Solar flight in space
15 June 2010
Last week I wrote about the Government’s commitment to the space industry, hoping that it knew why it was being committed (in a manner of speaking).
A number of people wrote in with different viewpoints to 'Space sector gets lift-off', some backing space exploration as a worthy pursuit in its own right. Others point out that the space industry is one of the drivers for technological advancement – for some reason the development of non-stick frying pans seems to be the key advancement directly attributable to the space industry, but I assume there are other breakthroughs of more significance.
Maybe news that the Japanese have spread their solar wings is such an advancement. Solar energy being used to provide photon pressure power allowing travel through the vacuums of space. The Japan Aerospace Exploration Agency (JAXA) began to deploy the solar sail of the Small Solar Power Sail Demonstrator ‘IKAROS’ on 3 June. On 10 June, it was successfully expanded and was generating power through its thin film solar cells at about 770 km from the Earth.
The IKAROS was launched on 21 May 2010 from the Tanegashima Space Center. The aim is to measure and observe the power generation status of the thin film solar cells, accelerate the satellite by photon pressure, and verify the orbit control through that acceleration. Through these activities, the ultimate goal is to acquire navigation technology through the solar sail.
A Solar Sail gathers sunlight as propulsion by means of a large membrane while a Solar ‘Power’ Sail gets electricity from thin film solar cells on the membrane in addition to acceleration by solar radiation. If the ion-propulsion engines with high specific impulse are driven by such solar cells, it can become a ‘hybrid’ engine that is combined with photon acceleration to achieve fuel-effective and flexible missions.
JAXA is studying two missions to evaluate the performance of the solar power sails. The project name for the first mission is IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun). This craft was launched with the Venus Climate Orbiter ‘AKATSUKI’, using an H-IIA launch vehicle. This will be the world's first solar powered sail craft employing both photon propulsion and thin film solar power generation during its interplanetary cruise.
A solar sail can move forward without consuming propellant as long as it can generate enough energy from sunlight. This idea was born some 100 years ago, but it had lots of technical hurdles such as the appropriate material and deployment method for the sail. Recent progress has finally seen some prospect of using this technology practically. The sail of the IKAROS is a huge square some 20 metres in a diagonal line, as thin as 0.0075 mm, and made from polyimide resin. On the membrane of the sail are not only thin film solar cells but also an attitude control device and scientific observation sensors. This thin and light solar sail membrane will be deployed using the centrifugal force of spinning the main body of the IKAROS before its tension is maintained. The deployment is in two stages. The first stage is carried out quasi-statically by the onboard deployment mechanism on the side of the main body. The second stage is the dynamic deployment. As this deployment method does not require a strut such as a boom, it can contribute to making it lighter, thus can apply to a larger membrane.
My first thoughts about this is that it sounds great if the limit of our space travels is to be no further than, say, Mars, possibly Jupiter or at a push Saturn. But surely there is a limit when solar energy cannot be generated because the solar source is too far away. However, the technology being used by JAXA is clearly pioneering and maybe, returning to the theme at the beginning of this column, there may be practical applications closer to home.
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