Lunar Space Debris
Published on Oct 28, 2008 at 8:52 pm.
5 Comments.
Filed under moon, space debris, space exploration.
From the time that the first satellite was placed into orbit around the Earth, we have had space debris. The rocket that launched Sputnik also orbited Earth. Every launch creates more debris. Old satellites still orbit Earth. In the early days, when satellites needed to use cameras to image Earth, they simply jettisoned their lens caps. But, all that stuff stays orbiting for a long time. The feeling at first was that space is so big that the likelihood of coming across anything else was pretty slim. But, putting satellites into space came to be more than a rare event. Even here on Earth, if you travel to remote places you can find debris left by others (bottles, cans, paper, etc) almost anywhere on the planet. But, in space, these items are moving very fast. If their orbits don’t exactly match your own, then coming across them can be disastrous. Today, we know this, and there are international agreements to limit the amount of space debris generated by the various space programs.
NASA came to realize the potential hazard of space debris a long time ago. At least large pieces of debris, though, can be tracked by radar, and so spacecraft can take evasive action to make sure that they don’t come dangerously close to the debris. That is a bit tougher to do around the Moon. We don’t have large radar dishes there to monitor things in lunar orbit. So, untracked debris in lunar orbit could pose a greater risk than it would in Earth orbit.
So, to keep the lunar orbit safe for future manned missions, NASA’s early lunar missions never left anything in orbit. The Rangers slammed into the Moon. The Surveyors landed. The Lunar Orbiters orbited, of course, but at the end of their missions they were to be deliberately de-orbited into the lunar surface (De-orbited sounds much better than “crashed.”). That left the space around the Moon safe. This presented a point of contention, though, between the United States and the Soviet Union, because the Soviets did not de-orbit all of their lunar orbiters. That left them orbiting the Moon in a poorly defined orbit, so NASA had no idea where they were. The chance than any of them would slam into an Apollo craft, destroying it, was small, but was not zero.
As it turns out, though, most of the things that were left orbiting the Moon decades ago, during our first phase of lunar exploration are probably no longer there. Satellites and debris orbiting Earth eventually reenter our atmosphere. There are a variety of reasons that orbits decay, but for low Earth orbit, the biggest reason is that Earth’s atmosphere drags on objects in orbit. Earth’s atmosphere is a gas, so it doesn’t have a sharp edge. Rather, it just gets thinner and thinner. There are still air molecules and atoms at the altitude of the Space Shuttle, the Hubble Telescope, and the International Space Station. These bodies running into those atoms gradually slows them down, causing them to spiral downward to lower altitudes, where they run into more of them. The Space Shuttle is not in orbit long enough for this to matter. The ISS must be routinely boosted to a higher orbit to avoid falling back to Earth. The Hubble Telescope is also designed to be boosted every three years to a higher orbit by the Space Shuttle. Without a boost, it will come back on its own in an uncontrolled reentry. That is one reason that we must send another shuttle mission to it. Eventually, NASA will have to send an unmanned rocket to it to either boost it to a far higher orbit until we know what to do with it, or to send it crashing back to Earth somewhere safe (such as over the Pacific Ocean). All of the other debris in low Earth orbit will also fall back to Earth, given time. Satellites and debris reenter all the time, typically putting on quite a show. When these bodies reenter, they look much like meteors, only much slower and often breaking apart. I’ve seen space debris reenter, and it is impressive. Most of the debris burns up in the atmosphere. Some of it, however, manages to make it to the ground. There have been instances where people have had near misses with space debris, and there have been international incidents where radioactive material from one nation has fallen onto another.
But, the Moon doesn’t have a dense atmosphere like Earth. So, satellites and space debris won’t be crashing into the Moon, right? Actually, they still do. It turns out that most lunar orbits are unstable. You don’t have atmospheric drag, of course, but there are other things that can affect orbits. The Moon, it turns out, has very large differences in densities across its surface. Some part of the Moon, particularly beneath the lunar seas, have a bit larger gravitational pull than other places on the Moon. This is because of localized mass concentrations (perhaps as a result of the impacts that caused the seas to form there in the first place). Other impact sites also are areas of mass concentration. These mass concentrations were discovered by the early lunar orbiting spacecraft, and studied in greater depth during the manned Apollo missions. Since saying “mass concentrations” over and over again is a bit of a mouthful, a shorthand term was developed: mascons. As a satellite or piece of space debris orbits the Moon in low lunar orbit, these mascons pull on the orbit, changing its eccentricity. Eventually, with repeated random pulls, the mascons cause the orbit to become so elliptical that it runs into the lunar surface. But, since the Moon doesn’t have a thick atmosphere, these bodies slam into the lunar surface at orbital velocities without burning up or breaking up beforehand. Thus, the orbits of most of the things left in orbit around the Moon decades ago have likely already decayed, and so they pose no hazard to astronauts returning to the Moon.
But, if we really do go back to the Moon with manned missions, we’ll be generating more space debris. Today, there are already several unmanned lunar missions studying the Moon from lunar orbit. Even more are planned. Soon, there will be a number of missions in lunar orbit, each with the potential for more space debris. Recent missions have continued the practice of ending the mission by deliberately crashing the spacecraft into the lunar surface, but there is no guarantee that all lunar missions will end that way. With many international parties involved, and perhaps some private lunar missions, there is always the possibility that somebody will leave debris in orbit. Even if they don’t mean to do so, there is the possibility that a spacecraft will fail before it is sent into its final orbit into a controlled crash into the lunar surface. This could pose a threat to astronauts on the lunar surface. In a story that I read on Space.com, it is clear that NASA officials have thought of this. There is a program to limit any possible space debris. But, of course, this program will require cooperation of all space agencies, government and private, sending spacecraft towards the Moon. I am glad, though, that they are at least thinking of this.
-Astroprof
Image courtesy of NASA
Sili on October 29, 2008 at 3:44 pm: 1
Stupid question:
If we know where all the *good* satellites are, can’t we conceivable blast big lasers at the sky while they’re not around?
Of course - we don’t have lasers like that, yet, and it’d have to be done for quite a while to get all of the junk, but given all the other crazy ideas floating around for space exploration, it doesn’t seem altogether impossible to evaporate away the ‘bad’ stuff up there.
Astroprof on October 29, 2008 at 10:06 pm: 2
The problem is that you might just blast the debris into a cloud of more debris instead of just vaporizing it. But, you might be onto something. If we had the technology to vaporize the things, it would sure be a nice solution.
Sili on October 31, 2008 at 11:01 am: 3
There’re no problems that can’t be solved with giant lasers.
metallic purple on March 6, 2010 at 9:41 am: 4
can the debris though decompose
after million and million of years the way it happened to dinosaurs?
Ella on November 21, 2011 at 8:32 pm: 5
to Astroprof-
you should make your point and make the writing shorter because i’m bored to death