Vatican Observatory

Why did Jesuit Bob Macke Measure the Asteroid Bennu?

Earlier this fall, NASA’s OSIRIS-REx mission brought samples of the asteroid Bennu back to Earth. Soon NASA held a press conference announcing the first findings regarding those samples. The news reports concentrated on the chemistry of the samples, rich in carbon and water.

That was what scientists expected. That’s why we went to that asteroid. It would have been shocking if the samples didn’t contain those materials. The fact is, we can detect chemicals in distant objects by looking at their spectra (the rainbow of colors that we see when we disperse their light with a prism).

Who first measured the composition of stars and planets by looking at their spectra? The Italian Jesuit, Angelo Secchi, from his observatory atop St. Ignatius Church in Rome, in the 1860s. At the new Specola Vaticana visitor center in Castel Gandolfo, there is a model of that observatory, made by the Specola’s own Br. Bob Macke during the Covid shutdown.

The interior of the Observatory of p. Angelo Secchi.

Br. Macke makes things both as a hobby and in his work as curator of the Vatican’s meteorite collections. Most recently he has been installing one of the things he has made, a pycnometer, at the Johnson Space Center in Houston to measure the density and porosity of the Bennu samples. (A pycnometer measures the volume of irregularly-shaped, and even porous solids, through the displacement of a gas and the resulting changes in its pressure.)

What has him excited about the samples from Bennu is not that the rocks are full of water and carbon but that they are rocks indeed! The shaking and rattling that they encountered, first when they were pulled off the surface of Bennu, and then when the capsule was coming down onto the Utah desert, might have broken these samples into a pile of dust.

NASA invited Br. Macke and his instrument to do these measurements because he is the world’s leading expert on these measurements, which began at the Vatican Observatory. Here he had certain advantages that no other lab on earth could boast: the combination of a large collection of meteorites (rocks from space that have fallen to Earth, and thus are expected to be very similar to the material found in asteroids) and the freedom to spend the years necessary to develop and perfect the techniques needed to measure their physical properties… time not available to those who live on three-year government grants.

Left: Model of Fr. Angelo Secchi’s 19th-century observatory atop St. Ignatius Church in Rome.
Right: Br. Bob Macke in the OSIRIS-REx clean room at NASA’s Johnson Space Center during the installation of his pycnometer.

But if we already have meteorites in our collections similar to what we think were found at Bennu, why was it necessary to go to the asteroid at all? Br. Macke explains:

A sample return mission like OSIRIS-REx provides invaluable material for studying asteroids and the early solar system. Unlike meteorites (most of which also come from asteroids)… we know exactly where they came from. Also unlike meteorites, returned samples have not been altered by the violent and destructive process of atmospheric heating and ablation when entering Earth’s atmosphere. They are as pristine as we can get. Furthermore, they allow us to compare the actual surface of an asteroid (from the rocks themselves) with the data we get from asteroid spectroscopic and other observations from Earth, performed in observatories that are the modern successors of Secchi’s.

It is a great honor to have been invited to work on the OSIRIS-REx project. I am doubly privileged to have been able to see the specimens with my own eyes. Pictures are great, but seeing the real thing is amazing.

The meteorites of the Vatican Observatory are ours thanks to donations by collectors, by dealers, by friends, and on occasion by those who have given us the funds to purchase special samples we could not get otherwise. That freedom doesn’t come free, of course; it’s possible thanks to the support of the Vatican and from our friends around the world. And thanks to that support, the Specola is now playing an important role in the OSIRIS-REx mission.

Guy Consolmagno, SJ and Christopher M. Graney

Director of the Vatican Observatory, the Astronomical Observatory of the Holy See

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