The banner shows the transparent impact shielding of Firefly. Heated by the constant shock of Interstellar gas, it is slowly eroded by the impact of dust particles. Interstellar space is mostly empty; over its lifetime, the shield will be hit by 0,13 kg of gas and less than 0,01 kg of dust.
Kelvin A Long, Michel Lamontagne
The original Daedalus shield design, by Martin(1) was a Beryllium plate, 9mm thick. The plate protected the payload from impacts from Interstellar gas and dust. Their combined impacts heated the plate up to 200K (-73 C) and the dust impacts slowly evaporated the Beryllium, which was chosen for its high heat of sublimation and low density. A little less than half the mass of Beryllium was expected to evaporate over the time required for the trip to Barnard’s star.
To create a shield for an Interstellar vehicle, we must take into account that they can be designed for a lower velocity than Daedalus: 7-8% of the speed of light for Firefly, rather than 14% for Daedalus, for example. Therefore the dust impacts will have less effect. In fact, since the effect is to the square of the velocity, a reduction by a factor of two yields a reduction in wear by a factor of four. The dust shield for Firefly can therefore be lighter than the one for Daedalus.
A review of the paper by Hoang(2) about shielding the project Starshot vehicles shows little significant changes in the evaluation of the risks to interstellar vehicles since the original Daedalus study. However, although the numbers for gas density and average dust concentration have remained the same since Daedalus was designed, the number of larger dust grains may be higher than expected by the Daedalus team. Some changes in the shield design may therefore be required.
A new paper by Long(3) integrates all the latest data into a complete evaluation of shielding requirements for the various Icarus designs. This chapter summaries the information from this paper.
Table 1: Shield material properties
Material
Density
kg/m3
Specific heat capacity
Latent heat of fusion
Latent heat of sublimation
Aluminium
Beryllium
1850
Boron
Carbon
Lithium
Table 2:
Although the distance traveled by a Starship is huge, the ship is relatively small. The volume of space crossed by a starship will be like a tremendously long spaghetti; but when it is brought into scale, the volume is quite a bit less than, for example, the volume of the Earth.
The volume crossed by a Starship with a 25m radius shield going to Alpha Centauri is a very long cylinder, where:
V=Volume = A*L (m3)
A= Front area = radius2 x pi = 25m2 x pi
L = Distance = 4.3 light years = 4.068e16 m
V= 8e19 m3, or about 7% of the volume of the Earth (1,1e21m 3)
The density of interstellar gas is d=1,67e-21, therefore the ship will hit 8e19 x 1,67e-21 = 0,13 kg of gas.
The density of interstellar dust is much lower, d= 6.2e-24, and the ship will encounter about 8e19 x 6.2e-24 = 0,0005 kg of dust, or about ½ a gram.
The total quantity of dust hitting The Starship will be small, in the order of ½ gram over a century of travel. Nevertheless, the impact of the dust particles is non trivial. In particular, the larger dust grains in Interstellar space could mass as much as 10e-8 kg, and have a kinetic energy at 0.1c of 4.5×106 J. This is about the energy from the impact of a 10 tonne truck at 100 km/hr. This might well puncture the simple one layer shield that was proposed for Daedalus, and lead to damage to the ship itself.
A multi layer structure, similar to the Whipple shield used for space stations near the Earth, is proposed instead. Despite the more complex structure, the unit mass of the shield could be lighter, as the overall erosion will be less since the ship is traveling more slowly. Replacing the beryllium with carbon fiber would also provide some reduction in mass. Carbon fiber has seen widespread use in all areas of industry since the time of the original Daedalus report and is a well known technology. It has similar heat of sublimation as Beryllium, as well as higher strength.
A multilayer assembly of carbon fiber sheets, with a combined thickness of 5mm is proposed for Firefly. The volume for a 50m diameter shield would be 12 m3, and the mass about 25 tonnes, so half the mass of the 50 tonnes shield planned for Daedalus. Other protection schemes are possible, they will be explored in detail in the individual ship chapters.
Cutaway view of the carbon fiber Whipple shield
Whipple shield equations?
The Daedalus team also concerned itself with the encounter phase, providing the ship with a protection system that consisted of a dust cloud generated in front of the ship, that was periodically renewed during the encounter. The cloud would pulverize any object in front of the ship, turning it into harmless plasma.
However, an exploration mission enters the target system at much lower velocity and eventually stops altogether. The risks and consequences of impacts for the encounter period are greatly reduced, and the protection system designed for Daedalus is not required. However, the Proxima Centauri flyby probe would face the same risks as Daedalus and a similar solution might required.
Shielding example
1 Project Daedalus, final report, bombardment by Interstellar material and its effect on the vehicle, Martin
2 Project Daedalus, final report, Target system encounter protection, Bond
3 Hoang, T., Lazarian, A., Burkhart, B., & Loeb, A. (2016). The interaction of relativistic spacecrafts with the interstellar medium. arXiv preprint arXiv:1608.05284.
4 Dust grain damage to Interstellar vehicles, Early
5 New paper by Kelvin A. Long