Our solar system is a busy place — in addition to the large bodies we are all familiar with exist an uncountably large number of rocky asteroids and icy comets. These mostly stay put in their orbits far from Earth but many also roam around the solar system. As they do, they shed some material due to collisions, deformations or heating. Due to this, the Earth is surrounded by small particles we call interplanetary dust. By investigating the size and composition of the interplanetary dust, astronomers can indirectly investigate the activity and makeup of the parent bodies.

“When in space, interplanetary dust is practically invisible. However, around 1,000 kilograms falls to Earth every day in the form of tiny meteors which appear as bright streaks in the night sky,” said astronomer Ryou Ohsawa from the Institute of Astronomy at the University of Tokyo. “We can observe these with ground-based radar and optical instruments. Radar is useful as it can cover wide areas and gather vast readings, but optical telescopes can give more detailed information useful for our studies. So we set out to bridge this gap to boost our observational capacity.”

Ground-based radar is very good at detecting the motion of meteors, but it does not reveal much information about the mass or composition of the meteors. Optical telescopes and sensors can infer those details based on the light given off by falling meteors due to interaction with the atmosphere. However, telescopes have a limited field of view and until recently lacked the sensitivity to see faint meteors at all. Ohsawa and his team wished to imbue radar observatories with the powers of optical ones. After a few years, they have finally succeeded.

“We thought that if you could observe enough meteors simultaneously with both radar and optical facilities, details of the meteors in the optical data may correspond to previously unseen patterns in the radar data too,” said Ohsawa. “I am pleased to report this is in fact the case. We recorded hundreds of events over several years and have now gained the ability to read information about meteor mass from subtle signals in radar data.”

In 2009, 2010 and 2018, the team used the Middle and Upper Atmosphere (MU) Radar facility, operated by Kyoto University and located in Shigaraki, Shiga Prefecture, and the Kiso Observatory, operated by the University of Tokyo, on the Nagano Prefecture side of Mount Ontake. They are 173 kilometers apart, which is important: the closer the facilities, the more accurately the data from them can be correlated. MU points directly upwards, but Kiso can be angled, so it was pointed 100 km above the site of MU. The team saw 228 meteors with both facilities and this was plenty to derive a statistically reliable relationship to connect radar and optical observations.

“Data analysis was laborious,” said Ohsawa. “A sensitive instrument called the Tomo-e Gozen wide-field camera mounted to the Kiso telescope captured over a million images a night. This is too much for us to analyze manually so we developed software to automatically recognize faint meteors. From what we’ve learned here we hope to extend this project and begin using radar to investigate the composition of meteors. This could help astronomers explore comets and aspects of solar system evolution like never before.”

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“We felt like Sherlock Holmes,” says Mordechai (Moti) Feingold, the Kate Van Nuys Page Professor of the History of Science and the Humanities at Caltech, who explains that he and his former student, Andrej Svoren?ík (MS ’08) of the University of Mannheim in Germany, spent more than a decade tracing copies of the book around the world. Feingold and Svoren?ík are co-authors of a paper about the survey published in the journal Annals of Science.

Moreover, by analyzing ownership marks and notes scribbled in the margins of some of the books, in addition to related letters and other documents, the researchers found evidence that the Principia, once thought to be reserved for only a select group of expert mathematicians, was more widely read and comprehended than previously thought.

“One of the realizations we’ve had,” says Feingold, “is that the transmission of the book and its ideas was far quicker and more open than we assumed, and this will have implications on the future work that we and others will be doing on this subject.”

In the Principia, Newton introduced the laws of motion and universal gravitation, “unifying the terrestrial and celestial worlds under a single law,” says Svoren?ík.

“By the 18th century, Newtonian ideas transcended science itself,” says Feingold. “People in other fields were hoping to find a similar single law to unify their own respective fields. The influence of Newton, just like that of Charles Darwin and Albert Einstein, exerted considerable influence on many other aspects of life, and that is what made him such a canonical figure during the 18th century and beyond.”

Principia Found Behind the Iron Curtain

Svoren?ík says that the project was born out of a paper he wrote for a course in the history of science taught by Feingold. Originally from Slovakia, Svoren?ík had written a term paper about the distribution of the Principia in Central Europe. “I was interested in whether there were copies of the book that could be traced to my home region. The census done in the 1950s did not list any copies from Slovakia, the Czech Republic, Poland, or Hungary. This is understandable as the census was done after the Iron Curtain descended, which made tracing copies very difficult.”

To Svoren?ík’s surprise, he found many more copies than Feingold had expected. The summer after the class, Feingold suggested to Svoren?ík that they turn his project into the first-ever complete, systematic search for copies of the first edition of the Principia. Their ensuing detective work across the globe turned up about 200 previously unidentified copies in 27 countries, including 35 copies in Central Europe. Feingold and Svoren?ík even came across lost or stolen copies of the masterpiece; for example, one copy found with a bookseller in Italy was discovered to have been stolen from a library in Germany half a century earlier.

“We contacted the German library to let them know, but they were too slow to make a decision to buy back the copy or apprehend it somehow, so it ended up back on the market,” says Feingold.

A Rare, Collectible Item

According to the historians, copies of the first edition of the Principia sell today for between $300,000 and $3,000,000 via auction houses like Christie’s and Sotheby’s as well as on the black market. They estimate that some 600, and possibly as many as 750 copies of the book’s first edition were printed in 1687.

