How scientists went to an asteroid to sample the Sun

It was, Dante Lauretta told his audience at an American Geophysical Union meeting in San Francisco, like seeing an old friend after “seven years [and] a journey of 3bn miles”. Sitting in front of him in the Utah desert on the morning of September 24th, “a little charred and worse for wear”, was a capsule about the size of a washing-machine drum. The last time he and his colleagues had seen it had been when they had packed it up ready to be launched into space from Cape Canaveral in the summer of 2016.

In the intervening years the capsule had travelled to Bennu, a small asteroid in an orbit which crosses that of the Earth, as part of a mission called OSIRIS REx. In 2020 the mission’s main spacecraft briefly descended to Bennu’s surface and loaded the capsule with perhaps 140g of material. It then returned to the vicinity of the Earth, cast off the capsule and flew off to study another asteroid named Apophis. The capsule plunged into the atmosphere like an incoming meteorite; it failed to deploy its parachute in the way that had been planned (thus giving Dr Lauretta conniptions) but ended up safe and sound on the surface.

On December 11th Dr Lauretta provided the gathering with some preliminary results from his team’s analysis of its contents. Perhaps the most important was a negative one. The sample was pristine. Though bits of asteroids fall to the Earth every day as meteorites, they are not protected from the heat of re-entry or later contamination at the surface. The Bennu sample was.

This matters because asteroids like Bennu are the most primitive objects in the solar system, remnants of the cloud of gas and dust that collapsed to form the sun. They are thought to have changed hardly at all in the billions of years since. This means that the elements they contain are present in the same relative abundances as would be found in the young sun itself, and in the raw material of the rest of the solar system. The Bennu sample is “the largest pristine reservoir of this material on Earth,” Dr Lauretta said. “This alone, in my opinion as a cosmochemist, makes the whole mission worthwhile.”

There is much more to do with the sample than just tot up elemental abundances. The boulders seen on Bennu came in various forms, some hummocky, some angular. The grains of rock in the sample, despite being hundreds of times smaller, showed a similar variation of types, a peculiarity which may take some explaining. Though the minerals in the sample are more or less the same as those which OSIRIS REx saw all across the surface of the asteroid, some grains are coated with mysterious crusts rich in magnesium, sodium and phosphate, with which Dr Lauretta seems rather taken. There are clearly years, perhaps decades, of further work for him and his team to enjoy.

One result from that September morning, though, is already in. As the capsule streaked through the sky a flotilla of eight highflying balloons observed its passing, not by means of light, but by listening to the very-low-frequency sound, known as infrasound, produced by its shock wave. Their instruments marked its passing as a double-bang, heard first directly, then a second time as it bounced off the desert floor below.

Why go to such trouble to listen to an incoming rock? Because infrasound is tricky stuff, and it is not always possible to know what one is hearing when listening to its strange rumblings. The return from Bennu gave scientists devoted to such listening a sound-alike for an incoming meteorite. If they find such double bangs in their data they will know what they are hearing.

The eventual aim of such research, says Siddharth Krishnamoorthy of the Jet Propulsion Laboratory, the centre in California which does most of NASA’s planetary science, is to build up a catalogue of all sorts of infrasound signals, including those from various types of earthquake. Such a catalogue might find applications off Earth as well as on it. Infrasound-equipped balloons floating in the upper atmosphere of Venus, for instance, would have a chance of picking up earthquakes taking place below them.

The ability to diagnose Venus-quakes from afar would be extremely helpful. Designing seismometers capable of working in the hellish conditions of Venus’s surface is a tall order, so being able to hear what was going on in the planet’s crust from high above its clouds would be a ballooning boon. And making the most of it will require being able to recognise other sources of infrasound for what they are. The ability to recognise the distinctive “knock knock” of an incoming extraterrestrial body will thus be crucial. It should also help scientists assess the background rate of meteorite impacts on Venus, which could prove interesting in and of itself.

No one is expecting there to be flotillas of infrasound-sampling balloons in the skies of the morning star any time soon. But all scientific observers know that a good opportunity to categorise background noise should not be wasted. On top of that, there is something about turning a side-effect of one bit of solar-system exploration into a test-run for another that adds to the appeal of the whole endeavour. ■