How do spacecraft orient themselves in the absence of magnetic poles? Is there any truth to the system they use on Star Trek? —S. Messick, Oklahoma City

Christopher Potts, a navigation engineer at the NASA Jet Propulsion Laboratory in Pasadena, Calif., shows the way:

Without an ever present magnetic field to rely on, as compass users have on Earth, those of us responsible for spacecraft navigation must utilize a three-dimensional Cartesian coordinate system, or frame, of our own devising.

One common frame currently used in deep space is called the Earth Mean Equator and Equinox of Epoch J2000, abbreviated as EME2000. Its name is so involved because it captures the many elements required to define a three-dimensional coordinate system: a reference body (Earth); a reference plane (the mean equator, an idealized equator that does not include the small nodding motion, or nutation, of Earth’s axis); a reference direction (the vernal equinox, a line from Earth to the sun on the first day of spring); and a reference time (J2000, or January 1, 2000, at 12:00:00 Ephemeris Time, a uniform timescale used for planetary motion calculations). The reference body and reference plane define the x-y plane of the frame. The z-axis is perpendicular to the plane, generally along the body’s axis of rotation. A reference time is required because the reference planes experience subtle motion caused by gravitational forces of the other bodies in the solar system.

Using the defined coordinate frame, a spacecraft must be able to both determine and control its orientation. Instead of a compass, spacecraft sensors use the sun and stars to determine the craft’s orientation relative to the coordinate frame. Desired directions can be specified in several ways with respect to the defined frame, but two angular measurements are commonly used. In astronomy, right ascension and declination identify directions in the sky. Right ascension is an angular measurement in the reference plane, and declination measures the angle above or below the reference plane.

Although the specifics may vary, determining directions in spaceflight relies on the basic principles of defining a reference frame and using measurements to determine orientation relative to that frame. As for the system on Star Trek (such as heading 294, mark 37), I doubt this method finds any current use in deep-space navigation. But by specifying two measurements, at least there is enough information to properly aim the warp drive.

How long will global uranium deposits fuel the world’s nuclear reactors at present consumption rates? —G. Peck, Seward, Alaska

Steve Fetter, dean of the University of Maryland’s School of Public Policy, supplies an answer:

If the Nuclear Energy Agency (NEA) has accurately estimated the planet’s economically accessible uranium resources, reactors could run more than 200 years at current rates of consumption.

Most of the 2.8 trillion kilowatt-hours of electricity generated worldwide from nuclear power every year is produced in light-water reactors (LWRs) using low-enriched uranium (LEU) fuel. About 10 metric tons of natural uranium go into producing a metric ton of LEU, which can then be used to generate about 400 million kilowatt-hours of electricity, so present-day reactors require about 70,000 metric tons of natural uranium a year.

According to the NEA, identified uranium resources total 5.5 million metric tons, and an additional 10.5 million metric tons remain undiscovered—a roughly 230-year supply at today’s consumption rate in total. Further exploration and improvements in extraction technology are likely to at least double this estimate over time.

Using more enrichment work could reduce the uranium needs of LWRs by as much as 30 percent per metric ton of LEU. And separating plutonium and uranium from spent LEU and using them to make fresh fuel could reduce requirements by another 30 percent. Taking both steps would cut the uranium requirements of an LWR in half.

Two technologies could greatly extend the uranium supply itself. Neither is economical now, but both could be in the future if the price of uranium increases substantially. First, the extraction of uranium from seawater would make available 4.5 billion metric tons of uranium—a 60,000-year supply at present rates. Second, fuel-recycling fast-breeder reactors, which generate more fuel than they consume, would use less than 1 percent of the uranium needed for current LWRs. Breeder reactors could match today’s nuclear output for 30,000 years using only the NEA-estimated supplies.