MMOD Impacts

What is MMOD? It is the acronym for Micro-Meteoroid and Orbital Debris. Space debris is a risk to other spacecraft both manned and unmanned. It includes derelict craft, fragments from their disintegration and collision, paint flecks, frozen liquids expelled from spacecraft, and unburned particles from solid rocket motors. The debris comes in a wide range of sizes from microscopic to bigger than a car. Most of it is small. There are estimated more than 128 million pieces of debris smaller than 1 cm (0.4 in), about 900,000 pieces of debris 1–10 cm, and around 34,000 of pieces larger than 10 cm (3.9 in) were estimated to be in orbit around the Earth.

The smallest size of debris like paint flecks and rocket exhaust particles are grouped with the small micrometeoroids from space in a group called MMOD. They pose a definite risk. Collisions with debris cause damage similar to sandblasting on spacecraft surfaces, to solar panels, and to optics like telescopes or star trackers. These small fast particles can puncture thin metals. Collision speed between 10 – 14 km/s (6 – 8.4 mi/s) are likely.

Several years ago I was evaluating science lessons for students who were preparing to take exams. One of these lessons from NASA posed some questions about the number of MMOD impacts felt by the Hubble Space Telescope. During the 2009 STS-125 Shuttle mission, the astronauts removed the Wide- field Planetary Camera 2 (WFPC2) and replaced it with the Wide Field Camera 3 (WFC3). Protecting the WFPC2 while it was aboard Hubble was a curved rectangular aluminum plate covered with white paint. A number of blemishes were observed from a distance on the painted surface and photographed with a telephoto lens from the Shuttle window. They are circled in this image. They were impacts from MMOD recorded during its 16 year exposure in space. More impacts were present but were too small to see from a distance.

Closeup inspection of several impacts of WFPC2 revealed these details. Many were actual puncture holes ranging in size from 3.8 mm to 5.9 mm.

The lesson asked questions about the impacts. The students were asked to compute the area of the curved plate (1.8 m2) struck by the 20 impacts. From those values, the impact density was computed (20 impacts / 1.8 m2 = 11 impacts/m2). The impacts by MMOD took place over 16 years meaning about 0.69 impacts/m2 occurred each year for this size of MMOD. It doesn’t sound like much of a problem with that low number.

The lesson next described the solar panels used to power the Hubble telescope. Their area of 1,632 m2 is exposed to MMOD. The students were asked how many impacts per year would they experience. (1632 m2 x 0.69 impacts/m2 per year = 1130 impacts/yr)

Knowing there are 8760 hrs/year and 1130 impacts/yr, the students divided 8760 by 1130 to get an average of 7.8 hrs between impacts to the solar panels by small MMOD. These impacts gradually degrade the performance of the panels over time. So far, no large objects have cause significant damage or destruction.

Astronauts doing a spacewalk also have a risk to their safety from MMOD. Their surface area exposed by the suit is very small and they are out for only a few hours. So, the risk is minimal, but not zero. High velocity small projectile testing of spacesuit materials has been done. This image shows the damage to a suit by a 0.5 mm aluminum sphere going 4.25 mi/s. The suits do offer an acceptable level of protection.

What about the threat of MMOD to a much larger object like the International Space Station ISS? Look at all that surface area exposed. It is about 12,600 m2. Using calculations from the student activity about Hubble, the average time between impacts can be estimated. (12,600 m2 x 0.69 impacts/m2 per year = 8700 impacts/yr)

Knowing there are 8760 hrs/year and 8700 impacts/yr, the ISS is possibly struck every hour by small MMOD.

Much of the structure housing people and essential hardware is shielded against these impacts. The solar panels are not. There have been several documented places where they have been struck and still function in spite of the puncture hole. Hand rails that guide astronauts on spacewalks have been struck leaving a sharp edge of metal. The gloved hand fabric was torn slightly on a spacewalk but not enough to cause a serious leak.

An avoidance maneuver is typically performed for the ISS if the chance of a strike with a larger object is greater than 1-in-10,000. As of January 2014, sixteen maneuvers were done in the fifteen years the ISS had been in orbit. In the event of a possible collision with an even larger more dangerous object, the crew can shelter in docked Russian Soyuz-capsules.


6 thoughts on “MMOD Impacts

  1. Nice research.

    We’ve known about the space debris in Earth orbit for a long time but we don’t seem capable of removing it, all we do is create more, leaving the issue to future generations to deal with. A bit like knowing that fossil fuel causes a greenhouse effect and not doing enough to stabilise emissions let alone reduce them. We seem blind to the dangers.

      • Will we “wake up and change?” if it actually happens. Good question, Jim. I would say that we will, but how is murky. It will depend not only on engineering analysis but on cost and both national and international politics. Such is the nature of modern problems.

        • It seems today that problems less about things to solve than a way to cast blame on someone else. We aren’t working together. We are capable of amazing results when work together.

  2. Also like plastic in the ocean. The way we approach things like this reminds me of the way we engineer cars. They are engineered for assembly, not repair.

  3. In the future the biggest threat won’t be space junk it will be humans warring and taking out satellites. I am surprised it hasn’t happened yet. Very fine post.

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