Naturally Occurring Hazards to High-Speed Interstellar Spacecraft

Ancient cartographers creating charts of distant seas might have felt the need to embellish them with fanciful hazards but Team Icarus has no need to create dragons.  The real interstellar environment is not quite the empty void of popular imagination, its parameters are generally known, and the hazards it poses can be predicted by looking at spacecraft now operating within the Solar System.  Modern astronomy has shown that we are surrounded by a sea of dilute gas and dust bathed in cosmic radiation and a tenuous interstellar magnetic field.  This article discusses the environment between the stars and the hazards it might pose to an interstellar mission like Icarus.

The Project Icarus terms of reference state that the vehicle will be designed to reach its stellar destination “within as fast a time as possible, not exceeding a century and ideally much sooner.”  Ian Crawford discussed likely target star candidates for Icarus in his post entitled “Targets for Icarus: Planets within 15 light-years of the Sun”.  Taking a hint from his title we will be looking at the interstellar environment within 15 light years.

Between the stars of our galaxy we find primarily a diffuse mixture of ions, atoms, molecules, dust, cosmic rays, and magnetic fields called the interstellar medium.  General parameters of this medium appear below.

Composition of interstellar medium

90% hydrogen 8% helium 2% heavier nuclei

Mass percentages of gas and dust

99% gas 1% dust

Interstellar gas number density (galactic average)

1 atom per cubic centimeter

Galactic cosmic ray count (at Earth orbit)

Average galactic magnetic field

1 microgauss (1 nanotesla)

The gas and dust probably represent the most significant environmental hazard to Icarus.  Compared to the Earth’s atmospheric number density of roughly 1019 per cubic centimeter, one atom per cubic centimeter doesn’t sound so bad.  The problem, of course, comes from the fact that Icarus will be moving through the interstellar medium at a relative velocity of 15% of the speed of light for a decade or more.  Any surface facing along the velocity vector will experience heating and erosion as the impinging dust and gas strike it during transit.  Furthermore, some parts of the interstellar medium that exist are nearly completely ionized plasmas.  Such plasmas can cause spacecraft to build up charges on their surfaces both directly by depositing charge into the spacecraft and indirectly by ionizing atoms in its structures.  Both of these effects are seen in Earth orbiting spacecraft.  Unless we drive Icarus through an ionization region, we will likely be facing much cooler plasma than typically found in orbit around our Sun.  That means that the primary source of charging will be ionization by collision with the interstellar medium.  There is reason to believe that this will still present a problem, however.  The amount of energy required to remove the first electron from an atom is called “the work function”.  The typical work function for spacecraft materials is around 3-5 eV.  The energy of a proton impacting on Icarus at 15% of lightspeed would be around 10.7 million eV.

Recent discoveries about the interstellar medium give us a much better picture of our local dust and gas environment than Team Daedalus had to work with.  Our star system exists in a cavity or void in the galactic interstellar medium known as the Local Bubble.  This bubble is about 300 light years across and has a significantly lower density than the galactic average (about 0.05 atoms per cubic centimeter).  You can see a map of our galactic neighborhood within 1500 light years by Linda Huff and Priscilla Frisch here. The Local Bubble is the irregular black void surrounding the Sun.  An important feature on this map is the fact that the green Gum nebula (the closest ionization region) is roughly 1000 light years away.   This is many times farther than any planned Icarus science targets, and so confirms the idea that we won’t be sending the spacecraft through any highly ionized regions.

The dust and gas picture gets more complicated when one “zooms in”.  A map of the Local Interstellar Cloud (same authors) appears here.  For scale, Alpha Centauri (upper right) is about 4.4 light years away and Altair (left) is 16.7 light years away.  The Sun is seen to be embedded in a cloud dubbed the “Local Interstellar Cloud” or “Local Fluff” by astronomers.  This cloud has a higher density than that of the Bubble of about 0.1 atoms per cubic centimeter.  Recent measurements by the Voyager probes also indicate an unexpectedly high magnetic field strength in the Fluff of 4-5 microgauss (0.4-0.5 nanotesla).

Besides running Galactic cosmic rays are an omnipresent source of radiation and a hazard to all spacecraft operating within or outside of the Solar System.  They are not really “rays” at all but rather consist of particles (mostly protons but also including some heavier nuclei) accelerated to very high energies by a variety of processes.  The exact composition, flux, and origin of galactic cosmic rays are matters of some debate because the Sun’s magnetic field acts to prevent some of them from entering the Solar System and complicates measurements.  Important data on galactic cosmic ray fluxes will be obtained from the Cosmic Ray Subsystems aboard Voyagers 1 and 2 as these spacecraft continue to exit the Solar System.  These instruments seem to be averaging roughly 20 – 30 particles with greater than 0.5 MeV per nucleon per second as I write this but remain inside the influence of the Sun’s magnetic field.  These highly energetic particles would not be a threat to Icarus’ structure but rather to computers and other electronic components that have small clearances or rely on sensitive voltage measurements for operation.  They are the source of the infamous single event upsets or “bit flips” that occasionally plague spacecraft.  These are random bit changes in a spacecraft computer system which typically interfere with its mission and require extensive recovery actions.

References:

W.J. Larson and J.A. Wertz, Space Mission Analysis and Design 3rd ed., Microcosm Press, 1999.

E.T. Benedikt, ‘Disintegration barriers to extremely high-speed space travel’, Advances in the Astronautical Sciences, vol. 6, pp 571-588, 1961.