Relativistic spacecraft, like those proposed by the NASA Starlight program and the Breakthrough Starshot Initiative, will have to survive radiation production that is unique when compared to that experienced by conventional spacecraft. In a relativistic interstellar spacecraft’s reference frame, the interstellar medium (ISM) will look like a nearly monoenergetic beam of charged particles which impinges upon the leading edge of the spacecraft. Upon impact, ISM protons and electrons will travel characteristic lengths through the spacecraft shield and come to a stop via electronic and nuclear stopping mechanisms. As a result, bremsstrahlung photons will be produced within the spacecraft shield. In this work, we discuss the interstellar environment and its implications for radiation damage on relativistic spacecraft. We also explore expected radiation doses in terms of onboard device radiation tolerance.

The Breakthrough Initiatives Project Starshot proposes to send a gram-scale laser driven spacecraft to the Alpha Centauri system in a 20 year mission travelling at v~0.2c. One of the challenges of this mission as the spacecraft moves through the interstellar medium is the presence of dust and gas (mostly hydrogen). The dust has a typical matter-density of 2.57 × 10-27 g/cm3 with typical particle mass being 3 × 10-13 g although some of the largest particles may be 5 × 10-9 g in mass. These dust particles will deposit ~1012-1016 MeV onto the spacecraft with an energy flux of order ~0.3 J/sm2. We consider the erosion of the spacecraft frontal area due to dust and also heating effects. We attempt to characterise the likely environment for the Starshot mission and estimate the particle bombardment shielding requirements in terms of mass and thickness of material. Current analysis estimates that the likely erosion rates are of order ~10-11-10-8 g/s and that the frontal area temperature for the models examined in this paper will be ~135.2 K depending on the ratio of frontal area to radiating area. For an assumed shielding material with atomic number range 3-13 (Lithium to Aluminium), and for spacecraft geometries with radii ~1 mm and cylindrical length ~5 mm, over a 21.5 year mission duration, this would suggest a shielding thickness of ~1.4-3 mm. This would also suggest a shielding mass in the range ~0.01-0.05 g; depending on the material choice, spacecraft size and chosen geometry. This would represent between ~1–5% of the total mass, assuming a spacecraft mass of 1g (driven by a ~102 GW laser power). We also examine the additional effects of charged particles and estimate the stopping power and penetration range for different materials. Finally, we briefly examine the potential to use the incoming energy flux as a power source for the transmission of an optical laser deep space communication system. The work presented highlights the close coupling in the Project Starshot spacecraft design between the vehicle geometry and the particle bombardment requirements.

Accelerating one ton to one-tenth of the speed of light requires at least 450 PJ or 4.5 ×1017 J or 125 billion kWh, not accounting for losses. The source of energy has to be carried, since solar panels do not work far from the Sun and other stars. The magnitude of this energy may make interstellar travel impossible.[3] One engineer stated “At least 100 times the total energy output of the entire world [in a given year] would be required for the voyage (to Alpha Centauri)”.[3] Interstellar medium interstellar dust and gas may cause considerable damage to the craft, due to the high relative speeds and large kinetic energies involved. Larger objects (such as bigger dust grains) are far less common, but would be much more destructive. . Travel time The long travel times make it difficult to design manned missions. The fundamental limits of space-time present another challenge.[9] Also, interstellar trips would be hard to justify for economic reasons. It can be argued that an interstellar mission which cannot be completed within 50 years should not be started at all. Instead, the resources should be invested in designing a better propulsion system.