Interstellar Medium

Interstellar medium refers to the gas and dust particles that are found between the stars in our galaxy, the Milky Way. Up until about three decades ago, the interstellar medium was considered to be an uninteresting void. Today, through advances in astronomy (especially infrared and radio astronomy), we know that the interstellar medium contains a rich and interesting variety of atoms and molecules as well as a population of fine grained dust particles. More than 100 interstellar molecules have been discovered to date, including many organic molecules considered essential in the development of life. Interstellar dust (which “reddens” the visible light from the stars behind it because of its preferential scattering of shorter-wave-length photons) is considered to consist of very fine silicate particles, typically 0.1 micrometers (µm) in diameter. These interstellar “sands” sometimes may have an irregularly shaped coating of water ice, ammonia ice, or solidified carbon dioxide.

The properties and behavior of the interstellar medium must be carefully analyzed before any kind of transport in possible. In general, matter transport between star systems in a galaxy; specifically, travel beyond our own solar system is known as interstellar transportation. The “matter” transported may be (1) a robot interstellar probe on an exploration mission, (2) an automated spacecraft that carries a summary of the cultural and technical heritage of a civilization (e.g., the plaques on the Pioneer 10 and 11 spacecraft and the “recorded message” on the Voyager 1 and  2 spacecraft), (3) a “crewed” starship on a long-term round-trip voyage of scientific exploration, or even (4) a giant “interstellar ark” that is designed to transport a portion of the human race on a one-way mission beyond the solar system in search of new suitable planetary systems to explore and inhabit.

An interstellar probe is a highly automated interstellar space craft sent from our solar system to explore other star systems. Most likely this type of probe would make use of very smart machine systems capable of operating autonomously for decades or centuries. Once the robot probe arrived at a new star system, it would begin a detailed exploration procedure. The target star system would be scanned for possible life-bearing planets, and if any were detected, they would become the object of more intense scientific investigations. Data collected by the “mother” interstellar probe and any mini-probes would be transmitted back to Earth. There, after light-years of travel, the signals would be intercepted and analyzed by scientists, and interesting discoveries and information would enrich our understanding of the universe. Robot interstellar probes also might be designed to carry specially engineered microorganisms, spores, and bacteria. If a probe encountered ecologically suitable planets on which life had not yet evolved, then it could “seed” such barren but potentially fertile worlds with primitive life-forms or at least life precursors. In that way human beings would not only be exploring neighboring star systems but would be participating in the spreading of life itself through some portion of the Milky Way galaxy.

Questions to Ponder

• Why is the Cosmic Microwave Background not absorbed by the Hydrogen present in the interstellar medium?
• Can the ‘Voyager’ spacecraft be considered as an Interstellar probe? What was its purpose?