My last post noted the increased hand-wringing over the space congestion/debris problem, which is one of the biggest challenges to space situational awareness (SSA). I also defined SSA, using a motorcyclist’s experience to make it more relatable. This post tries to provide some perspective on the debris problem with a little thought experiment.
First, orbital debris isn’t something we should put our heads in the sand about. The orbiting used rocket bodies and upper stages, the bits of satellites from accidents and intercept tests, and the disused/abandoned satellites–these do represent a potential problem. Just like polluted water or polluted air, space debris is something we should be cognizant about and try to minimize. But is the human-created space debris situation as bad as some write? You’ve likely encountered some of the posts:
There are a lot of numbers thrown about within these posts meant to depict just how bad the debris problem is. However, the true scope of the problem hasn’t been very well defined. These experts can’t give an accurate number because equipment used to detect debris smaller than 10cm (4 inches) isn’t in use yet. Also, note these estimates keep increasing, which is funny because the initial estimates are just, well, estimates.
Let’s start, then, with researched numbers from decent sources first. The number of “active” satellites on orbit as of December 2016? That number is 1,459, according to the Union of Concerned Scientists’ satellite database. The number of objects with a 10 cm or greater cross-section orbiting the Earth, including active satellites and debris? There are a bit more than 21,000 objects, according to the U.S. military trackers quoted in this post. Satellite speed in a geosynchronous orbit (which is 42,164 km–a little over 26,000 miles–from the Earth’s center)? 3.07 kilometers per second (1.91 miles per second–nearly 6,900 mph). Satellite speed in a low Earth orbit–the kind the space shuttle used? That would be 28,800 kmh (18,000 mph). For these last two bits of information, I used this post: Geostationary orbit: Are satellites faster than the space shuttle?
Yep, that’s right–satellites in orbits of differing altitudes around the Earth have different speeds.
Let’s put those numbers in context and use something hopefully familiar to many people: the Earth. Let’s consider the Earth’s surface, including its oceans, as a hard-shelled orbit, with an “altitude” from the Earth’s center of 6,378 km (3,963 mi–Satellites helped get us that accurate number). At that altitude, surface orbital speeds are generally between 1,673 kilometers per hour (1,040 mph) and 0, depending on where a person is on the Earth’s surface. One surface orbit has a period of 23 hours, 56 minutes, and 4 seconds.
This means ships, cars, and people, while sitting still on the Earth’s equator, are moving at 1,673 kmh. Unlike satellites, which can be put into many different types of orbits and directions, objects sitting on the Earth’s surface will move in the same direction. There are a lot of “objects” on the Earth’s surface, but collision is never really a problem at this “orbital altitude.” Unless they are an object that moves, like a car or a ship.
Ships, such as tankers and container ships, are useful to help with this perspective. These are behemoths. The biggest container ships carry over 20,000 twenty foot long containers (see this link for one of the biggest). That’s about 20,000 of the biggest satellites ever put into space. That’s just one kind of ship, part of a combined commercial fleet of over 180,000 ships (by the way, that number is getting more accurate, thanks to satellites). These ships are cruising the Earth’s oceans and seas at about the same orbital altitude. Potentially, there are 180,000 very big objects moving around the Earth’s center at a little over 1,600 kmh at an altitude of 6,378 km.
That sounds like a lot of objects, some of the biggest man-made objects around. Yet, if you’ve ever cruised the oceans, you might not see any ship for hours, even though they are basically all orbiting the Earth at the same altitude as you. It’s also, thankfully, relatively rare for collisions to happen with these ships. Compare the number of 180,000 commercial ships to the relatively small number of tracked objects orbiting the Earth–21,000.
Let’s, literally, expand that perspective.
The Earth’s radius is 6,378 km. The Earth’s diameter, then, is twice its radius–12,756 km 97,926 mi). We also know a satellite in geosynchronous orbit is 42,164 km from the Earth’s center or 35,786 km from the Earth’s equator. This means the Earth could fit nearly three times between the Earth’s surface and a geosynchronous orbit point. The diameter of a geosynchronous orbit would be about 7 Earths across.
Remember, also, that diameter is just one axis of many possible ones going from one geosynchronous point, through the Earth’s center, to another geosynchronous point. Including debris, 21,000 tracked objects (all much smaller than a commercial ship on Earth) are orbiting the Earth in various altitudes within that volume of space, from low Earth orbit to the geosynchronous belt. That’s a lot of space.
Hopefully this post has given some perspective about the spaces on Earth and around it. Don’t worry–we’re not done with the experiment. There’s still a bit more to think about in the next post (or two).
The Mad Spaceball 