Category Archives: Nebula

Astronomy, Nebula, Physics, Space Travel, Stars

At what speed does the interstellar medium become lethal to high speed flight?

The dilute interstellar medium permeates space at a density of about one hydrogen atom per cubic centimeter. This image shows an all-sky map of this hydrogen observed by the Wisconsin H-Alpha Mapper (WHAM) Northern Sky Survey (Haffner, L. M. et al, 2003, Astrophysical Journal Supplement, 149, 405).. The Wisconsin H-Alpha Mapper is funded by the (US) National Science Foundation.

We do not really know what the interstellar medium looks like at the human-scale. If it is just stray hydrogen atoms you will just experience a head-on flow of ‘cosmic rays’ that will collide with your spacecraft and probably generate secondary radiation in the skin of your ship. This can be annoying, but it can be shielded so long as the particles are not ultra-relativistic. At spacecraft speeds of 50-90% the speed of light, these particles are not likely to be a real problem. At speeds just below the speed of light, the particles are ultra-relativistic and would generate a very large x-ray and gamma-ray background in the skin of your ship.

As it turns out, our solar system is inside a region called the Local Bubble where the density of hydrogen atoms is about 100 times lower that in the general interstellar medium. This Bubble, produced by an ancient supernova, extends about 300 light years from the Sun but has an irregular shape. There are thousands of stars within this region which is enough to keep us very busy exploring safely. Here is one version of this region by astronomers at the Harvard-Smithsonian Center for Astrophysics.

Interstellar space also contains a few microscopic dust grains (micron-sized is common) in a region about a few meters on a side. At their expected densities you are probably in for a rough ride, but it really depends on your speed. The space shuttle, encountering flecks of paint traveling at 28,000 mph (about 6 miles/second or 0.005 percent the speed of light) is pitted and pierced by these fast moving particles, but dust grains have masses a thousand times smaller than the smallest paint fleck, so at 0.005 percent light speed, they will not be a problem.

At 50 percent the speed of light which is the minimum for interstellar travel you will cover enough distance in a short amount of time, that your likelihood of encountering a large interstellar dust grain becomes significant. Only one such impact would be enough to cause severe spacecraft damage given the kinetic energy involved.

A large dust grain might have a mass of a few milligrams. Traveling at 50% the speed of light, its kinetic energy is given non-relativistically by 1/2 mv^2 so E = .5 (0.001 grams) x (0.5 x 3 x 10^10 cm/sec) = 1.1 x 10^17 ergs. This, equals the kinetic energy of a 10 gram bullet traveling at a speed of 1500 kilometers per second, or the energy of a 100 pound person traveling at 13 miles per second! The point is that at these speeds, even a dust grain would explode like a pinpoint bomb, forming an intense fireball that would melt through the skin like a hot poker melts a block of cheese.

The dust grains at interstellar speeds become lethal interstellar ‘BB shots’ pummeling your spacecraft like rain. They puncture your ship, exploding in a brief fireball at the instant of contact.

Your likelihood of encountering a deadly dust grain is simply dependent on the volume of space your spacecraft sweeps out. The speed at which you do this only determines how often you will encounter the dust grain in your journey. At 10,000 times the space shuttle’s speed, the collision vaporizes the particles and a fair depth of the spacecraft bulkhead along the path of travel.

But the situation could well be worse than this if the interstellar medium contains lots of ice globules from ancient comets and other things we cannot begin to detect in interstellar space. These impacts even at 0.1c would be fatal…we just don’t know what the ‘size spectrum’ of matter is between interstellar ‘micron-sized’ dust grains, and small stars, in interstellar space.

My gut feeling is that interstellar space is rather filthy, and this would make interstellar, relativistic travel, not only technically difficult but impossible to boot! Safe speeds for current technology would be only slightly higher than space shuttle speeds especially if interstellar space contains chunks of comet ice.

This is an issue that no one in the science fiction world has even bothered to explore! The only possible exception is in Star Trek where the Enterprise is equipped with a forward-directed ‘Brussard Deflector’ (that big blue dish just below the main saucer) which is supposed to sweep away particles before they arrive at the ship. This is very dubious technology because hydrogen atoms are not the main problems a ship like that would have to worry about, especially traveling inside a planetary system at sub-light speeds. It’s dust grains!

Astronomy, Nebula, Stars

Why doesn’t the Sun blow up?

In fact, the Sun is doing a slow-motion explosion. It is shedding about 600 million tons every second in light energy, and it is loosing about 100 trillionth of its mass every year in the so-called solar wind. Here is a satellite photo of one of these mass ejections seen by the NASA/ESA SOHO satellite on December 2, 2003. These are dramatic events and often eject ‘a billion tons’ of plasma every few weeks or months. As impressive as they are, the sun is far more massive by a factor of a billion-billion times (1018).

But the sun will never blow up the way we think of a genuine explosion. It is the wrong kind of star to be either a nova or a supernova. It has no companion star for mass-transfer, and its mass is well below the 6-8 solar-mass limit when supernova detonations start to occur.

The energy of the Sun, the thermonuclear fusion which produces all the heat and light, is occurring in the core of the Sun. The weight of all the mass in the Sun in the overlying layers is so enormous that the Sun is in an equilibrium state where the internal thermal pressure is balanced by the gravitational pressure directed inwards.

Eventually, this balance will cease as the core depletes its hydrogen fuel. The core will collapse and heat up causing the outer layers to expand as a planetary nebula like the one shown here: NGC 6720 (Credit:ESA). This is still not a detonation that shatters the sun into interstellar space. In fact, more than 90% of its mass is left behind as a white dwarf ,which is a stable configuration of matter.