Heavy metal in outer space

Asteroids are the overlooked jewels of the solar system: Much smaller than planets, less prominent than moons, not as dazzling as icy comets. Yet the asteroids, hundreds of thousands of which orbit the sun between Mars and Jupiter, may still hold countless secrets. One of the biggest that remains to be cracked is: How did the planets in our solar system form?

According to current knowledge, they probably originated from smaller planetary precursors: Protoplanets. These, in turn, were formed when smaller chunks randomly clustered together and melted under the heat of their collisions. In the glowing, viscous mass, the heaviest elements sank into the interior: The metals. All larger protoplanets are likely to have had a heavy metallic core. Today, these metals are firmly encased in the interior of the planets, and are therefore practically inaccessible to researchers.

However, what if some protoplanets had never made it to being a large planet? Such planetary fossils could have survived unnoticed for billions of years. One candidate for such a celestial body is the asteroid "(16) Psyche". Its size and presumed composition are consistent with a bare core of a protoplanet. A violent collision could have stripped it of its mantle of rock and other light compounds a long time ago. However, researchers are not in agreement as to whether the metal-rich asteroid was really formed in this way.

The key to this question also bears the name "Psyche": A space probe that was recently launched into space to study the asteroid of the same name. Initial indications obtained with telescopes and theoretical calculations suggest that the asteroid may not consist largely of rock or carbon compounds, as most others do, but are instead composed almost entirely of metal. According to everything that is known about the distribution of metals in the solar system, this is likely to be largely iron and nickel, but also rarer precious metals such as gold, silver and platinum. Psyche would be the largest and heaviest of the comparatively small group of metallic asteroids. Shaped like a potato, it is around 225 kilometers in size.

The space probe has several scientific instruments on board to study the asteroid, including two cameras for different wavelength ranges of light, a measuring device for high-energy radiation and magnetic field sensors. Researchers at the Institute of Planetary Research at the German Aerospace Center (DLR) are already waiting to use the image data from the cameras to create a three-dimensional terrain model of the asteroid. The measurements from the devices should also allow conclusions to be drawn about the asteroid's magnetic field and internal composition. The magnetic field of the small planet can provide information on whether it has ever been molten (like the interior of a protoplanet) or not (like most of the smaller celestial bodies). The cameras will reveal whether the surface of the asteroid is also made of metal or whether it could be covered with rock, ice or other materials. In addition, minimal distortions in the radio waves received by antennas on Earth from the space probe will be analyzed. These analyses will provide information about the gravity, and therefore also the internal composition of Psyche.

However, the research team will have to be patient: The space probe is not due to reach the asteroid until 2029, before orbiting it for around one and a half years. Such a long flight time is nothing out of the ordinary in planetary research, as Psyche belongs to the growing group of research probes with ion propulsion. This propulsion system does not rely on the classic chemical combustion of fuel. Instead, individual atoms are electrically charged, ("ionized") and accelerated away from the probe using electric and magnetic fields. As unbelievable as it sounds, the probe is propelled by the recoil of individual atoms that are thrown away. It draws the electrical energy required for this from its solar panels. Drives of this type are highly efficient and therefore consume very little fuel. However, they can only generate a very low thrust, meaning the journey is economical but lengthy.