FEATURE ARTICLE
The Galactic Environment of the Sun
The heliosphere appears to protect the inner solar system from the vagaries of the interstellar medium
Priscilla Frisch
An Interstellar Probe
Our current understanding of the sun's galactic environment is derived almost entirely by remote methods, from earth-based telescopes or near-earth spacecraft. However, the best way to explore the local galactic environment is with an interstellar spacecraft, launched into the upwind direction, beyond the "nose" of the heliosphere, and into pristine interstellar space. Preparatory studies for such a spacecraft—known as the Interstellar Probe—were recently conducted at the Jet Propulsion Laboratory in Pasadena, California. The Interstellar Probe would be able to directly assess the interaction of a star (our sun) with its environment. By collecting information on the physical properties of the interstellar cloud surrounding the solar system, we will make the first-ever in situ measurements of gas and dust in interstellar space.
A direct assessment of the interstellar medium is not merely a luxury but a necessity. When we observe interstellar gas with telescopes, generally we sample clouds many light-years long. Consequently, we cannot be sure which part of the cloud we are actually measuring. On the other hand, observations of interstellar matter within the solar system reveal its nature at the entry point into the heliosphere. A comparison of the telescope data and the interstellar matter inside the solar system is not always meaningful because they don't come from the same part of the cloud. To overcome this problem and truly understand the physical properties of the interstellar cloud feeding material into the solar system, we must directly sample the cloud with instruments on board an interstellar spacecraft.
The Interstellar Probe is being designed to explore the nature of the interstellar medium and its interaction with the solar wind and the solar system. It will provide detailed information on the composition, ionization state, magnetic-field strength and other physical properties of the cloud surrounding the sun. The result will be a quantitative understanding of how interstellar gas and dust interact with the solar wind to determine the heliosphere's properties and thus how changes in the interstellar medium would affect the heliosphere.
Results from such a mission may be forthcoming sooner than one might think. Solar-sail propulsion methods are now being considered that could propel a spacecraft with a speed of about 14 AU per year, so that the probe would emerge into the interstellar medium (about 150 AU away) within 15 years of being launched! Such a spacecraft would establish a new era, when humankind finally escapes the bounds of the solar system and looks out on the stars, rather than up at them.
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