When most of us think of scientists, we have visions of men in white coats working in laboratories full of shining tubes and pumps going pockita-pockita. Clearly, that is not what it is like to be an astronomer these days. For one thing, about half the graduate students in Planetary Sciences are now women, not men. What’s more, nobody I know owns a white coat. And it’s hard to do “experiments” on stars or planets; for one thing, they are a little too big to fit into our labs.
And yet, here I am in a lab at Boston College this week. Right behind me is a pump going pockita-pockita. I am setting up to do experiments on materials that may have a lot to do with our understanding of stars and planets.
I am working with Cy Opeil, another Jesuit scientist (like myself) whose lab here at BC measures the physical properties of all sorts of exotic materials. I’ve brought along some exotic materials of my own: not strange compounds created in a lab, the sort of thing he normally measures, but tiny bits of meteorite from our collection in Castel Gandolfo.
The hope is that we will be able to derive very accurate measurements of the physical properties of these meteorites. Knowing how they respond to heat — their thermal conductivity, heat capacity, and thermal inertia — will not only let us calculate more accurately models for their heating and melting. But it turns out that the very motions of asteroids can be affected by the way they respond to sunlight.
About a hundred years ago, a Russian scientist named Yarkovsky realized that a spinning asteroid, which obviously will be hotter on the sunlit side than on the side facing away from the sun, will actually have its “afternoon” side slightly warmer than its “morning” side since its spin will carry the warmer part of the asteroid away from the exact point directly under the Sun. But as an asteroid absorbs sunlight and heats up, it must also radiate that heat away as infrared light (thus balancing the heat coming in with the heat going out, and so maintaining an even temperature). Since the afternoon side is hotter, it emits more energy than the morning side; and this slight imbalance in energy can actually perturb the orbit of the asteroid.
But that depends on how much the heat actually gets carried about by the material in the asteroid: its thermal inertia. If the inertia is high, then the heat can be carried quite a ways and the effect is very strong. Lots of different things can affect the thermal inertia: the presence of dust vs solid rock, the amount of metal vs rock, and so forth. Our measurements in this lab, we hope, will let us put some limits on the thermal inertia of the different materials that make up the asteroid, to help sort out some of these differences.
The Yarkovsky effect has an odd history. Yarkovsky himself apparently published his insight in a pamphlet around 1900, but I don’t know anyone who’s actually seen that pamphlet. The Estonian/Irish astronomer Ernst Öpik recalled reading this pamphlet many years later, and gave him credit for the idea around 1950. When I was a grad student at MIT in the 1970s, a fellow student named Charlie Peterson revisited the idea and worked it out in some detail, but he then left the field and the idea lay fallow for another twenty years. Finally it was revived by a number of scientists starting around 1990 (notably David Rubincam) and it is now well established as an important force in controlling the position and spin of asteroids… and the way they can move pieces into orbits that eventually hit the Earth as meteorites.
A lesson from that: it’s not enough to have a great idea; you have to “campaign” it, keep telling people about it, until the message gets through that you have come up with something important. (Only, sometimes, the opposite message gets back to you… that the idea wasn’t so great, after all, and it is time to look somewhere else!)
With luck, my work in the lab this week will give me something new to “campaign” at meetings of meteorite scientists and planetary astronomers this fall.
