NS&T : Frequently Asked Questions : Space

Space : Future

Frequently Asked Questions : Space


What is Space Nuclear Power? What Types of Radiation are Found in Space? What is radiation? What is the difference between ionizing and non-ionizing radiation? What are the measurements of radiation and radiation dose? What is the effect of radiation on the body? What dose of radiation if fatal? How much radiation do I receive? What is an RTG? What about RTGs reentering the atmosphere and spreading plutonium throughout the atmosphere? What if an RTG did burn up in re-entry? What about reactor cores burning up on reentry?

What is Space Nuclear Power?
	Using nuclear technology to power and/or propel space vehicles. The main application that is used extensively is a radioisotope thermoelectric generator (RTG) that provides electrical power where chemical batteries and solar power cannot be used because of large power concerns or weight restrictions. Reactors have been designed and tested for larger scale power production on space missions. The soviets have even launched space vehicles with reactors on board. Also, nuclear rocket technology has been developed and tested, even though it has never been used.

What Types of Radiation are Found in Space?
	There are three naturally occurring sources of space radiation: trapped radiation, galactic cosmic radiation (GCR), and solar particle events (SPE). Trapped Radiation: The rotation of Earth's metallic core causes the formation of magnetic field lines around the Earth. The Sun causes a constant stream of ionized particles, which are mostly electrons and protons, to stream out into space. The Earth's magnetic field protects the surface from the particles from the sun by deflecting them. However, not all of the particles are deflected. Some if the particles are trapped in one of two doughnut-shaped magnetic rings surrounding the Earth called the Van Allen radiation belts. These particles then circle around Earth's magnetic lines. Most manned missions stay well below the Van Allen radiation belts, letting Earth's magnetic field protect them. Galactic Cosmic Radiation: GCR comes from outside of the solar system. It is made of ionized atoms that travel very close to the speed of light and can produce intense ionization when they pass through matter. The magnetic field protects us from most cosmic radiation except over the north and south poles. Solar Particle Events: These are energetic electrons, protons, alpha particles, and heavier particles shot into space by the sun (and other stars). These particles can arrive within 10 minutes or over the course of a day after solar flares, and cause a temporary rise in radiation levels in interplanetary space around the magnetosphere. They also may penetrate to low altitudes in the polar regions. These increases in radiation can cause displays of light in the atmosphere known as the Northern (and Southern) Lights.

What is radiation?
	Radiation is a generic term that refers to all electromagnetic waves (radio-waves, microwaves, gamma rays, X-rays, etc.) and atomic and subatomic particles (protons, neutrons, mesons, alphas, etc.).

What is the difference between ionizing and non-ionizing radiation?
	Non-ionizing radiation is all around us and we use it every day to talk on cellular phones, cook food, and watch T.V. Non-ionizing radiation does not have sufficient energy to create ion pairs that damage the DNA of living organisms. Examples of non-ionizing radiation are radio waves and microwaves. Ionizing radiation has sufficient energy to displace, and damage genes contained in DNA. Examples of ionizing radiation are X-rays, gamma rays, neutrons, protons, alpha particles, and beta particles. We are exposed to ionizing radiation during our daily lives, some comes from space and some is emitted from naturally occurring radioactive sources in the earth such as radon.

What are the measurements of radiation and radiation dose?
	Traditionally in the United States, rads and rems are used to measure absorbed dose and dose equivalent. Rads measure energy deposition per unit mass (100 ergs/g) and are independent of the material in which the radiation deposits its energy. Rems modify a dose to reflect its affect on a person. In fact rem originally stood for Radiation Equivalent in Man. The factor that multiplies a dose in rads to get the dose equivalent in rems is called the quality factor or Q. For gamma and beta radiation the Q is 1, for neutrons of unknown energy the Q is 10, and for alpha particles the Q is 20. Although, it should be noted that alpha particles emitted from outside the body are stopped by the dead layer of skin and essentially give no radiation dose, and it is only when alpha materials are inhaled or are otherwise taken into the body that alpha radiation is a problem. In other countries, units of Sieverts and Grays are used. A Sievert is 100 rems and one Gray is 100 rads.

What is the effect of radiation on the body?
	Radiation ionizes molecules in the body that then can cause damage to DNA. The effect of low doses on people is not well understood. This is because there is no measurable change in the body and it is impossible to determine whether cancer is radiation induced or caused by some other phenomenon. However, The Nuclear Regulatory Commission (NRC) estimates 0.0004 health effects per rem.

What dose of radiation if fatal?
	The lethal dose of radiation varies from person to person and depends on whether or not they receive medical treatment. However, the LD 50 dose (dose at which 50 percent of the population exposed will die) is about 500 rems. This dose is assumed to be whole body and to be absorbed in a short time. A dose of 1000 rem is sufficient to kill 100% of those exposed.

How much radiation do I receive?
	The average value for background radiation on Earth is around 0.36 rems per year. The dose does depend on where you live, how much time you spend on an airplane, your number of medical x-rays, and your occupational dose if applicable. The Nuclear Regulatory Commission imposes an annual limit for radiation dose of 5 rems (this is an additional dose above background).

What is an RTG?
	RTG stands for Radioisotope Thermoelectric Generator. Usually they contain Pu-238 or Pu-239 depending on the application. These isotopes decay by emitting an alpha particle, which is easily stopped, and the heat recovered. This heats two dissimilar metals that are fused together and creates a voltage.

What about RTGs reentering the atmosphere and spreading plutonium throughout the atmosphere?
	Several modern RTGs have re-entered the atmosphere and since the material is contained in high temperature ceramics they maintain their integrity upon re-entry, and don't release radioactive material. Examples are the Apollo 13 Lunar Module, and the Nimbus B-1 satellite. Neither released any radioactivity, and the radioactive material from Nimbus B-1 was recovered and reused in future missions

What if an RTG did burn up in re-entry?
	In 1964 the Transit 5BN navigational satellite reentered the atmosphere and the satellite along with the RTG burned up in the atmosphere. However, this RTG was designed by the U.S. Navy before the philosophy of containing the radioactive material upon reentry was adopted, and instead was designed to burn up in the upper atmosphere about 75 miles up. There was a small amount of material that was spread over a large area so it didn't create any environmental impact.

What about reactor cores burning up on reentry?
	The Soviets had several vessels with reactors on board re-enter the atmosphere and burn up. Several of these released radioactivity into the atmosphere, but it was released high in the atmosphere and spread over a large area. Only the reentry of the Kosmos 954 left contamination on the ground, and the area in Canada was cleaned up and currently there is no detectable radioactivity.