Mars Climate Orbiter Lost, September 23, 1999: The Metric Mix-Up

NASA lost the Mars Climate Orbiter on September 23, 1999, as the spacecraft fired its engine to slip into orbit around Mars. It came in far too low and was destroyed in the Martian atmosphere. A week later, NASA’s Jet Propulsion Laboratory announced the likely cause: one team had used English units and another had used metric units for the same critical data.

What Happened on September 23, 1999

The orbiter had been traveling for 286 days since its launch from Cape Canaveral on December 11, 1998. Early on the morning of September 23, it began a planned 16-minute, 23-second engine burn to slow down and be captured by Mars’s gravity. Signals showed the burn starting at about 5:01 a.m. Eastern Daylight Time, as received on Earth.

Contact was lost at 09:04:52 Universal Time, about 49 seconds earlier than the flight team expected, and the spacecraft was never heard from again.

The flight plan called for the orbiter to pass about 140 to 150 kilometers above the surface. Later analysis showed it had actually dipped to about 57 kilometers, deep enough in the atmosphere to destroy it. Engineers had considered about 80 kilometers to be the lowest altitude the spacecraft could survive.

How a Units Error Sent the Orbiter Off Course

During the long cruise to Mars, the spacecraft’s small thrusters fired from time to time. Each firing nudged its path slightly, so navigators needed accurate figures for the impulse those thrusters produced.

Ground software supplied by Lockheed Martin, which built the spacecraft, reported the thruster impulse in pound-force seconds. The navigation software at JPL expected the figures in newton-seconds, the metric unit NASA’s system was built around. One pound-force second equals about 4.45 newton-seconds, so every firing was underestimated by that factor.

The error was small with each firing but added up over months of flight. In its September 30, 1999, announcement, JPL described the problem as a failure to recognize and correct an error in information passed between the spacecraft team in Colorado and the navigation team in California.

Warning Signs Before the Loss

The trajectory problem did not go completely unnoticed. After the fourth trajectory correction maneuver on September 15, navigators’ estimates of the arrival altitude kept dropping. A day before arrival, the projected closest approach was about 110 kilometers, lower than planned but still above the survival limit, according to the later investigation.

At least two navigators raised concerns, but their worries were set aside because they had not been filed through the project’s formal procedures. A fifth correction maneuver was discussed and never carried out.

Timeline: Mars Climate Orbiter and Mars Polar Lander

• December 11, 1998: Mars Climate Orbiter launches on a Delta II rocket from Cape Canaveral.
• January 3, 1999: Its companion, Mars Polar Lander, launches.
• September 15, 1999: The fourth trajectory correction maneuver sets up the arrival.
• September 23, 1999: The orbiter is lost during Mars orbit insertion.
• September 30, 1999: JPL announces the likely cause, a units mismatch.
• November 10, 1999: NASA’s Mishap Investigation Board releases its Phase I report.
• December 3, 1999: Mars Polar Lander is lost during its landing attempt.

The orbiter’s launch appears on the 1998 calendar and its loss on the 1999 calendar. The days between dates calculator confirms the 286-day gap between them.

NASA’s Response: “Not the Error”

NASA did not simply blame its contractor. Edward Weiler, the agency’s associate administrator for space science, said people sometimes make errors and that “the problem here was not the error, it was the failure of NASA’s systems engineering” to catch it. JPL director Edward Stone said the lab’s inability to recognize and correct “this simple error has had major implications.”

The loss had a second cost. The orbiter was meant to study Mars from orbit and to relay communications for Mars Polar Lander. That lander was then lost on December 3, 1999, when a false touchdown signal most likely caused its software to shut off the descent engines too early. NASA’s cost figures for the two-spacecraft Mars Surveyor ’98 program total $327.6 million.

Thomas Young, who chaired an independent review of the Mars program, concluded that the Mars Surveyor ’98 effort had been “under funded by at least 30%.” Losing both spacecraft within about 10 weeks put NASA’s Mars program under close review.

Why the Mars Climate Orbiter Still Matters

The Mars Climate Orbiter is a lasting example of why units must be stated and checked wherever data passes from one team or program to another. The mistake was a single missing conversion between two pieces of software, one built by a contractor in Colorado and one used by navigators in California.

NASA’s own officials framed the deeper lesson as one of process. The error happened, but the loss occurred because reviews, testing and the navigators’ concerns all failed to catch it before the spacecraft reached Mars. Readers comparing mission records should also note that NASA logs times in Universal Time, while the summary here uses Eastern Daylight Time; the guide to US time zones explains the difference.