Psyche is going to an asteroid that appears to be largely metal. Most small bodies are rock or ice; 16 Psyche, some 220 kilometres across, reflects radar like a metallic object, and the leading hypothesis has been that it is the exposed iron-nickel core of a protoplanet whose rocky exterior was stripped away by collisions in the early solar system.
If that is right, it is the only place a core can be examined directly. Earth's own core lies 2,900 kilometres beneath our feet and will never be sampled; a mission to Psyche would be, in effect, a visit to the inside of a planet. More recent estimates of its density suggest it may be less metallic and more of a rubble mixture than first thought, which is precisely why it needs visiting.
The spacecraft uses solar-electric propulsion, with Hall-effect thrusters that expel xenon ions and glow blue, accumulating an enormous velocity change over years of near-continuous operation. It flew past Mars for a gravity assist in 2026 and arrives in 2029.
It also carries a communications experiment of considerable significance. Deep Space Optical Communications encodes data in an infrared laser rather than radio, and during the cruise it has transmitted at rates vastly beyond radio capability from tens of millions of kilometres — including, in a much-publicised test, streaming a video of a cat from deep space.
The mission's launch was delayed a year when an independent review found the flight software behind schedule and the team overstretched, prompting an unusually public institutional reckoning at JPL about workload and staffing. The delay was used to fix the problems rather than to press on, which is a healthier outcome than several missions in this list managed.
