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 doesn't produce much power but accomplishes what solar cells and accumulators inherently fail to do: provide continuous 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 performance was probably the least conspicuous aspect. No nuclear reactor, no kilowatt source, no radioactive generator powering a spacecraft across interplanetary distances. Instead, the CubeSat demonstrates a technology whose output is more suited to the world of ultra-low-power electronics: a tritium-based betavoltaic power source.

Still, this mission might hold greater significance than its performance data initially suggests. BOHR is not just a technology demonstrator. The mission is also a test of whether nuclear micropower can transition from the exclusive realm of government space programs to commercial spaceflight in the future.

City Labs describes BOHR as the first commercial 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 aims 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 those functions that can never rely solely on sunlight.

A Battery That Doesn't Need to be Recharged

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

A radioisotope generator uses the heat generated during radioactive decay. A thermoelectric converter then generates electrical energy from it. Betavoltaics skip this thermal detour. The basic principle is more akin to that of a solar cell.

Instead of photons, beta particles hit 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, in principle, operate over very long periods. However, their major weakness is the low power output.

This is precisely why they have so far been of interest primarily for applications that require little energy but demand an extremely long-lasting power source—for example, 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 comparatively low energies. This is precisely what makes them interesting for betavoltaic applications. A fundamental problem with a high-energy beta emitter is that the particles can penetrate deep into the semiconductor and cause damage there. Additionally, the structure of the converter must be designed in such a way that as much of the decay energy as possible is deposited in the region from which 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, along with nickel-63, is considered one of the most promising candidates for long-lasting betavoltaic power sources. At the same time, 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 capacity to generate large amounts of electrical power from a compact converter. A tritium betavoltaic is therefore no 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, but with the problem that a battery eventually runs out.

The Real Problem Is Called Power Density

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

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

First, the decay energy must even reach the active semiconductor. With an unfavorable geometry, the radioactive material absorbs part of its own beta radiation—a phenomenon 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 align as closely as possible with the geometry of the depletion layer.

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 shows that even small changes in geometry and layer thickness can significantly impact 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 perform significantly below these levels.

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 for extended periods, activating only for a few milliseconds. Sensors sometimes require only micro- or milliwatts. Wireless transmissions can be buffered and conducted at larger intervals. Energy from a continuous source can be stored in a capacitor or battery and then briefly used for a power-intensive task.

As a result, the nuclear battery does not need to deliver peak current but rather 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. During this period, a continuous micropower source charges an energy storage device. After hours or days, the microcontroller is activated, measures, processes data, and sends a short radio signal. Then the cycle starts again. The nuclear battery simply needs to never stop supplying power.

From Flat Chip to Three-Dimensional Nuclear Cell

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

Therefore, the geometry of the converter is becoming increasingly important. Microstructured semiconductors with pores, pillars, 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 interweave 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 in a way that minimizes self-absorption, shielding, and radiation damage?

Current work on high-volume and multi-stage 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 where the BOHR mission becomes interesting. Technologically, betavoltaics in space is not entirely revolutionary. Radioisotopic energy sources have been part of space exploration for decades. Large interplanetary missions use RTGs or other radioisotopic systems when solar energy is insufficient or unavailable.

The difference lies in scaling and commercialization. BOHR is intended 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 an 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. In the long term, this could even be more important than the electrical performance of the demonstrator.

Because a technology can function physically and still remain commercially insignificant if every single application represents a lengthy regulatory special case. BOHR could therefore be understood as a sort of demonstrator.

Where the Sun Actually 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. Betavoltaics will not change that. Their potential significance begins where solar cells encounter physical or mission-related limitations.

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

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 necessary solar areas become correspondingly larger.

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

Instead of a nuclear revolution, BOHR represents a practical solution for a growing gap in space exploration: through miniaturized electronics, the energy demand of autonomous systems has decreased to a range that is too long-lasting for batteries, too sunlight-independent for solar power, and too small for generators. Whether betavoltaics will become a commercial market despite physical limits on power density and efficiency depends primarily on the regulatory precedent. The mission's key breakthrough, therefore, lies not in the amount of electricity generated but in proving that a nuclear micropower source can overcome the hurdles for commercial launches.