Energy Supply in Space When the Sun Is No Longer Enough: The Return of the Nuclear Battery to Space

A CubeSat with a tritium battery is on a mission in space. The battery does not generate much power but achieves what solar cells and accumulators inherently fail to do: continuously supply energy for years, regardless of the sun's position.

When the small satellite BOHR was launched into space aboard SpaceX Transporter-17 in July 2026, its electrical power was probably the least remarkable thing about it. No nuclear reactor, no kilowatt source, no radioactive generator powering a spacecraft over interplanetary distances. Instead, the CubeSat demonstrates a technology whose output power belongs more to the realm of ultra-efficient electronics: a tritium-based betavoltaic power source.

Nevertheless, this mission could have greater significance than its performance data initially suggests. Because BOHR is not just a technology demonstrator. The mission is also a test of whether nuclear micropower can transition from the exclusive world of state space programs to commercial spaceflight in the future.

City Labs describes BOHR as the first commercially nuclear-powered satellite and the first nuclear CubeSat. However, an important distinction must be made: the satellite bus itself continues to be conventionally powered by solar cells. The nuclear power source serves as an independent power supply for the payload and is intended to validate the NanoTritium technology in orbit for the first time. This hybrid architecture is particularly interesting, as it demonstrates how nuclear micropower can be used not as a replacement for solar energy but as a complement for functions that can never afford to depend solely on sunlight.

A Battery That Doesn't Need to Be Charged

The term "nuclear battery" inevitably evokes associations with radioisotope generators and large space missions. In fact, a betavoltaic cell operates fundamentally differently from an RTG.

A radioisotope generator uses the heat generated during radioactive decay. A thermoelectric converter then converts it into electrical energy. Betavoltaics skip this thermal detour. The basic principle is more similar to a solar cell.

Instead of photons, beta particles strike a semiconductor. These high-energy electrons generate electron-hole pairs along their trajectory. If these charge carriers are within or close enough to a suitable p-n junction, the electric field of the depletion layer can separate them. A current is generated.

The fundamental chain is thus: radioactive decay → beta electrons → electron-hole pairs → charge separation → electrical energy

The idea is by no means new. Betavoltaic energy converters have been studied for decades. Their great advantage lies in an unusual combination of properties: they require neither light nor an external energy supply and can, by design, operate over very long periods. However, their major weakness is the low power output.

This is precisely why they have so far been particularly interesting for applications where not much energy is needed, but the energy source must be available for an extremely long time—such as for autonomous sensors, hard-to-access systems, or certain medical applications. Scientific literature accordingly describes betavoltaics as a niche technology with potentially very long lifespans, whose practical use has so far been mainly limited by low power density and limited conversion efficiency.

Why Tritium Specifically?

BOHR uses tritium, the radioactive hydrogen isotope ³H. Tritium decays via beta-minus decay into helium-3: ³H → ³He + β^- + antineutrino

The emitted beta electrons have relatively low energies. This is precisely what makes them interesting for betavoltaic applications. With a high-energy beta emitter, a fundamental problem arises: the particles can penetrate deeply into the semiconductor and cause damage there. Additionally, the structure of the converter must be designed to ensure that as much of the decay energy as possible is deposited in the region where the generated charge carriers can still be collected.

Tritium, on the other hand, is a relatively low-energy beta emitter. Its electrons have a short range and therefore place lower demands on shielding. In scientific literature, Tritium is considered, alongside Nickel-63, one of the most promising candidates for long-lasting betavoltaic power sources. Additionally, Tritium can be bound in metal hydrides or tritides, for example, which can be relevant for compact power sources.

However, the low energy of the beta particles has a downside. The lower their energy, the smaller the potential to generate large amounts of electrical power from a compact converter. A tritium betavoltaic is therefore not a competitor to a lithium-ion battery when watts or kilowatts are needed in the short term. It competes with something else—not with the battery itself, but with the problem that a battery eventually runs out.

The Actual Problem Is Called Power Density

Here lies the key difference between a spectacular technology and a universally applicable energy source. A betavoltaic cell can theoretically supply energy for years or decades. However, its instantaneous output remains low. This is not merely a development issue but partially a direct consequence of physics.

A single beta emission can indeed generate numerous electron-hole pairs in the semiconductor. However, the particle flux is low compared to the photon flux of a solar cell. Accordingly, the power density is significantly lower than that of photovoltaic systems. Additionally, several loss mechanisms come into play.

First, the decay energy must reach the active semiconductor. With an unfavorable geometry, the radioactive material absorbs part of its own beta radiation—an effect known as self-absorption. Then, the generated charge carriers must be collected before they recombine. Therefore, the depth at which the beta electrons deposit their energy must match the geometry of the depletion layer as closely as possible.

This creates a challenging optimization problem between:

• Activity and quantity of the radioactive material,
• Range of the beta particles,
• Thickness of the semiconductor,
• Depth and width of the active zone,
• Doping profile,
• Diffusion length of the charge carriers,
• Surface geometry and
• Radiation damage to the material.

