Zero-drag satellites or drag-free satellites are

satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...

s where the payload follows a

geodesic In geometry, a geodesic () is a curve representing in some sense the locally shortest path ( arc) between two points in a surface, or more generally in a Riemannian manifold. The term also has meaning in any differentiable manifold with a conn ...

path through space only affected by

gravity In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...

and not by non-gravitational

force In physics, a force is an influence that can cause an Physical object, object to change its velocity unless counterbalanced by other forces. In mechanics, force makes ideas like 'pushing' or 'pulling' mathematically precise. Because the Magnitu ...

s such as drag of the residual atmosphere,

light pressure Radiation pressure (also known as light pressure) is mechanical pressure exerted upon a surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of ...

and

solar wind The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...

. A zero-drag satellite has two parts, an outer shell and an inner mass called the ''

proof mass A proof mass or test mass is a known quantity of mass used in a measuring instrument as a reference for the measurement of an unknown quantity. A mass used to calibrate a weighing scale is sometimes called a ''calibration mass'' or ''calibration ...

''. The proof mass floats freely inside the outer shell, while the distance between the outer shell and the proof mass is constantly measured. When a change in the distance between the outer shell and the proof mass is detected, it means that the outer shell has been influenced by non-gravitational forces and moved relative to the proof mass. Thrusters on the outer shell will then reposition the outer shell relative to the proof mass so that its distance is the same as before the external influence changed it. The outer shell thus protects the proof mass from nearly all interactions with the outside that can cause acceleration, except those mediated by gravity, and by following the proof mass, the outer shell (which is to say, the rest of the spacecraft, carrying instruments, etc.) itself follows a geodesic path. One way to think about a zero-drag satellite is to see the shell/proof mass setup as being an

accelerometer An accelerometer is a device that measures the proper acceleration of an object. Proper acceleration is the acceleration (the rate of change (mathematics), rate of change of velocity) of the object relative to an observer who is in free fall (tha ...

, measuring the acceleration of the outer shell. The input from the accelerometer is then used to control the satellites thruster to exactly compensate for the measured acceleration, ensuring that over time the satellite has zero acceleration. Since the proof mass is floating free within the outer shell, neither the initial drag nor the thruster's compensation for it is experienced by the proof mass.

Applications

Zero-drag satellites are used when it is instrumental for the satellite's mission that the payload remains on a near perfect geodesic path. Two such missions were

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

and

Stanford University Leland Stanford Junior University, commonly referred to as Stanford University, is a Private university, private research university in Stanford, California, United States. It was founded in 1885 by railroad magnate Leland Stanford (the eighth ...

Gravity Probe B Gravity Probe B (GP-B) was a satellite-based experiment whose objective was to test two previously-unverified predictions of general relativity: the geodetic effect and frame-dragging. This was to be accomplished by measuring, very precisely, t ...

(2004–2005) created to measure spacetime curvature near the Earth, and the

ESA The European Space Agency (ESA) is a 23-member international organization devoted to space exploration. With its headquarters in Paris and a staff of around 2,547 people globally as of 2023, ESA was founded in 1975 in the context of European ...

GOCE The Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) was the first of European Space Agency, ESA's Living Planet Programme satellite, heavy satellites intended to map in unprecedented detail the Earth's gravity field. The spacecr ...

spacecraft (2009–2013) which measured variations in the Earth's gravitational field. Planned zero-drag satellites include the STEP experiment, and the

LISA Lisa or LISA may refer to: People People with the mononym * Lisa (Japanese musician, born 1974), stylized "LISA" * Lisa, stagename of Japanese singer Lisa Komine (born 1978) * Lisa (South Korean singer) (born 1980) * Lisa (Japanese musician, b ...

and

DECIGO The DECi-hertz Interferometer Gravitational wave Observatory (or DECIGO) is a proposed Japanese, space-based, gravitational wave observatory. The laser interferometric gravitational wave detector is so named because it is designed to be most sen ...

gravitational wave observatories In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force be ...

References

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