Why is it so hard to figure out what's going on?

The material that the Earth and the solar system are made of has been changed and rearranged during the billions of years since its creation, so measuring its complete composition, or makeup, is difficult. We have good evidence that the first elements to appear in the early universe were the lightest ones, hydrogen and helium. Most of the gas found between the stars in our Milky Way galaxy and in other galaxies throughout the universe as well are hydrogen and helium. We also know that the Sun is made chiefly of hydrogen and helium. We understand that stars, including our Sun, shine by the process of combining, or "burning", lighter elements into heavier ones, hydrogen into helium, and helium into carbon, and so on. In fact, in the outer layers of the Sun, we can see the light emitted by heavy elements like carbon, silicon, and iron. These elements were formed by earlier generations of stars.

Scientists have attempted to answer questions about where this matter came from and how it evolved in a variety of ways. Meteorites that have hit the Earth can be studied since, in some respects, they seem to be like the solar system when it formed. Another way to study the material is by going into space, above Earth's atmosphere, to study particles that come from the Sun. Early in this century, scientists learned that matter from space is bombarding the Earth. With the advent of space missions, we learned that it comes not only from the Sun, but also from the distant reaches of the Galaxy. It has been recently discovered that some of these particles come from the gas clouds outside our solar system.

Science has barely scratched the surface in examining the actual source of the particles traveling through space around us. Knowledge of the many processes that created the current order is difficult to attain but rewarding. Theories are upheld or upset, and new theories take their place. As new information becomes available, from both spacecraft and Earth-based instruments, the picture becomes clearer.

Top Ten Reasons for a Mission to the Interstellar Medium

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