A thicker semiconductor structure is not automatically better. If the energy is deposited too far from the collection region, electrons and holes recombine before they can contribute to the electrical current. At the same time, the active structure must not be so thin that a significant portion of the decay energy passes through unused.

Modeling therefore shows that even small changes in geometry and layer thickness can significantly affect achievable efficiency. For tritium-silicon structures, theoretical and analytical efficiency limits in the single-digit to low double-digit percentage range have been discussed. However, real systems often fall significantly below these values.

Why Modern Semiconductors Are Making the Technology Interesting Again

The renewed focus on betavoltaics is due to advancements in semiconductor technology. Microcontrollers can operate in deep sleep mode for extended periods, only becoming active for a few milliseconds. Sensors now require only micro- or milliwatts of power. Wireless transmissions can be buffered and carried out at longer intervals. Energy from a continuous source can be stored in a capacitor or battery and then used briefly for a power-intensive task.

As a result, the nuclear battery does not need to deliver peak current but can accumulate enough energy over hours or days to enable the next work cycle.

A sensor, for example, might require virtually no power most of the time. A continuous micropower source charges an energy storage device during this period. After hours or days, the microcontroller is activated, measures, processes data, and sends a brief wireless signal. Then the cycle begins again. The nuclear battery simply needs to never stop supplying power.

From the Flat Chip to the Three-Dimensional Nuclear Cell

The classic betavoltaic cell has a geometric limitation. A flat radioactive source emits in all directions. If the semiconductor is only on one side, a significant portion of the beta radiation is lost. Additionally, the range of particularly low-energy tritium electrons is limited.

Therefore, the geometry of the converter is becoming increasingly important. Microstructured semiconductors with pores, columns, trenches, or other three-dimensional structures can massively increase the active surface area. Instead of placing a radioactive source in front of a planar semiconductor surface, a three-dimensional architecture could integrate the source and the converter much more closely.

The goal with such cells is to deposit more decay energy in the active semiconductor volume without extending the path of the generated charge carriers to the collection structure. This makes betavoltaics increasingly a matter of micro- and nanofabrication.

How can three-dimensional p-n structures be fabricated? How deep can a structure be? Which surface maximizes energy absorption? How does recombination change? And how can a radioactive source be integrated to minimize self-absorption, shielding, and radiation damage?

Current work on high-volume and multi-layered betavoltaic structures shows that better utilization of the active volume can enable significant improvements in power density. At the same time, the challenge remains to combine such concepts with realistic manufacturing, long-term reliability, and suitable encapsulation.

BOHR Does Not Yet Solve the Power Problem—But Possibly the Approval Problem

This is exactly where the BOHR mission becomes interesting. Technologically, betavoltaics in space are not completely revolutionary. Radioisotope energy sources have accompanied space exploration for decades. Large interplanetary missions use RTGs or other radioisotope systems when solar energy is insufficient or unavailable.

The difference lies in scaling and commercialization. BOHR is meant to demonstrate that a small tritium-based energy source can be integrated into a commercial CubeSat and launched on a commercial rideshare flight. Equally important is the regulatory pathway.

According to City Labs, the mission received FAA Payload Authorization in September 2025. The company views BOHR as a demonstration that commercial spaceflight companies can integrate tritium-based systems within the existing U.S. regulatory framework—provided that safety analyses, containment, and regulatory documentation meet the respective requirements. This could even be more important in the long term than the demonstrator's electrical performance.

Because a technology can physically work and still remain commercially insignificant if every single application represents a lengthy regulatory exception. BOHR could therefore be understood as a kind of demonstrator.

Where the Sun Truly Becomes a Problem

In low Earth orbit, solar energy is an excellent solution for most satellites. It is available, established, and provides high power densities. A betavoltaic will not change that. Its potential significance begins where solar cells encounter physical or mission-related limits.

For example, on the dark side of the Moon or in deep craters near the poles. These areas receive no direct sunlight for extremely long periods. At the same time, such regions are considered scientifically interesting, partly due to potential water ice deposits. A traditional power supply there would require either an external cable, a large energy storage system, or an alternative power source. Here, a nuclear micropower source could permanently power sensors, measurement systems, or ultra-energy-efficient electronics.

The same applies to deep-space missions. As the distance from the sun increases, the available solar power decreases with the square of the distance. At great distances, the required solar arrays become correspondingly larger.

The Real Question Now Is: Will a Niche Become a Market?

Instead of marking a nuclear revolution, BOHR represents a practical solution for a growing gap in space exploration: As miniaturized electronics reduce the energy requirements of autonomous systems to a level that is too long-lasting for batteries, too light-independent for solar energy, and too small for generators. Whether betavoltaics will become a commercial market despite physical limits on power density and efficiency largely depends on the regulatory precedent. The mission's crucial breakthrough, therefore, lies not in the amount of electricity generated but in demonstrating that a nuclear micropower source can overcome the hurdles for commercial launches. 

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