From the December 2015 issue

Has NASA kept track of the orbits of the Mariners and Rangers that missed the Moon? Do they know where they are and if they would return to Earth?

Calvin Remmers, Pierz, Minnesota
By | Published: December 28, 2015 | Last updated on May 18, 2023
Mariner spacecraft
Only one failed Mariner spacecraft is still in solar orbit. The dead spacecraft missed Mars by millions of miles.
The world’s space agencies do not actively track defunct spacecraft — that is, spacecraft with dead radios — beyond Earth orbit, though sometimes they can work out where they should be. The problem is, a spacecraft in solar orbit is almost always too small to observe and things can happen that can change its course. For example, if it includes a pressure vessel — say, a propellant tank — that has not been intentionally depressurized, there’s a good chance it has by now burst or vented. That means it acted like a rocket and changed the spacecraft’s course. Sunlight, solar wind, and even peeling paint can nudge a spacecraft’s course over decades.

So Rangers that missed the Moon — no Mariners were launched to the Moon — are lost in solar orbit. So are a host of Pioneers and Explorers and miscellaneous rocket stages and some Apollo hardware. That takes into account only some lost U.S. spacecraft. Dead Soviet spacecraft and rocket stages are scattered around the inner solar system, too, along with derelict hardware from other countries.

Incidentally, only one failed Mariner is in solar orbit. Mariner 1 (a Venus probe) and 8 (a Mars orbiter) were lost during launch and fell back to Earth. Mariner 3 (a Mars probe), on the other hand, escaped Earth, but its solar arrays became tangled in its streamlined launch shroud, so it could not make electricity. The dead spacecraft missed Mars by millions of miles.

David S. F. Portree
USGS Astrogeology Science Center
Flagstaff, Arizona