Units  (Page 2/4)

A notable engineering failure: the nasa mars climate orbiter

In 1999, NASA experienced the failure of its Mars Climate Orbitor spacecraft because a Lockheed Martin engineering team used English units of measurement while a NASA engineering team used the more conventional metric system for a key spacecraft operation. (NASA, 1999). This mismatch in units prevented the navigation information from transferring properly as in moved between the Mars Climate Orbiter spacecraft team in at a Lockheed Martin ground station in Denver and the flight team at NASA’s Jet Propulsion Laboratory in Pasadena, California.

Working with NASA and other contractors, Lockheed Martin helped build, develop and operate the spacecraft for NASA. Its engineers provided navigation commands for Climate Orbiters thrusters in British units although NASA had been using the metric system predominantly since at least 1990.

After a 286 day journey, the spacecraft neared the planet Mars. As the spacecraft approached the surface of Mars, it fired its propulsion engine to push itself into orbit. Instead of the recommended 276 kilometer orbit, the spacecraft entered an orbit of approximately 57 kilometers. Because the spacecraft was not in the proper orbit, its propulsion system overheated and was subsequently disabled. This allowed the Mars Climate Orbiter to plow through the atmosphere out beyond Mars. It is theorized that it could now be orbiting the sun

The primary cause of this discrepancy was human error. Specifically, the flight system software on the Mars Climate Orbiter was written to calculate thruster performance using the metric unit Newtons (N), while the ground crew was entering course correction and thruster data using the Imperial measure Pound-force (lbf). This error has since been known as the metric mixup and has been carefully avoided in all missions since by NASA.

Unit conversion procedure

The process of transforming from one unit of measure to another is called unit conversion . One can easily perform unit conversion using the procedure that will be presented in this section. You will soon discover that performing unit conversion can be reduced to multiplying one measurement by a carefully selected form of the integer 1 to produce the desired measurement.

Prior to presenting the procedure of unit conversion, it is important to understand a simple fact. Numbers with units such as 25.2 kilometers or 36.7 miles can be thought of and treated in exactly the same manner as coefficients that multiply variables, such as 25.2 x or 36.7 y . Of course here, x and y are variables.

From Algebra, we know that we can always multiply a quantity by 1 and retain its value. The key idea of unit conversion is to choose carefully the form of 1 that is used. We will illustrate this idea by means of an example.

Suppose that we wish to convert 25.2 kilometers to miles. In order to accomplish this conversion of units, it is important that one know the following information

Let us take this equation and divide each side by the term 1 km , as shown below