November 14, 1999

Mars Orbiter Lost Due to Mistake in Units

Mars OrbiterMost physics and chemistry textbooks start with a chapter dedicated to units, precision, and significant digits. You may discover that your science teacher will not accept a value as a correct answer if there are no units attached. The fate of NASA's $125 million Mars orbiter shows that teachers are not just being picky, they are practicing good science.

The Mars orbiter had completed a 9-month journey and on September 23 was ready to be placed in orbit around Mars. While the engineers sent signals to the orbiter to adjust its path, the spacecraft moved too close to the planet's atmosphere and broke into pieces or burned. At first the engineers did not know what became of the orbiter. An investigation into the mishap discovered that one team of engineers working on the orbiter had used metric units while another team had used English units for a key spacecraft operation. Therefore, information did not transfer between the teams at the critical moment, and the orbiter was lost.

Read an account: NASA's metric confusion caused Mars orbiter loss or Mars Climate Orbiter Team Finds Likely Cause of Loss

Read about America's problem with the metric system: Mars probe mishap shows metric system still tripping up Americans

Learn about the Problem

This week, sends you to other Web sites to learn about units and significant digits. Note that the Chemistry Gateways series does have a unit conversion tool available in all activities.

  • How Many? A Dictionary of Units of Measurement
    Offers background information on the International System (SI), the English system, and the metric system. The dictionary defines units of measurement from all areas of human endeavor and from many different cultures.

  • Science Made Simple's Unit Converter
    Online unit converters where you input a value and a unit and choose the target unit for conversion. You receive immediate answers.

  • Significant Figures
    Covers significant figures, scientific notation, math operations. Gives examples and has some practice problems with explanations of the answers.

Think about the Problem

Consider how confusing it would be if the following common items did not provide units:

  • a cookbook (Why did my cake turn out so salty?)
  • instructions for building a birdhouse, not from a prepared kit (Why is my birdhouse as big as my bedroom?)
  • the price of gasoline at the filling station (What do you mean the price is really $0.99 a quart?!!)

Throughout the day, notice the things around you that are labeled with units, e.g., light bulbs, download speeds on the Internet.

For the Teacher: Extending the Problem

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