National Aeronautics and Space Administration

This is the third in a series of posts on orbital debris and what it means for science.



This object exceeds the 2-pound frozen chicken rule

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.

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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.

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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.
  

Space junk plugged this hole in the Solar Max satellite.

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.

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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…

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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.

This is the second in a series of posts on orbital debris and what it means for science.


According to a recent article in the Space Review, humans had launched 6,854 spacecraft as of December 31, 2009. That is an average of 132 spacecraft a year since 1957!

The spacecraft go up, but they don’t always come back down — at least, not for a long, long time. A satellite placed in low Earth orbit (200 to 2,000 kilometers above the surface) can take much more than 25 years to drift down into the atmosphere and burn up. In the much higher geosynchronous orbits, spacecraft usually remain for centuries.

It is sometimes possible to speed up the process. If a satellite has thrusters, it can deliberately “de-orbit” into the atmosphere. In that case it becomes a really bright shooting star as it burns up.

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Blogolicious space junk facts: recent orbital crack-ups and explosions

• The Brazilian CBERS-1 satellite exploded February 18, 2007.
• The next day, February 19, a Russian Briz-M booster rocket exploded.
• On January 11, 2007, China destroyed one of its own spacecraft with an anti-satellite weapon.
• On February 10, 2009, the Iridium 33 and Cosmos 2251 satellites collided.

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While a spacecraft is cooling its heels for decades waiting to reenter, it may produce additional debris. The reason, orbital debris engineer Scott Hull explains, is that propellant tanks and batteries in old spacecraft can explode. (Hull recently discussed the orbital debris issue at the regular Systems Engineering seminar at Goddard Space Flight Center.)

On February 18, 2007, a propellant tank aboard the Brazilian CBERS-1 satellite burst, spewing about 425 trackable pieces of debris (softball sized and larger) and scores of smaller bits. The next day, a Russian Briz-M booster rocket exploded in space, leaving an estimated 150 trackable chunks.

Collisions in space
But here is the worrying part: Even if we stopped launching stuff into space right now, the number of individual chunks of orbital debris would continue to increase. The reason: In the crowded dark mosh pit that is low Earth orbit, spacecraft and other objects have begun to smash into each other.

Remember the big brouhaha last year when the Iridium 33 and Cosmos 2251 communications satellites collided over northern Siberia? That added 1,650-plus big chunks to low Earth orbit.

Taking (out) the A Train
Just our luck! The Iridium-Cosmos collision of 2009 occurred right above the 700-kilometer orbit of NASA’s “A Train” of Earth-observing satellites. The five satellites, which will eventually grow to seven or more, travel together in a constellation commonly referred to as the Afternoon (or A Train) Constellation.

In coming decades, collision debris will sink lower and cross paths with the A Train satellites. We can track the larger pieces and duck when needed. In fact, says Hull, “The A-Train members already have to dodge debris several times per year.”

Recently A-Train member Aura “experienced a momentary disturbance consistent with a small debris hit,” Hull says. There is a small risk — about a few percent chance per year — that an A Train satellite will sustain a fatal strike by a small and untrackable piece of debris.

Space junk: here to stay
Even if everyone follows guidelines for reducing on-orbit explosions and other sources of new debris release, one study predicts as many as 8 or 9 collisions within the next 40 years, or roughly 1 every 5 years.

Space News recently quoted U.S. Air Force General Kevin Chilton: “The growing problem of space debris could make this essential domain untenable for man or machine.”

Hull’s own prediction is less dire. “If current trends continue, I think the region below 1,000 km will become so crowded with debris that even science missions will need to spend a sizeable percentage of their mass on shielding, just to accomplish the mission.”

The next post in this series looks at what NASA does to reduce the growth of orbital debris and the risks it poses to spacecraft.

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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…

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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.

This is the first in a series of posts about orbital debris and what it means for space science.



This visualization shows space junk now being tracked in low Earth orbit.

When Scott Hull hears someone use that well-worn phrase “the sky is falling,” I wonder if he thinks: “Yeah, if only it would!”

Hull and fellow orbital debris engineer Ivonne Rodriguez are the orbital undertakers of Goddard Space Flight Center. They offer advice on how to design and operate satellites to produce the least amount of space junk possible after they reach the end of their operational lives.

“Right now, there are at least 19,000 pieces of debris this big or bigger on orbit,” Hull said, holding up a softball, as he talked about the orbital debris issue at the regular Systems Engineering seminar at Goddard Space Flight Center.

“Any of those that hits your spacecraft could pretty much destroy it immediately,” he continued. “Most of these puppies are traveling about 7 kilometers per second or more.”

Translation: Imagine a softball-sized chunk of metal pitched at your friendly neighborhood spacecraft at nearly 16,000 mph.

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Blogolicious space junk facts:

• There are at least 19,000 pieces of orbital debris the size of a softball or bigger orbiting Earth.
• There are at least 300,000 pieces of debris at least 1 centimeter across.
• At current rates of increase, space junk could make some orbits too dangerous for new satellites.

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Radar can track objects the size of a softball (about 10 centimeters) or larger. NASA satellites — not to mention the International Space Station — can and do dodge chunks of space junk from time to time.

Then Hull held up a glass chip about the size of a piece of hard candy.

“There’s more than 300,000 this size. They can really hurt you, maybe end the mission, and probably generate more debris when they hit. But we can’t track them.”

Where does all this junk come from? From Earth, of course. And the problem is growing, and could someday pose a serious threat to the whole business of launching and operating scientific satellites.

Hull talks more about the risks of space junk in the next post in this series. Stay tuned.

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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…

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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.