The primary person behind the book’s publication was Edmond Halley, a well-known English scientist who made several discoveries about our solar system, including the periodicity of what later became known as Halley’s Comet. Feingold explains that, before the Principia was written, Halley had asked Newton for some calculations regarding elliptical orbits of bodies in our solar system. When Halley saw the calculations, “he got so excited, he rushed back to Cambridge and basically forced Newton to write the Principia,” says Feingold. In fact, Halley funded the publication of the book’s first edition.

Soon after its publication, the book was recognized as a work of genius. “Because Halley had already prepared the public for what was to come,” says Feingold, “there was a widespread recognition that the Principia was a masterpiece.” Later, a “mystique” about Newton started to develop, according to Feingold, exemplified in a story about two students walking in Cambridge and spotting Newton on the street. “‘There goes a man,’ one of them said, ‘who wrote a book that neither he nor anybody else understands,'” says Feingold.

The idea that the Principia was incomprehensible and little read is called into question with the new survey results. Not only does the research show that there was a larger market for the book than once thought, it also demonstrates that people were digesting its contents to a greater extent than realized.

“When you look through the copies themselves, you might find small notes or annotations that give you clues about how it was used,” says Svoren?ík, who has personally inspected about 10 percent of the copies documented in their census. When traveling to conferences in different countries, he would make time to visit local libraries. “You look at the condition of the ownership marks, the binding, deterioration, printing differences, et cetera.” Even without inspecting the books up close, the historians could trace who owned them through library records and other letters and documents, and learn how copies were shared.

“It’s harder to show how much people engaged with a book than simply owned it, but we can look at the notes in the margins and how the book was shared,” says Feingold. “You can assume that for each copy, there are multiple readers. It’s not like today, where you might buy a book and are the only one to read it. And then we can look for an exchange of ideas between the people sharing copies. You start to put together the pieces and solve the puzzle.”

Svoren?ík and Feingold hope that their census, which they call preliminary, will yield information about other existing copies tucked away with private owners, book dealers, and libraries. Continuing this line of research into the future, the historians plan to further refine our understanding of how the Principia shaped 18th-century science.

This form of technology, known as “cold spray,” results in mechanically robust, porous structures that are 40% stronger than similar materials made with conventional manufacturing processes. The structures’ small size and porosity make them particularly well-suited for building biomedical components, like replacement joints.

The team’s paper, “Solid-State Additive Manufacturing of Porous Ti-6Al-4V by Supersonic Impact,” published Nov. 9 in Applied Materials Today.

The paper’s lead author is Atieh Moridi, assistant professor in the Sibley School of Mechanical and Aerospace Engineering.

“We focused on making cellular structures, which have lots of applications in thermal management, energy absorption and biomedicine,” Moridi said. “Instead of using only heat as the input or the driving force for bonding, we are now using plastic deformation to bond these powder particles together.”

Moridi’s research group specializes in creating high-performance metallic materials through additive manufacturing processes. Rather than carving a geometric shape out of a big block of material, additive manufacturing builds the product layer by layer, a bottom-up approach that gives manufacturers greater flexibility in what they create.

However, additive manufacturing is not without its own challenges. Foremost among them: Metallic materials need to be heated at high temperatures that exceed their melting point, which can cause residual stress buildup, distortion and unwanted phase transformations.

To eliminate these issues, Moridi and collaborators developed a method using a nozzle of compressed gas to fire titanium alloy particles at a substrate.

“It’s like painting, but things build up a lot more in 3D,” Moridi said.

The particles were between 45 and 106 microns in diameter (a micron is one-millionth of a meter) and traveled at roughly 600 meters per second, faster than the speed of sound. To put that into perspective, another mainstream additive process, direct energy deposition, delivers powders through a nozzle at a velocity on the order of 10 meters per second, making Moridi’s method sixty times faster.

The particles aren’t just hurled as quickly as possible. The researchers had to carefully calibrate titanium alloy’s ideal speed. Typically in cold spray printing, a particle would accelerate in the sweet spot between its critical velocity — the speed at which it can form a dense solid — and its erosion velocity, when it crumbles too much to bond to anything.

Instead, Moridi’s team used computational fluid dynamics to determine a speed just under the titanium alloy particle’s critical velocity. When launched at this slightly slower rate, the particles created a more porous structure, which is ideal for biomedical applications, such as artificial joints for the knee or hip, and cranial/facial implants.

“If we make implants with these kind of porous structures, and we insert them in the body, the bone can grow inside these pores and make a biological fixation,” Moridi said. “This helps reduce the likelihood of the implant loosening. And this is a big deal. There are lots of revision surgeries that patients have to go through to remove the implant just because it’s loose and it causes a lot of pain.”

While the process is technically termed cold spray, it did involve some heat treatment. Once the particles collided and bonded together, the researchers heated the metal so the components would diffuse into each other and settle like a homogeneous material.

“We only focused on titanium alloys and biomedical applications, but the applicability of this process could be beyond that,” Moridi said. “Essentially, any metallic material that can endure plastic deformation could benefit from this process. And it opens up a lot of opportunities for larger-scale industrial applications, like construction, transportation and energy.”

Co-authors include doctoral student Akane Wakai and researchers from MIT, Polytechnic University of Milan, Worcester Polytechnic Institute, Brunel University London and Helmut Schmidt University.

The research was supported, in part, by the MIT-Italy global seed fund and Polimi International Fellowship.