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Type of aerostat that gains buoyancy from air heated by the Sun

A solar balloon is a balloon that gains buoyancy when the air inside is heated by solar radiation, usually with the help of black or dark balloon material. The heated air inside the solar balloon expands and has lower density than the surrounding air. As such, a solar balloon is similar to a hot air balloon. Usage of solar balloons is predominantly in the toy market, although it has been proposed that they be used in the investigation of planet Mars, and some solar balloons are large enough for human flight. A vent at the top can be opened to release hot air for descent and deflation.

Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter (W/m2). Direct insolation is the solar irradiance measured at a given location on Earth with a surface element perpendicular to the Sun's rays, excluding diffuse insolation (the solar radiation that is scattered or reflected by atmospheric components in the sky). Direct insolation is equal to the solar constant minus the atmospheric losses due to absorption and scattering. While the solar constant varies with the Earth Sun distance and solar cycles, the losses depend on the time of day (length of light's path through the atmosphere depending on the Solar elevation angle), cloud cover, moisture content, and other impurities. Over the course of a year the average solar radiation arriving at the top of the Earth's atmosphere is roughly 1,366 watts per square meter (see solar constant). The radiant power is distributed across the entire electromagnetic spectrum, although most of the power is in the visible light portion of the spectrum. The Sun's rays are attenuated as they pass through the atmosphere, thus reducing the insolation at the Earth's surface to approximately 1,000 watts per square meter for a surface perpendicular to the Sun's rays at sea level on a clear day. A black body absorbs all the radiation that hits it. Real world objects are gray objects, with their absorption being equal to their emissivity. Black plastic might have an emissivity of around 0.95, meaning 95 percent of all radiation that hits it will be absorbed, and the remaining 5 percent reflected.

The system is in equilibrium when the energy lost from the balloon through convection, radiation and conduction, equals the energy received through radiation from the Sun.

In 1972, Dominic Michaelis, a British architect and the inventor of many solar utilities and projects, invented and built the first solar balloon, with a clear external surface and dark, heat catching internal walls.

The first human carrying pure solar balloon flight was made on 1 May 1973 by Tracy Barnes in his balloon 'Barnes Solar Firefly Tetrahedron'. This balloon was made from a spiral tube of fabric that was formed into a tetrahedron. Dominic Michaelis is recorded as having owned the first pure solar balloon in Europe. This balloon was flown by Julian Nott across the English Channel. Records compiled for the FAI show that on 6 February 1978 Iranian Frederick Eshoo also made a solar flight in a balloon called Sunstat. This used a standard balloon design, but used clear plastic on one side, allowing the Sun's radiation to reflect off the inner surface, heating the inside air.

The first 100% solar weather probe, named Ballon ORA, was launched from the French Antarctic Dumont d'Urville Station in January 2011 by a joint team of students, scientists and engineers. The idea was to assess the feasibility of using solar balloons as probes in remote area, where saving the use of lifting gas, helium or hydrogen, would be precious. The flight was a success, approaching 46000 feet (14021 m). The savings do not only concern the lifting gas in itself. The ORA Balloon alleviates the need for the transportation, in and out, of the heavy gas canisters.

California Institute of Technology's Jet Propulsion Laboratory has conducted a study on the use of solar balloons on several planets and moons in the solar system, concluding they are a viable option for Mars, Jupiter and Saturn.

Planning and airspace permission may be required by local or national airspace authorities. Manned flights carry special risks. Unexpected clouds pose a serious risk, akin to regular hot air ballooning without reserve fuel. Solar balloons can descend rapidly when cooling occurs, making ballast very important.