Dark Endeavours

AMS-2 on the ISS, artist concept

Artist concept of AMS-2 mounted on the ISS.When NASA's Space Shuttle Endeavour lifted off on its 25th and final mission to space on Monday morning, it carried in its cargo bay more than just supplies for the International Space Station. Alongside the containers of TV dinners, oxygen bottles, water tanks, and other various and sundry necessities was a bus-sized, two billion dollar observatory that will probe some of our Universe's deepest, darkest secrets: dark matter, and the unaccounted antimatter.

The Alpha Magnetic Spectrometer 2 (AMS-2), like other space-borne observatory satellites, will make its observations outside of Earth's atmosphere, attached to the International Space Station—but unlike "conventional" satellite observatories such as the Hubble Space Telescope, Kepler, and the Solar Dynamics Observatory, AMS-2 will not observe electromagnetic radiation (light), but rather cosmic rays.

Cosmic rays are energetic, electrically charged subatomic particles whizzing through space, originating from various places such as the Sun, distant stars and supernovae, and other high-energy sources from the most distant reaches of the known Universe. Most cosmic rays are simply high-speed protons (hydrogen nuclei) and alpha particles (helium nuclei). Less than 1% of cosmic rays are heavier atomic nuclei, and electrons. All of these particles are things familiar to us on Earth, the main differences being their exotic origins and their extremely high speeds—often approaching the speed of light.

A tiny fraction of cosmic rays are exotic particles indeed: antimatter, in the form of positrons (the positively charged antimatter counterpart of electrons) and anti-protons.


What is AMS-2 looking for? In a nutshell, it's looking for what we cannot see…. Only about 5% of the Universe is composed of "ordinary" matter—the stuff we are made of, and which we can see with telescopes out in the universe by virtue of the light it emits: stars, galaxies, nebulae, giant molecular clouds, and more.

An estimated 95% of the Universe's bulk is made up of "dark" stuff—dark matter (about 20%) and dark energy. Some of this dark matter may be accounted for by massive objects that we can't see, such as black holes, but the primary constituent probably consists of exotic particles that defy direct detection. A number of particles that fit this bill have been theorized, like neutrinos, and more recently neutralinos. In theory, interactions between neutralinos should produce charged particles in the form of cosmic rays that AMS-2 should be able to detect. If it does, then we'll have observational evidence for the existence of this exotic particle, which would shed some light onto some of the Universe's dark mystery….

AMS-2 will also look for antimatter. The Big Bang theory (the theory, not the TV show) of the formation of the Universe suggests that there should be equal parts matter and its counterpart antimatter, but so far we've mostly seen only the former. While there are positrons and anti-protons flying about that can be accounted for by processes involving nuclear interactions, if AMS-2 can detect a more complex anti-particle, like an anti-helium nucleus (an atom composed of two anti-protons, two anti-neutrons, and two positrons), then we'll have an example of antimatter that formed by more complex processes than a random nuclear collision or decay.

AMS-2's "lens" is not made of glass, but of magnetic fields. Conventional telescopes bend and focus light with glass lenses or curved mirrors, but AMS-2 will observe electrically charged cosmic rays, which can be collected and sorted with magnetic force. AMS-2 will count cosmic rays, determine what types of particles they are, and how much energy they possess (how fast they are moving).

From Earth's surface observing cosmic rays is nearly impossible, at best. The particles interact with the nuclei of atoms in our atmosphere, forming a different subatomic particle and a burst of "secondary" cosmic radiation—which can be detected by ground-based instruments, but only as "second hand" news. In fact, it is cosmic ray interactions with ordinary carbon (carbon 12) in Earth's atmosphere that transform them into the radioisotope carbon 14, which scientists take advantage of to determine how long a sample of formerly living material has been dead (as in carbon dating).

So, stay tuned for news on what scientists discover as they peer into the very dark darkness of a currently unknown realm of existence. Should be exciting….

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