A practical use of space energy: energy transmission to probes on Venus

A few weeks ago, a team of Caltech scientists announced that they had successfully transmitted energy from an Earth-orbiting satellite. It wasn’t a lot of energy, but it proved it was possible.

Eventually, we may be able to beam energy from solar satellites back to Earth, making solar energy available almost everywhere and helping fight climate change. But there’s another potential use: to power surface probes on Venus.

Everyone knows Venus. She has killed multiple landers with her extreme heat and crushing atmospheric pressure. The old Soviet Union sent a number of probes to the planet’s surface, and most failed. The most successful was Venera 13, which survived for just over two hours at 457°C (855°F), and was subjected to 9.0 MPa (89 standard atmospheres) of pressure.

Despite the brief but significant success of Venera 13, the planet has kept its secrets and we are drawn to its surface to reveal them. That’s why NASA wants to send a lander to the surface as part of its DAVINCI+ (Deep Atmosphere Venus Investigation of Noble gas, Chemistry, and Imaging.) mission.

But there is the question of how to power a lander on Venus’s unique and treacherous surface, assuming we can build one that won’t easily succumb to Venus’s poor conditions. The usual methods: solar energy, batteries, radioisotope thermoelectric generators are not up to the task. This according to new research published in the journal Acta Astronautica.

The research article is “Feasibility of power radiating through the atmosphere of Venus”. The corresponding author is Erik Brandon of the Jet Propulsion Laboratory.

‘State-of-the-art space energy technologies including solar panels, batteries and radioisotope thermoelectric generators are unable to function on the surface of Venus, limited by the high temperatures, high pressures and corrosive environment,’ the authors explain.

Venus is closer to the Sun, but its thick atmosphere means that not much solar radiation reaches the surface. About 75% of the solar energy is reflected by the clouds of Venus and only about 2.5% of the incident solar flux in the upper atmosphere reaches the surface. Above the clouds, solar energy is abundant. Venus receives twice as much solar radiation in its upper atmosphere as Earth does in its upper atmosphere.

Could this abundant energy be harnessed by solar collectors above the clouds and then beamed to a lander/rover? It should make it through lots of thick clouds. “The feasibility of such an approach and other related mission concepts are discussed here from the perspective of atmospheric absorption and radiated energy dispersion,” the paper states.

The transfer of energy from one place to another is called wireless energy (or power) transfer. There are two types: near field and far field. Near-field is the short-distance transfer of energy like the type used in charging pads for mobile devices. Far-field energy transfer is also called power transmission and uses microwaves or lasers to transmit power from a producer to a receiver.

One problem with transmitting energy from an orbital solar collector to a surface vehicle is complications in a geostationary orbit of Venus. The planet rotates so slowly that the geostationary orbit is at a great distance from the planet, making the orbit unstable. Somehow, a solar collector should be closer to the planet. Above the upper clouds, at about 60 or 70 km altitude, a collector would receive essentially all available sunlight. Mission design may need to keep the harvester, or group of harvesters, at the right altitude and position.

An alternative solution is to beam a portion of the energy to a lander on each orbit, which may be sufficient. ‘Hundreds of Wh (Watt hours) of energy could be received during several orbital passes of the lander,’ the authors explain.

But these are bigger issues than the overall mission architecture. This research assumes that there are solutions to this problem. In this work, the authors focus on how to radiate energy and receive it, something that has not been studied extensively. ‘However, to date, there has not been an in-depth study on the feasibility of power transmission at relevant wavelengths, whether a suitable platform and mission architecture could be devised and implemented,’ the authors write.

The problem is that Venus’s atmosphere is thick and contains chemicals that interfere with the energy transmitted by microwaves. CO2 concentrations are a particular problem.

Lasers might be a better option. While there are problems with the thick atmosphere, there are some “frequency windows” in the atmosphere that could allow for power beaming with lasers. “Counterintuitively, power transmission via laser sources may be possible on Venus despite continued cloud cover, given some optical/infrared ‘windows’ present in Venus’ atmosphere, which are not available using microwave power transmission,” they write. the authors.

Lasers also have other advantages, such as reduced beam scattering compared to microwaves. This means that the receiving antennas don’t have to be that big. A one-meter receiver might suffice and wouldn’t be so bulky as to interfere too much with a lander’s design.

While solar energy is abundant in the upper atmosphere of Venus, radiating it throughout the entire atmosphere may not be the best approach. Instead, a balloon or some other vehicle could position itself near the center of the atmosphere. There it would receive enough solar energy to be viable and would only need to radiate the energy through part of the atmosphere.

“These calculations indicate a plausible approach for teleportation
to Venus, using transmission from an aerial platform operating near the
cloud base”.

from “Feasibility of power radiating through the atmosphere of Venus”

Research shows that the altitude of 47km is significant. There is a cloud base at that altitude, and below that the radiated energy is subject to less leakage. It also shows that from 47km, the highest transmission factor is at 1022 nanometers, where about 20% of the radiated energy would reach a surface lander.

‘These calculations indicate a plausible approach for transmitting power to Venus, using transmission from an aerial platform operating close to the cloud base,’ the authors write.

But is there the technology to do it?

The paper does not discuss what type of vehicle or platform could be used at an altitude of 47km. They focus on the radiating power itself and whether the calculations show that it is possible. But they also talk about the laser technology available and whether it is up to the task.

According to the researchers, we still don’t have the right kind of laser.

Researchers are busy developing them, though. Ytterbium-doped fiber lasers (YDFLs) operating in the near-infrared (NIR) window are under development and can also operate at high power. Unfortunately, they don’t operate at the ideal wavelength for use on Venus: 1022. Instead, they are limited to two other ranges: 970-980nm and 1030-1100nm. But lasers are the focus of many researchers around the world and progress is constant.

The task of keeping some sort of aerial platform stable and in the correct position is fundamental to any transporter mission. But researchers are already working on balloons and other aerial platforms for use on Venus. Assuming they can be developed, the authors are confident that an energy transmission scenario could be up to the challenge and create successful missions to the Venusian surface.

“Furthermore, while engineering and mission design challenges exist with regards to controlling and pointing such an air vehicle platform used for power transmission and in overall thermal management, this analysis shows that these optical windows could be exploited to allow for sufficient power levels to enable the mission to be beamed to the surface of Venus.

We need a better understanding of Venus’ atmosphere before we can design a specific system. DAVINCI+ has three main scientific goals and one of them is to understand the atmosphere as it passes through it.

His findings will help scientists understand what obstacles they face in transmitting energy to the planet’s surface. If it can be done reliably, then Venus will be open to exploration.

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