Gregory Benford's Starborn

Interstellar bow shock "moisturization"

In his 2005 novel Starborn, Benford suggests that if by some mischance the Sun's interstellar bow shock (the boundary between the 50km/sec, 1/cm3 interstellar hydrogen and the sun's magnetic field) moved in as far inward as the earth, the effects would be troublesome - destruction of the ozone layer, precipitation of atmospheric oxygen as water. Let's do the math.

1/cm3 is 1E6 / m3. So that is 5E10 atomic hydrogens per square meter per second, or 8.3E-17 kg/m2-s (6.022E26 hydrogens/kg), or 2.6E-9kg/m2-year. That is coming from one direction, and the disk-to-surface ratio of a sphere is 1/4, so that is 6.5E-10kg/m2-year averaged over the Earth's surface.

Keep in mind that the Earth swims in a sea of very hot and thin (denser than interstellar!) solar hydrogen - that (slower) hydrogen is banging into the top of the atmosphere all the time, so the claim that the thinner interstellar stuff can have noticable short-term effects is questionable.

If the interstellar hydrogen intersecting the Earth's disk all entered the atmosphere, combined with atmospheric oxygen, and turned to water, that would be 5.9E-9kg/m2-year of water created (9x the mass of the hydrogen). For comparison, the atmosphere averages about 25kg/m2 of water in the tropospheric air column, and precipitation averages about 1000 kg/m2-year.

Water freezes and (slowly) falls at the top of the troposphere, which defines the boundary with the stratosphere. The stratosphere is dry, not because water can't get there, but because it freezes and falls down into the troposphere.

Meanwhile, the air column weighs about 10,000 kg/m2, and 2,000 kg/m2 is oxygen. About 1% of it is in the stratosphere (and a tiny bit above in the mesosphere). So even if the stratosphere was not constantly getting replenished with oxygen from below, it would take 3.4 billion years before the stratosphere was emptied of oxygen by hydrogen appearing in a "galactic wind". In fact, it is replenished from the reservoir below, so it would take closer to 340 billion years.

But what about the ozone?

The stratosphere reaches from 10 to 50km altitude. Ozone forms in the lower stratosphere (it is destroyed by sunlight UV in the upper stratosphere) and is maximum at 20km altitude (about 5E18 molecule/m3). In my "US Standard Atmosphere 1976", I do not have hydrogen number densities for the stratosphere, but at 150 km the hydrogen density is 4E11/m3, the mean free path is about 100 meters, and the temperature is 270K, ( v(vert) = sqrt ( kT/m ) = 1100 m/s for H2 ). So the hydrogen around earth produces 4E11 collisions per second at that very high altitude ( slower but denser than the interstellar wind ), about 30 times the the atomic flux rate of the interstellar wind. This does not destroy the ozone (down at 20km), where the hydrogen is even denser. Keep in mind that the earth orbits around the sun at 30 km/s, so it sweeps through interplanetary hydrogen nearly as fast as the solar system travels through the interstellar medium.

Comparison to Coronal Mass Ejections

Coronal mass ejections from the Sun have roughly similar density and speed, about 3 protons/cm3 and 500 km/sec . These can result in geomagnetic storms, causing field changes at the surface of the earth of 5 microteslas per second. The field changes can induce DC voltages and currents in power lines, causing transformers to saturate. However, these terrestrial problems are caused by a change in the particle flux. So the real problem would a rapid onset in particle flux (CMEs do this) and a similarly rapid magnetic change. It is hard to imagine a multi-AU sized event having the same rapid onset as a bubble of fast moving solar coronal plasma.

What if the interstellar density increased greatly?

This is not suggested in the novel, but if the earth passed through a very dense hydrogen cloud (say, just before star formation), that would have more significant effects. The field of the sun's magnetic dipole drops off as 1/R3. The magnetic pressure is the square of the field. So if the density was enough to push the bow shock in from 230AU to 1AU, that suggests the cloud density is 2306 times larger, about 1.5E14 times denser than 1/cm3, or about 1.5E20 hydrogens per m3. That is 250 mg/m3, about the density at 60 km altitude. If the cloud was a uniform sphere (unlikely) massing as much as the Sun at this average density (2E30 kg), it would be about 250 million kilometers across, a bit smaller than the Earth's orbit. At 50km/sec, the Earth would pass though this sphere in less than 2 months.

A very exciting two months to be sure. The pressure of the passage is the mass density times the velocity squared, 625 KPa, six atmospheres. The power flow is mass density times velocity cubed, 30 gigawatts per square meter. The earth's atmosphere and upper oceans would be stripped away, and the top of the crust would be vaporized or melted. This would be a brief and violent foretaste of the what will happen when the sun expands into a red giant - that will be a lot less dense and hot, but the earth will erode away in a million years or so as it is enveloped in the outer corona and the orbit decays.

Very likely the Earth would miss all such clouds entirely, 1AU in the immensity of the galaxy is a very small bulls-eye, and such clouds will be very nonuniform as they collapse into a sun-sized ball of gas.


I have great respect for Dr. Benford as an author and as a plasma physicist, but I think the needs of the story (something that noticably affects the Earth) won out over the physics.

Starborn (last edited 2012-10-27 17:31:46 by KeithLofstrom)