A meteorite is a natural object, often a fragment from an asteroid or comet, that survives passage through Earth’s atmosphere and lands on the surface. A common question is whether these celestial visitors pose a radiation hazard. All meteorites are radioactive, but the levels are generally quite low, often comparable to common terrestrial rocks. This slight radioactivity results from two distinct processes: original elements present since the solar system formed and a secondary effect from their long journey through space.
Inherent Radioactivity from Formation
Every rock, whether terrestrial or extraterrestrial, contains natural radioactivity, known as primordial decay. This inherent radioactivity comes from long-lived isotopes incorporated into the meteorite’s matrix when the solar system coalesced 4.5 billion years ago. These elements decay extremely slowly over immense geological timescales, allowing them to persist today.
The primary long-lived isotopes found in meteorites are Potassium-40 (K-40), Uranium-238 (U-238), and Thorium-232 (Th-232). The concentrations of these primordial elements are often lower in meteorites than in Earth’s continental crust. Terrestrial geological processes can concentrate elements like uranium into higher-activity deposits, a process generally absent in most space rocks. Therefore, the radiation from this source is typically less than what is measured from many common Earth rocks.
Induced Radioactivity from Cosmic Rays
The second source of radioactivity is induced, created while the space rock travels through the solar system. The meteoroid is constantly bombarded by high-energy galactic cosmic rays, primarily protons and heavier ions. This energetic particle flux causes nuclear reactions within the structure, a process called spallation or activation, which creates new, short-lived radioactive isotopes known as cosmogenic nuclides.
The production rate of these elements depends on the meteoroid’s size and depth, as outer layers shield the interior. Analyzing these nuclides allows scientists to determine how long a body was exposed to cosmic rays before falling to Earth. Examples include Aluminum-26 (Al-26), Sodium-22 (Na-22), and Cobalt-60 (Co-60).
These isotopes have relatively short half-lives, meaning their induced radioactivity fades over time once the meteorite is shielded on Earth. Freshly fallen meteorites exhibit the highest induced activity because these short-lived radionuclides have not yet had time to decay.
Hazard Assessment and Safe Handling
The combined radiation from both inherent and induced sources in a typical meteorite presents a negligible health risk to the public and collectors. Most meteorites do not emit radiation levels substantially above the natural background radiation found everywhere on Earth. Even in a freshly fallen sample, the induced radioactivity is concentrated in short-lived isotopes that rarely pose a danger, though they can be measured in a laboratory.
Handling a meteorite requires more caution to protect the space rock itself than to protect the person from radiation. Human hands transfer oils, moisture, and microbes that can contaminate and degrade the surface over time. To preserve the scientific value of a discovery, handle the specimen with clean gloves, tongs, or aluminum foil to prevent contamination.