The Johns Hopkins University Applied Physics Laboratory (JHU/APL), with the support of NASA’s Meteoroids Environments Office, conducted a dust study as a part of NASA’s Solar Probe Plus mission to explore the sun’s outer atmosphere.

The study provided information about the meteoroid environment the spacecraft will encounter. JHU/APL incorporated these findings into a model of the dust environment the probe will experience and used the model to make recommendations that influenced the design of the spacecraft to ensure it’s well protected throughout its mission.

“A very small particle can hurt the spacecraft unless it’s properly designed,” said Bill Cooke, Meteoroid Environments program manager at NASA’s Marshall Space Flight Center. “Unlike in the movies, we don’t worry about a big rock hitting a spacecraft and blowing it to tiny pieces. What is more likely is a small meteoroid, less than a millimeter across, will cut a wire or hit it and cause it to jerk, disrupting its science operations.”

Despite common perception, meteor showers are a relatively small risk to spacecraft — the sporadic background meteors present 90 percent of the risk. Fortunately, because these meteors are always there, project personnel can take them into account and protect the spacecraft.

However, no spacecraft has ever explored the sun’s outer atmosphere, so very little is known about the area, including the meteoroid environment.

“It’s sort of like the old explorers before there were maps,” explained Cooke. “They’d reach a point and say ‘here there will be dragons.’ This is us approaching the dragons.”

During Solar Probe Plus’ mission, it will sample the near-sun environment to provide a new understanding of coronal heating and the origin and evolution of solar wind. Findings from its scientific endeavors will help NASA characterize and forecast the radiation environment in which future space explorers will work and live.

Initially, the dust team’s biggest concern was dust damage to the thermal protection shield, which protects the spacecraft from the environment. Other concerns included dust perforating the water system used to cool the spacecraft, which could result in the loss of the system, and the perforation of other key spacecraft subsystems. To compensate for the lack of real experience with the sun’s outer atmosphere, JHU/APL turned to modeling techniques for information on how to best protect the spacecraft from these concerns.

“As you move away from the areas that too many people have been before, you get to an area where there aren’t a lot of measurements of dust,” said Doug Mehoke, lead engineer for the Solar Probe Plus dust study and group supervisor for mechanical systems in the Space Exploration Sector at JHU/APL. “We’re interested in two things: Where they [the meteoroids] are in space and what the size distribution is. [That’s] done through modeling.”

A Solution for the Lack of Real-Life Data  

Very few models of the solar environment exist and none pertain specifically to the dust environment beyond Mercury. Dust measurements typically are made optically — that is, by looking towards the area to be measured. Scientists take a picture of a particular area of the sky and then, based on optical characteristics, get an idea of the density of dust particles in the area. However, because the area to be measured in this situation was toward the sun, everything was what Mehoke calls “swamped out” by the sun’s light.

Although data and models on the sun’s outer atmosphere are lacking, the dust team wasn’t starting from scratch. The team looked at data from previous science missions in other areas of the solar system, models of these areas, and literature and test data on material properties and other factors. They then used this information to create a new model for Solar Probe Plus.

From these sources, scientists know that meteoroids behave differently near the sun than they do near Earth. Near Earth, meteoroids move at speeds around 20-40 kilometers per second, but near the sun speeds approach hundreds of kilometers per second to avoid falling into the star’s gravitational pull. In addition, the spacecraft is moving faster as well, also affecting the impact conditions.

These speeds further complicate attempts to model the environment, as modeling usually is based on test data, but test data is limited to the 7-10 kilometer per second range.

“[We’re] experiencing an environment you can’t really test to,” explained Mehoke.

As a result, the dust team had to develop a model with predictive capabilities. Unlike with traditional methods, the team did the analysis first and predicted what will happen at these higher velocities.

“We moved from an empirical testing approach to particle impact prediction to an analytical, predictive one,” said Mehoke.

These modeling and testing efforts were supported by the University of Texas at El Paso, Virginia Tech and the University of Dayton Research Institute. Field testing also occurred at NASA’s White Sands Test Facility.

The speeds also directly impact the size of meteoroids. Near the sun, particles from comets interact and collide with each other, which reduces them to interplanetary dust. When making design recommendations, the dust team needs to consider the diverse environment affecting the spacecraft.

“Solar Probe Plus is going through a dust environment its entire life,” said Mehoke. “As it gets closer to the sun, [we’ll] get faster particles, but smaller, and farther from the sun, [we’ll] get slower particles, but larger.”

Although Solar Probe Plus will spend a brief period of time in near-Earth orbit, the team’s primary concern is particles in the millimeter range — larger particles are so few they aren’t a concern, and smaller ones wouldn’t do much damage.

The team also knows that near Earth, particles are coming from all directions, but near the sun they are set in a more circular orbit around the star. Assumptions can be made about the direction the particles are orbiting and whether they are moving towards or away from the sun.

Throughout the dust study, JHU/APL relied on NASA’s expertise and vast experience with different space environments for guidance.

“We are trying to lean heavily on the extensive amount of work that NASA has already done,” said Mehoke.

Specifically, Mehoke references Cooke’s knowledge of existing environment models and the data that goes into each one. Cooke’s awareness of what other agencies are working on, like the European Space Agency’s dust modeling work, has been an asset. Mehoke also credits Eric Christiansen, NASA crewed vehicle Micrometeoroid and Orbital Debris protection lead at Johnson Space Center, and David Seal, Mission Engineering and Planning Group supervisor at the Jet Propulsion Laboratory, for their invaluable help.

“It all comes down to the data people have and what they do with it,” explained Mehoke. “[People make] claims based on that data, and [Cooke is] knowledgeable of which claims are valid and which need additional effort. The best information you have is from the spacecraft actually flying out there. [Cooke] pays close attention to the spaceflight community and what they experience.”

Together, these sources of information enabled the JHU/APL dust team to develop what it believes to be an effective and accurate predictive model of the dust environment Solar Probe Plus will experience during its mission.

“The combination has let us get to a point where we think we have a good model of predicting these things,” said Mehoke.

The dust team used the model to make recommendations on how to configure the spacecraft that directly impacted the design including the placement of key systems, the number of layers between the outer and inner pieces of the craft, and the distance between these layers. These design features will help protect Solar Probe Plus from the meteoroid environment throughout its mission, which is set to launch July 30, 2018.