What are Solar Sails and Solar Kites?

Some of you might remember Franklin’s Kite, the historic experiment conducted by Benjamin Franklin in 1752. Using a kite with a wet hemp string coming down to a key and a Leyden jar, Franklin could demonstrate that lightning is an electrical discharge from the thunder clouds. He could reproduce the electrical spark from the electric charge conducted down from the lightning clouds. That was a pivotal moment in understanding the nature of lightning and later led to development of lightning rods for protecting installations from lightning. Similar to that, in the context of a Solar Storm disturbing the Amateur Radio bands, N4JAW posted on X that this would be the best time to fly a Solar Kite!

As usual, that prompted a read up and I came across Solar Sails and Solar Kites which are a reality and not just hype. Solar sails, also known as light sails and photon sails, utilize the radiation pressure exerted by sunlight on large surfaces for spacecraft propulsion. IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) by Japan Aerospace Exploration Agency (JAXA) was the first spacecraft to make use of the technology in 2010. Launched on 20 May 2010, IKAROS flew by Venus at a distance of 80,800 km, on 8 December 2010. Thus it successfully completed its planned mission and entered its extended operation phase.

The Solar Sail of IKAROS made of polyimide measured 20m diagonally and had a mass of 2 kg. A thin film solar array was embedded in the sail. Based on the last data received from IKAROS in May 2015, it was calculated that it was at distance of 110 million km from Earth and 130 million km from the Sun. That is a fantastic performance for the first ever Solar Sail propelled spacecraft!

Though Solar Kites are supposed to be much smaller compared to Solar Sails, one project proposed at Worcester Polytechnic Institute some time back was to be a rectangle with 150 sq. m area! Kite would be covered with flexible photovoltaic cells and attached to a copper wire reinforced with carbon fiber and the cable attached to a generator on the ground. The kite would harness solar energy at a height of about 8 km. Seven of such kites together would make over 1 MW of energy. Though it appears like an excellent energy source, the cost of long cables and need for a ‘no fly zone’ were impediments to practical implementation.