"Where did I come from?" AdvancedAdvanced This question is usually one about life, but behind it are scien- CompositionComposition tific questions about the material of which we are made, the atoms and molecules of our bodies. ExplorerExplorer The answer to the question Energetic Particles "Where did the matter we are made of come from?" is not so The acceleration of charged particles easy to find. The Earth and the to extremely high energies takes solar system are made of materi- place throughout the universe: on the Sun, in interplanetary space, at the al that has been changed and edge of the solar system, at the blast rearranged during the billions of waves of supernovae, and in other years since its creation, so mea- powerful systems, like neutron stars suring its complete makeup, or and black holes. The diagram on the composition, is important to back of this page helps to illustrate understanding our origins. the origins of some of these particles, described below. Scientists have tried to answer these questions in many ways. From the Sun: Safety and communications during Early in this century, we learned Solar wind is the stream of elec- space flight may be seriously impact- that matter from space is bom- trically-charged particles flowing ed by major solar activity. The con- barding the Earth all the time. outward from the Sun at an average tinuous broadcast of solar wind, With the beginning of space 400 km/second (almost a million magnetic field, and SEP data from missions, we learned that it miles per hour!) although this speed ACE allows accurate forecasts of comes not only from the sun, but can vary from 200 to 1000 space weather up to one hour before- also from the far distant reaches km/second. The Sun's output in all hand to help us prepare. of the galaxy. The Advanced forms (for example, light, solar wind, and energetic particles) varies with Composition Explorer, or ACE for time and the position of the Sun. short, studies the many different These changes are called solar activi- types of speeding, energetic par- ty and are probably reflections of ticles in the heliosphere, the changes below the Sun's surface. immense magnetic bubble con- Sunspots and solar flares are some taining our solar system and the visible signs of solar activity. We can Sun's magnetic field. ACE pro- study the output of the Sun and how vides crucial information for it varies to probe its inner workings. understanding the sources, In addition to the solar wind, which acceleration and movement of is continually present, occasionally these high-energy particles. the Sun will emit particles (solar energetic particles, or SEPs) that are The solar wind near the Sun con- up to ten times more energetic than tains streams of high and low speed the solar wind, and which are associ- particles. The high-speed streams ated with coronal mass ejections or originate in coronal holes (cooler and solar flares. less dense areas in the solar surface) The Sun's activity causes the and extend toward the solar poles. "space weather" that affects Earth. The low-speed streams come from Magnetic storms on Earth due to near the Sun's equator. As the Sun solar activity produce the beautiful rotates, alternating streams of high Aurora borealis and Aurora australis and low speed can come out in the (as seen at right), but can also cause a same direction. The high-speed solar variety of highly undesirable wind streams then overtake the consequences, such as electrical cur- slower plasma (plasma is gas made rent surges in power lines, and inter- up of electrically-charged particles) ference in radio, television, as the particles move away from the telephone, and defense communica- Sun, producing corotation interac- tion signals. tion regions (CIRs)

The Science Behind the Advanced Composition Explorer Mission, page 1 ACE's instruments provide coordinated measurements of all these different sam- ples of matter.

The ACE spacecraft was launched on a Delta II launch vehicle on August 25, 1997 from the Cape Canaveral Air Station in Florida. ACE is part of the Explorer Program, the longest continuing program at NASA, and so is also known as Explorer 71. Its mission lasts a mini- mum of two years, with a goal of five.

The spacecraft is about 1.5 million km (about a million miles) from Earth toward the Sun. It orbits a point, the L1 libration point, where gravity helps keep it at an ideal location to mea- sure solar particles and plasma 24 hours a day, without being affected by the Earth's magnetic field. Its Out in interplanetary space, Now no longer neutral, the charged nine instruments sample the matter CIRs produce shocks with high particles can be picked up by the in the heliosphere that comes near density, pressure, and magnetic field Sun's magnetic field and carried out- the Earth, covering an unprecedent- ed range of particle types and strength. CIRs very effectively accel- ward to the solar wind termination energies. Simultaneous measure- erate particles, which then travel shock, a region outside the planets of ments from ACE's instruments are back toward us on Earth. Particles our solar system, but still inside the coordinated and then compared to accelerated by interplanetary shocks heliosphere. Some of these pickup information from other missions, past are also observed and reveal infor- ions undergo repeated collisions and present, to create a comprehen- mation about matter in interplane- with the shock, gaining energy in the sive picture of the energetic particles tary space. process. A fraction of these escape near us. from the shock and scatter back From the local interstellar medium: toward the inner heliosphere. These By identifying the particles coming Most of the matter in the uni- accelerated particles are then known from the Sun, the local interstellar verse heavier than hydrogen and as anomalous cosmic rays (ACRs). medium, and the galaxy, ACE pro- helium was created in stars, where vides us with a better understanding lighter elements are heated to very From the galaxy: of the composition and evolution of high temperatures and fused into Galactic cosmic rays (GCRs) are the universe. heavier ones. This process is called the particles that flow into our solar nucleosynthesis. Supernova explo- system from far away in the galaxy. sions result when the cores of mas- GCRs are atomic nuclei from which sive stars have exhausted their fuel all the surrounding electrons have supplies and burned everything into been stripped during their high- iron and nickel. Such explosions speed passage through the galaxy. eject products of nucleosynthesis They have been accelerated to nearly back into the interstellar gas. This the speed of light, possibly by super- means, for example, that the iron in nova shocks, and may have traveled our blood was produced in massive many times across the galaxy, stars that went supernova billions of trapped by the galactic magnetic years ago! field. When we measure these heavi- While the electrically-charged er particles coming from space, we particles of the interstellar gas are can learn what their likely history is kept outside the heliosphere by the based on their composition. Included interplanetary magnetic field, inter- in these cosmic rays are a number of ACE Lead Scientific Institution: California Inst. of Tech. http://www.srl.caltech.edu/ACE stellar neutral gas flows through out radioactive particles. Their decay ACE Project Management: NASA Goddard Space solar system, at a speed of about 26 allows us to determine the age of Flight Center http://www.gsfc.nasa.gov/ace/ace.html km/second. Collisions with solar that matter, as in the carbon-14 tech- ACE Real Time Solar Wind: NOAA http://sec.noaa.gov/ace/ACErtsw_home.html ultraviolet light or particles can case nique used to date archaeological Cosmic and Heliospheric Learning Center these atoms to lose an electron. artifacts. http://helios.gsfc.nasa.gov

The Science Behind the Advanced Composition Explorer Mission, page 2