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How did Sputnik I improve scientists’ ability to measure Earth’s shape?

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Diagram showing the deflection of the vertical from the geoid to the ellipsoid or spheroid, with labeled lines for geoid, spheroid, rock surface, ocean, and geoid height in a geology context. — geoid Deflection of the vertical from the geoid to the spheroid.

The launch of the Soviet Sputnik I in 1957 revolutionized the study of Earth's shape and its gravity field by providing a new method for observing the planet's physical characteristics from orbit.1

Prior to the satellite era, scientists relied on ground-based measurements to estimate Earth's figure. Sputnik I changed this by allowing researchers to observe how the planet's gravitational field perturbed the satellite's orbit. Because Earth is not a perfect sphere but rather an oblate spheroid that bulges at the Equator, its mass distribution is not uniform. As a satellite passes over the equatorial bulge, it experiences gravitational forces that cause its orbital plane to precess.12

By tracking these orbital changes, scientists were able to calculate a new, more accurate value for the flattening of the ellipsoid within weeks of the launch, superseding all previous estimates. This breakthrough established the use of orbiting satellites as a primary tool for refining the geoid—the model of Earth's shape that coincides with mean sea level—and mapping the complexities of the planet's gravitational potential.13