, dragging LEO satellites down faster; energetic particles can flip bits in electronics; and geomagnetic storms have brought down entire Starlink launches.

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Conclusion

From weather satellites and GPS to the crewed International Space Station, orbiting spacecraft have become quietly essential infrastructure for modern life — and every one of them sits directly in the path of the space weather this site tracks, making satellite operators some of the most attentive readers of solar activity forecasts.

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