What Are Small Satellites?

Small satellites are spacecraft with a mass below 500 kilograms, a threshold that separates them from conventional large satellites that historically required full dedicated launch vehicles and multi-year development programs. Within this broad category, the community recognizes several sub-classes by mass: minisatellites from 100 to 500 kilograms, microsatellites from 10 to 100 kilograms, nanosatellites from 1 to 10 kilograms, picosatellites below 1 kilogram, and femtosatellites below 100 grams. The most widely adopted standardized form within the nanosatellite range is the CubeSat, defined by a unit dimension of 10 centimeters by 10 centimeters by 11.35 centimeters and a per-unit mass limit of 2 kilograms, with common configurations scaling from 1U through 12U and beyond.

Small satellites emerged as a serious engineering platform in the late 1990s through a joint initiative by California Polytechnic State University and Stanford University, who developed the CubeSat standard to give university research groups affordable access to the space environment. The combination of commercial off-the-shelf components, standardized deployment hardware, and rideshare launch arrangements on rockets carrying primary payloads reduced per-mission costs by orders of magnitude compared to traditional satellite procurement, opening space experimentation to research institutions, startups, and government agencies with constrained budgets.

Platform Architecture and Subsystems

A small satellite integrates all spacecraft functions into a compact, mass-limited package. The electrical power subsystem typically uses body-mounted or deployable gallium arsenide or silicon solar panels feeding lithium-ion batteries, sized to sustain operations through eclipse periods. The attitude determination and control subsystem uses combinations of reaction wheels, magnetorquers, and star trackers to achieve pointing accuracy from a few degrees for technology demonstrations to arcseconds for Earth observation imagers. Communications rely on UHF and VHF links for telemetry and command at lower data rates, with S-band, X-band, or optical downlinks used when mission data volumes require higher throughput. Research on communication subsystem design for CubeSat missions has characterized the trade-offs between link margin, antenna gain, and the power constraints inherent in small platforms.

Propulsion and Orbit Raising

Early CubeSats were launched without propulsion, accepting the orbital altitude and inclination delivered by the rideshare. As mission requirements have grown more demanding, miniaturized propulsion systems have become essential for orbit raising, station keeping, formation flying, and deorbit compliance. Available technologies include cold-gas thrusters, electrospray and pulsed plasma thrusters, miniaturized ion engines, and green monopropellant systems using compounds such as ammonium dinitramide to replace conventional hydrazine. Purdue University researchers have demonstrated safer, inexpensive propulsion approaches for small satellites that reduce ground handling hazards while delivering useful thrust levels for orbital maneuvering. Propulsion capability transforms a passive CubeSat into an active platform capable of multi-year operational lifetimes in low Earth orbit altitudes where atmospheric drag would otherwise limit orbital residence to months.

Constellations and Coordinated Operations

Individual small satellites face limitations in coverage, data rate, and observation revisit time that single large satellites can often meet with higher power and larger apertures. The response has been the proliferation of small satellite constellations, in which hundreds to thousands of spacecraft distributed across multiple orbital planes collectively deliver continuous global coverage for broadband connectivity, Earth observation, and environmental monitoring. Commercial operators such as those catalogued in the Nanosats Database of satellite constellations and technologies have deployed or announced megaconstellations numbering in the thousands, requiring new approaches to spectrum coordination, space traffic management, and deorbit planning. The ESA program on CubeSats and small satellites tracks the evolution of these platforms from educational demonstrations into operational infrastructure nodes.

Applications

Small satellites have applications across a range of scientific, commercial, and government mission areas, including:

• Earth observation and remote sensing for agriculture, disaster response, and environmental monitoring
• Global broadband internet access through low-Earth orbit communication constellations
• Space weather monitoring and scientific experiments in heliophysics and geophysics
• Technology demonstration for components and systems intended for larger or deep-space missions