The Elemental and Isotopic Composition of Matter:
A major objective is the accurate and comprehensive determination of the elemental and isotopic composition of the various samples of "source material" from which nuclei are accelerated. Thus, ACE measurements will:

What Elements and Isotopes are found in space?
We really want to know, accurately and completely, which elements and what isotopes of those elements are found in the Sun, the local solar system, and in the galaxy as a whole. ACE measurements will help us do this by:

• Generate a set of solar isotopic abundances based on direct sampling of solar material;

• Telling us how many of each type of isotope is present in the sun, by taking samples of atoms in the solar wind;

• Determine the coronal elemental and isotopic composition with greatly improved accuracy;

• Finding out what elements and isotopes are found in the sun's corona [as opposed to the inner layers of the sun];

• Establish the pattern of isotopic differences between galactic cosmic ray and solar system matter;

• Seeing what the difference is between the isotopes present in galactic cosmic rays, and solar system matter;

• Measure the elemental and isotopic abundances of interstellar and interplanetary "pickup ions";

• Measuring the number and kind of elements and isotopes present in interstellar and interplanetary "pickup ions";

• Determine the isotopic composition of the "anomalous cosmic ray" component thought to represent a sample of the very local interstellar medium

• Finding out what isotopes are present in the unusual cosmic rays that we think might be representative of the area right around our solar system.

Origin of the Elements and Subsequent Evolutionary Processing:
Isotopic "anomalies" in meteorites indicate that the solar system was not homogeneous when formed, while other data suggest that the solar composition continues to evolve. Similarly, the galaxy is neither uniform in space nor constant in time due to continuous stellar nucleosynthesis. ACE measurements will:

Where did the elements come from, and how have they changed since they were formed?
The fact that the number and kind of isotopes found in meteorites are different from other sources we see suggests that the solar system did not have the same composition all over when it was formed. Other data points to the idea that the kind of elements present in the solar system continues to change. The galaxy is similar, in that it was also different in the past than it is now, because stars are constantly making heavier elements through fusion reactions. ACE measurements will:

• Search for additional differences between the isotopic composition of solar and meteoritic material;

• Look for other differences between the number and kind of isotopes in the sun's material and in meteors;

• Determine the contributions of solar-wind and solar flare nuclei to lunar and meteoritic material, and to planetary atmospheres and magnetospheres;

• Determine what differences in isotope composition within the moon, meteors, and the atmospheres and magnetospheres of planets are caused by nuclei in the solar wind and in solar flares;

• Determine the dominant nucleosynthesic processes that contribute to cosmic ray source material;

• Find out what fusion reactions produce the most cosmic rays, and how they are carried out;

• Determine whether cosmic rays are a sample of freshly sythesized material (e.g., from supernovae), or of the contemporary interstellar medium;

• Figure out if cosmic rays come from relatively recent stellar events, like supernovae, or if they are present in the interstellar medium pretty much all the time;

• Search for isotopic patterns in solar and galactic material as a test of galactic evolution models.

• Look for patterns in the number and kind of isotopes in materials from the sun and from the galaxy, to test current models of how the galaxy has changed through time.

Formation of the Solar Corona and Acceleration of the Solar Wind:
Solar energetic particles, solar wind, and spectroscopic observations show that the elemental composition of the corona is differentiated from that of the photosphere, although the processes by which this occurs, and by which the solar wind is subsequently accelerated, are poorly understood. The detailed composition and charge-state data provided by ACE will:

How is it that the Sun's corona is so different from interior layers, and how does the solar wind get so fast?
The study of solar energetic particles, the solar wind, and the sun itself show that the number and kinds of elements are different in the corona from the photosphere (the next layer down in the sun), but this fact, and the way the solar wind gets so fast, are not well understood. ACE will make much more detailed analysis of the elements and their ions, to help us:

• Isolate the dominant coronal formation processes by comparing a broad range of coronal and photospheric abundances;

• Find out what the most important process in the formation of the corona, by comparing a large range of particle abundances in the corona and the photosphere;

• Study plasma conditions at the source of the solar wind and the solar energetic particles by measuring and comparing the charge states of these two populations;

• Study the condition of the ionized gas known as plasma, where the solar wind and the solar energetic particles come from, by measuring and comparing the charge states of these two populations;

• Study solar wind acceleration processes and any charge or mass-dependent fractionation in various types of solar wind flows.

• Study the processes that cause the acceleration of the solar wind, and whether there is any separation of solar wind flows due to the charge or mass of particles being accelerated.

Particle Acceleration and Transport in Nature:
Particle acceleration is ubiquitous in nature and is one of the fundamental problems of space plasma astrophysics. The unique data set that will be obtained by ACE measurements will:

How are particles accelerated and moved around?
Particle acceleration happens all over in nature, and is a major question in the study of space plasmas. The data set that will be obtained by ACE instruments alone will:

• Make direct measurements of charge and/or mass-dependent fractionation during solar flare and interplanetary acceleration;

• Make direct measurements of any separation of solar wind flows due to the charge or mass of particles being accelerated during solar flares and the acceleration between planets;

• Constrain solar flare and interplanetary acceleration models with charge, mass, and spectral data spanning up to five decades in energy;

• Help to narrow the current models of solar flare and interplanetary acceleration by providing extensive charge, mass, and spectral data that will span up to five orders of magnitude (powers of ten) in energy;

• Test theoretical models for 3He-rich flares and solar gamma ray events;

• Test theoretical models for solar flares and gamma ray events, rich in a particular isotope of Helium (with two protons and one neutron);

• Measure cosmic ray acceleration and propagation time scales using radioactive `clocks`.

• Measure the acceleration of cosmic rays, as well as their movement through space, using radioactive "clocks" (which involves measuring the rates of decay of isotopes).