On Mars right now are two tiny robots on skis.
Weighing only about 4.5 kilograms (10 pounds), these Soviet-made Prop-M robots have been there since 1971, ready to explore its dusty surface. Unfortunately, they never will. Mars 2 crash-landed, while Mars 3 stopped communicating soon after landing.
The first human-made objects to reach the surface of Mars have been littering it for 50 years, their first 20 years as ignored failures. Mars 3’s first-ever photo of the Martian environment was judged to be of poor scientific value and didn’t come to light until the 1980s when a visitor to the Academy of Sciences in Moscow asked to see the photo. The spacecraft was later discovered on the Martian surface.
Are spacecraft like these disgusting “space junk” or valuable “space archaeology”?
What is space junk?
To most of us, “space junk” means defunct satellites clogging up low-Earth orbit, but the Moon, Mars, Venus and even a moon of Saturn contain plenty of human-made space hardware.
The presumed defunct NASA probe Pioneer 10, launched in 1972, is humanity’s farthest dead satellite at 131 astronomical units, though right now our farthest object left on another world is on Saturn’s giant moon Titan.
About the size of two refrigerators, the Huygens probe was jettisoned from NASA’s Cassini orbiter at Saturn on Jan. 14, 2005 to dive into Titan’s dense nitrogen atmosphere. It rains liquid methane on Titan, which may have caused the parachute to decay, but because methane doesn’t react with aluminum, the shell of Huygens should still be intact.
Space litter on Mars
If Titan is only lightly littered, that’s not true of the Red Planet.
“All landers on Mars have a parachute, but because the atmosphere is really thin, once it's on the surface, it stays where it is,” said Dr. Mark Paton, a research scientist at the Finnish Meteorological Institute studying how otherworldly winds can affect space junk. “There’s also a heat shield, but that’s jettisoned high above the surface so it lands far from the rover.”
The real worry is the Skycrane. Both the Curiosity (2012) and Perseverance (2021) rovers were lowered onto the Martian surface by this small spacecraft, which flies off to crash about 700 meters ( 2,300 feet) away.
“NASA is worried about the Skycrane because it carries some rocket fuel in its tanks,” said Paton. “When it crashes on Mars, some fuel gets released into the environment and possibly contaminates the soil.”
Space junk on Venus
Few worry about Venus landers, which most presume quickly melt into a puddle, but that’s not actually true.
“From 1970s Venera 7 onward, the cores of Venus landers were made using titanium to withstand 180 atmospheres and 450 degrees Celsius (840 degrees Fahrenheit), but titanium melts around 1,600 degrees Celsius (2900 degrees Fahrenheit),” said Alice Gorman, a space archaeologist at the Flinders University of South Australia’s Department of Archaeology.
Multiple Venus landers therefore still likely survive on Venus, as do the windows of the four probes deployed into the atmosphere during NASA’s Pioneer Venus mission in 1978, which were made using diamonds and sapphires.
“The rest may have decayed, but you could definitely go looking for jewels on the surface of Venus,” said Gorman.
What’s been left behind on the Moon?
There’s more treasure on the Moon, at least for a space archaeologist. There are hundreds of human-made objects there, from crashed spacecraft and lunar rovers to TV cameras and golf balls. There’s also almost 100 bags of human waste.
Gorman thinks that the trash-strewn Apollo landing sites on the Moon will one day come under threat of looting.
“People would pay huge sums of money for a piece of Apollo 11 — it will be some of the most collectible stuff ever,” she said. “So, when lunar surface activity ramps up, we’re going to have to protect these sites.”
That we already need heritage management guidelines for the Moon is a clue that space junk is a loose concept that seems only to refer to recently defunct things. It’s also a hugely subjective term.
“Litter is things that have been discarded — like confectionery wrappers in a national park — but that’s not true of anything in space, which has all been very carefully planned, and mostly, it’s for scientific purposes,” said Gorman. “We humans are as much a part of the solar system as anything else, and we’re creating new kinds of human landscapes."
Satellites and telescopes in orbit
There are more than 6,000 satellites orbiting Earth, about half of them defunct, while the Department of Defense's global Space Surveillance Network tracks around 27,000 pieces of orbital debris.
The oldest is Vanguard 1, launched in 1958 and only the second U.S. satellite ever launched. It will burn up in Earth’s atmosphere in about 300 years, but are these relics decaying?
“Space is full of micro-meteorites, cosmic and interplanetary dust and high-energy particles, so spacecraft surfaces do gradually erode,” said Gorman. “But they remain whole unless and until they collide with something else.”
The number of satellites in orbit around our planet is also growing rapidly. SpaceX's Starlink broadband internet satellites — which could number 30,000 — look likely to be the first of several such mega-constellation projects.
“Starlinks orbit at an average of 550 kilometers (340 miles), so they’re constantly reentering,” said Gorman, though their very existence increases collision risks. “There’s predicted to be 100,000 of them by the end of the decade, so the density out there is going to be radically increased.”
We’re not going to be entombed by a blanket of space garbage anytime soon, as envisaged by the Kessler/Cour-Palais Syndrome, but there’s another place in space where famous spacecraft go to live, work and die.
Welcome to the Sun-Earth Lagrange points. Here, the gravity of the Earth and Sun combine to create a stable location where spacecraft can remain in position while using very little fuel. One of these Lagrange points is the new home of the James Webb Space Telescope, at a distance of 1.5 million kilometers (1 million miles) from Earth, but what will happen to the telescope once its fuel runs dry in 20 years or so? Eventually, it will move off and — like Elon Musk’s Tesla Roadster, the first car launched into space in 2018 — go into an orbit of the Sun for millions of years.
When does space junk become pollution?
If and when commercial mining happens on the Moon or private missions are sent to Mars, we could begin to see industrial byproducts of the same kind that we see contaminating landscapes on Earth.
“The court of public opinion will probably turn out to have very strong feelings about where that dividing line is between what’s acceptable and what isn’t,” said Gorman.
But for now, whether you think it’s trash or treasure, space junk isn’t going away anytime soon.