Radiation is a primary concern for long-duration human spaceflight, such as the planned trips to Mars, which are the stated goal of organizations such as NASA and SpaceX. Shielding is the standard way to protect astronauts from radiation during those flights. However, shielding is heavy and, therefore, expensive when it is launched off the Earth.

What if, instead, astronauts could hitch a ride on a giant mass of shielding already in space that will take them directly to their destination? That is the basic thought behind a paper from Victor Reshetnyk and his student at Taras Shevchenko National University in Kyiv posted to the arXiv preprint server.

They looked at data collected by NASA’s Horizons service and analyzed the orbits of more than 35,000 Near Earth Objects (NEOs) for their trajectories to see if their paths would cross somewhere between the binary pairs of Earth-Venus, Earth-Mars, or Mars-Venus. If so, then in theory, they could be used as shielding from the deadly radiation astronauts would have to either suffer from or shield against on the trip.

Given the sheer amount of objects they looked at, they were bound to find some good candidates—and they did, with an estimated 525 making “fast” transfers of less than 180 days. They then further narrowed this list down to a reasonable speed during the approach to the planet they would start from—essentially to make sure that a crewed spacecraft could actually catch up to the asteroid without burning an absurd amount of fuel.

That lowered the total number of candidates down to 120, with the following breakdown:

• Earth to Venus: 44
• Earth to Mars: 17
• Mars to Earth: 13
• Mars to Venus: 2
• Venus to Earth: 38
• Venus to Mars: 6

In other words, there were plenty of options for hitching a ride. Granted, none of these would be exceptionally roomy—the largest is estimated to have a diameter of only 0.37 km. However, there is still plenty of room to fit a spaceship, as long as it’s not a Star Destroyer or Battleship from 40K.

Additionally, the authors found some asteroids that had more unique trajectories. Eleven had the possibility of doing “multiple” transfers, meaning they could go from Earth to Venus and then back or vice versa, but only one would do the same for the Venus to Mars trip. Two could even do a “double” transfer, meaning they could go from Earth to Venus to Mars or from Mars to Venus to Earth in less than one year. Anything beyond that wasn’t possible, though—they didn’t find any asteroids akin to an “Aldrin Cycler” that would go between the planets indefinitely on a known orbit.

That’s not to say that asteroid doesn’t already exist—we might just not have found it yet. NEO Surveyor, a NASA mission designed to launch in 2028 to find 90% of all NEOs larger than 140m in diameter, could increase the number of known NEOs by an order of magnitude.

Using any of them for a massive radiation shield for a crewed mission would take much more dedicated work, though. Any such transformation is decades away at least—but the place to start is to find the right ones, and this paper contributes to that effort.