Science and Space Junk III: The 2-pound frozen chicken rule and other ways NASA protects you from orbital debris
This is the third in a series of posts on orbital debris and what it means for science.
As far as we know, nobody has ever been killed by a piece of space junk. But a scrap of insulating blanket from a disintegrating Delta II rocket stage did brush the shoulder of one Lottie Williams of Tulsa, Oklahoma, in 1997. Fortunately the scrap wasn’t very heavy or moving very fast.
Anyway, if you’re concerned about falling space junk, NASA’s got your back. The official name is Standard 8719.14, Requirement 4.7-1. I call it the 2-pound frozen chicken rule.
The requirement states that the risk of significant injury from a piece of debris reentering Earth’s atmosphere must be no greater than 1 in 10,000. “Significant injury” is defined as a blow that delivers 15 joules of energy to the unprotected human body. “Fifteen joules is roughly the equivalent of a 2-pound frozen chicken falling out of your freezer on your foot,” says Scott Hull, an orbital debris engineer at Goddard Space Flight Center. “It’s going to hurt.”
If it hit you in the head, it could be fatal. But you’ve got a fighting chance. And by the way, the 15-joule threshold is based on a significant amount of actual research. It really is the amount of work you have to do to injure someone.
In a recent Systems Engineering seminar at Goddard, Hull summarized requirements that NASA spacecraft and mission engineers are required to follow to prevent new space junk and minimize the risks it poses to other spacecraft — and to you and me.
Blogolicious Space Junk Prevention Facts
- No NASA spacecraft may remain in low Earth orbit for more than 25 years after the mission ends or 30 years after launch..
- Most satellite missions and rocket stages in orbits 370 miles (600 km) or lower will reenter in less than 25 years.
- More than 2 million kg (4.4 million pounds) of debris exists in low Earth orbit as well as in geosynchronous orbit. That’s equivalent to the empty mass of about 25 space shuttle orbiters.
- NASA’s empty Skylab spacecraft returned to Earth on July 11, 1979, scattering debris over the Indian Ocean and the sparsely settled region of Western Australia.
Keeping space clean
NASA missions must fulfill requirements designed to reduce debris throughout a spacecraft’s lifetime. And debris prevention doesn’t end with the mission.
After the instruments are shut down, NASA requires a debris prevention safeguard called passivation. For example, leftover pressure in propellant tanks is vented so they can’t burst. Internal batteries are disconnected so they don’t accidentally overcharge, build up internal pressure, and explode.
In orbital debris lingo, “disposal” is how a spacecraft ultimately exits orbit. We don’t routinely pluck old rocket stages and spacecraft from orbit and return them to Earth, like a tow truck dragging a broken-down car off the highway. Instead, there are three options for spacecraft disposal:
- Park it in a higher “storage” orbit for a few centuries where it’s less likely to bump into anything.
- Deliberately steer the craft into Earth’s atmosphere so it can burn up over the ocean.
- Wait for the orbit to decay naturally to the point of reentry.
Hazards to humans
NASA mission designers have to show that a reentry will pose a minimum risk to people. They do this using computer models developed by the NASA Orbital Debris Program Office at Johnson Space Center.
“What we do is to examine each spacecraft design in detail, and estimate how we think it will break up during reentry,” Hull says. “Then we identify the dimensions, mass, and composition of each piece as it is breaking up, all the way down to very small pieces. ”
When a spacecraft reenters Earth’s atmosphere, most of it burns up. But some portions may survive long enough to reach the surface. This is where the 2-lb chicken rule comes in.
When a NASA spacecraft is slated for disposal via uncontrolled reentry, the people in charge of the mission must demonstrate that the risk of serious injury (a 15-joule blow) is less than 1 in 10,000. In effect, the spacecraft has to burn up pretty thoroughly so no large chunks can reach the ground.
“For an average mission,” Hull explains, “a 1 in 10,000 risk equates to only about 85 square feet of the whole Earth surface being at risk of being hit — pretty small.”
For a controlled reentry, the spacecraft operator must also meet the 1 in 10,000 risk rule. But in addition, the debris can’t fall within 300 miles of a foreign country, or within 30 miles of the United States, its territories, or (get this!) the permanent ice pack of Antarctica.
Penguins, polar bears, and seals rejoice: You are (pretty) safe from space junk.
Despite the best efforts of NASA and other space agencies, orbital debris continues to increase. The threat this poses to the future of space science and exploration is fueling many exotic schemes for capturing and removing debris from orbit. Harpoons, nets, lassos, and giant sticky beach balls are all on the table.
The next and final post in this series looks at the prospects for space junk removal.
Check it out:
- NASA’s Orbital Debris Program Office at Johnson Space Center…
- An article about space junk for students…
- A gallery of photos and images about space junk…
- European Space Agency web site about orbital debris…
OH AND DID I MENTION? All opinions and opinionlike objects in this blog are mine alone and NOT those of NASA or Goddard Space Flight Center.