Science · Physical Science
Chapter 1: Measurement and the Nature of Science
A number without a unit is not a measurement.
On 23 September 1999 the Mars Climate Orbiter fired its engine to go into orbit and was never heard from again. It had passed about 57 km above Mars, where the lowest safe height was about 80 km.
How does correct arithmetic lose a spacecraft?
Make your guess to turn the page.
Still to come: 9 pages · Lab · Words to Know · What We Learned · 4 questions · Explain It in Writing · Open Questions
The idea this lesson is working on — Students treat units as labels written on the answer, so a conversion becomes a guess about whether to multiply or divide. The book sets that against the orbiter, where every number was right and the units were wrong, and the lab, where an upside-down factor produces mL²/L. Ask: which units cancel, and what is left? The idea to reach is that units are part of the algebra.
Why the kilogram changed — Until 2019 the kilogram was a platinum-iridium cylinder near Paris. Over a century it drifted by about 50 micrograms against its official copies, and nobody could say which had changed, since the cylinder was the kilogram by definition. It is now fixed by the Planck constant, so any suitably equipped laboratory can realize it. The orbiter details are from NASA’s 1999 mishap board; the 57 km and 80 km figures are its estimates.