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The technology we use for space exploration gets more advanced all the time, but some of our most ambitious programs actually rely on optics invented in the 19th century for lighthouses.

Hosted By: Hank Green

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[♪ INTRO] Star Trek envisioned a high-tech  future of starships and space stations.

And more than ever, our real  technology is reaching for the stars to realize some of that sci-fi future. But some of the most ambitious  programs actually rely on technology that’s been around for hundreds of years.

Like the innovative 19th  century optics from lighthouses that could help to take us  to the moon… and beyond. Star Trek’s series ‘Deep  Space Nine’ aired in the 90’s, and portrayed life on board a huge  space station in the 24th Century. So maybe NASA was going for  a very early start on that when they launched their  Deep Space 1 mission in 1998.

The mission was a technology demonstration  that spent more than three years in space, visiting an asteroid and a comet  while it tested out a slew of brand new methods for the next generation  of space exploration. Notably, it was the first spacecraft  to be powered by an ion engine rather than traditional rocket fuel. Normally, spacecraft burn fuel to  push themselves around in space.

But that means that you'd have to take all  the fuel that you’d need for a long journey, which is heavy, so it requires more  fuel… you can see where this goes. So Deep Space 1 tried out a new  type of engine that used ions instead of conventional exhaust gases. The idea is that small quantities of  xenon gas could be electrically charged, and then accelerated by an electric field.

The individual ions would be  ejected from the spacecraft at speeds of up to 144,000 km per hour. In the test flight, that was enough to  accelerate DS1 by up to 4.3 km per second. The engineers think they  could have gone even faster, but there was more to DS1’s  mission than just going super fast!

Nonetheless, ion drives were a  breakthrough in spacecraft propulsion, especially for long-distance  or long-duration missions, as only tiny quantities of xenon are needed. But to make DS1’s enterprising ion engine work, the spacecraft needed a very strong  and reliable source of power. And that’s where the 200 year-old  lighthouse tech comes in!

In the mid-19th century, French  Engineer Augustin-Jean Fresnel figured out a way to use lenses to  make lighthouses more effective. A lens could theoretically bend  and collect the lighthouse’s light and make it visible from farther away. But a convex glass lens big enough  to do the job would have been far too heavy to mount at the top of a tower.

Fresnel figured out that you  didn’t need a big thick lens to get the same bending of light,  just its outer curved profile. So he effectively hollowed it out, and  then flattened the once-curved surface into a series of concentric circular prisms. The light passes through and is bent exactly  as it would be with a thick curved lens, but with just a fraction of  the materials and weight.

So these Fresnel lenses were used in  lighthouses to collimate the light: to take the diverging light rays from a lamp and straighten them out  into one concentrated beam. Collimating: make it into a column, I guess. This was a huge victory for  maritime safety and navigation.

But over the years, Fresnel lenses  also found other applications. They can be used as lightweight,  slimline magnifying sheets. And if they’re turned around, they can  focus the incoming light down to a point.

Which proved to be just the thing  to supercharge DS1’s ion engines. Because the spacecraft also sported brand new, so-called multi-junction solar panels. Made of different materials layered  together, they were able to absorb more of the sun’s wavelengths than  traditional silicon photovoltaic cells.

In fact, they can theoretically  produce the same wattage with just one eighth as much surface area. But to get the most out of them,  these revolutionary new cells needed to have a high intensity  of light falling onto them. So engineers turned to Fresnel lenses  to design what they called a solar concentrator with refractive linear  element technology, or SCARLET for short.

SCARLET was a component that sat  above the multi-junction cells with the sole purpose of  concentrating the sun’s light. NASA scientists made the already  slimline lenses even more lightweight, to reduce the cost of launching them into space. And they curved an extended  sheet of Fresnel lenses so that even more light could  be focused on a single point.

DS1 ended up using 720 of these lenses  to focus light onto 3600 solar cells. That was enough to generate about  2500 watts of continuous power, which is about the same as three microwave ovens. That might not sound like much, but it’s much more than most  solar system probes run on.

It was enough to power DS1’s instruments  and its groundbreaking ion engines. So it was thanks to these 200-year old lenses that DS1’s advanced technology mission  was considered a resounding success. The Fresnel-enhanced solar cells  were a revolution in space power, but back on Earth, it soon became much cheaper to make normal silicon photovoltaic cells.

So for most applications, the  traditional variety are used, rather than solar concentrators. But the high-efficiency cells are still  being considered for larger-scale projects, like solar power plants in sunny  places like Arizona and Colorado. In space, however, SCARLET’s Fresnel  lenses and DS1 did demonstrate the viability of a new kind of  propulsion using only solar electricity and a few high speed ions.

And these ion engines are now widely used for positioning satellites in Earth’s orbit. And also, they’re planned to propel the  Gateways support mission to the moon, in preparation for the future Artemis  program of manned lunar missions. And who knows, they may even  have a use on a space station 300 years in the future named Deep Space 9!

In the meantime, however, if you’d  like to experience Deep Space One up close and personal, you can, because  it’s this month’s Pin of the Month! From the time this episode goes  up through the end of February, you can pre-order this Deep  Space 1 pin for your very own. We’ll take pre-orders through the end  of the month, and then we will ship, and then we will never make that pin again.

Never fear though, we will  make another one in March. So look forward to the next one, and check out the link in the  description to learn more. [♪ OUTRO]