This could be Gold in the Sky by Alan E. Nourse, a novel that appeared in full in the September 1958 issue of Amazing Science Fiction Stories.
Chapter 4 is titled "Between Mars and Jupiter..." and contains the following conversation among the main characters:
"Hold it," Greg said, peering at the film reader. "There's something more on the last page, but I can't read it."
Tom blinked at the entry. "'Inter Jovem et Martem planetam interposui,'" he read. He scratched his head. "That's Latin, and it's famous, too. Kepler wrote it, back before the asteroids were discovered. 'Between Jupiter and Mars I will put a planet.'"
Greg and Johnny looked at each other. "I don't get it," Greg said.
"Dad told me about that once," Tom said. "Kepler couldn't understand the long jump between Mars and Jupiter, when Venus and Earth and Mars were so close together. He figured there ought to be a planet out here ... and he was right, in a way. There wasn't any one planet, unless you'd call Ceres a planet, but it wasn't just empty space between Mars and Jupiter either. The asteroids were here."
Tom's dad turns out to have buried a secret on an asteroid with an eccentric orbit.
Later, while they waited, they went over it to make sure that nothing was missing. "No wonder we couldn't spot it," the Major said. "We were looking for an asteroid in a standard orbit in the Belt."
"But there wasn't any," Tom said. "Dad's rock was isolated, nowhere near any others. And we were so busy thinking of the thousands of rocks in normal orbits between Mars and Jupiter that we forgot that there are a few eccentric ones that just don't travel that way."
"Like this one." The Major stared at the screen. "A long, intersecting orbit. It must swing out almost to Jupiter's orbit at one end, and come clear in to intersect Earth's orbit at the other end...."
"Which means that it cuts right through the Asteroid Belt and on out again." Tom grinned. "Dad must have seen it coming ... must have thought it was on collision course for a while. But he also must have realized that if he could hide something on its surface as it came near, it would be carried clear out of the Belt altogether in a few days' time."
The guys have to time their approach just right as the asteroid comes around. They land on it and search around for Tom's dad's secret.
And now his confidence began to fail. Supposing he had been wrong? They knew the rock had passed very close to Roger Hunter's asteroid, the astronomical records proved that. But suppose Dad had not used it as his hiding place at all? He pulled himself around another jagged rock shelf, staring down at the rough asteroid surface beyond....
... he heard a growl of static in his earphones, and then Greg's voice, high-pitched and excited. "Over here! I think I've found something!"
It was a metal cylinder, four feet long, two feet wide, and bluntly tapered at either end. In the sunlight it gleamed like polished silver, but they could see a hairline break in the metal encircling the center portion.
They had found Roger Hunter's bonanza.
But what's in the cylinder?
They stared down at it. The metal seemed to have a life of its own, glowing and glinting, focussing light into pinpoints on its surface.
It was a map.
At one side, a glowing ball with a fiery corona, an unmistakeable symbol that any intelligent creature in the universe that was able to perceive it at all would recognize as a star. Around it, in clearly marked orbits, ten planets. The third planet had a single satellite, the fourth two tiny ones. The sixth eleven. The seventh planet had ten, and was encircled by glowing rings.
But the fifth planet was broken into four parts.
Beyond the tenth planet there was nothing across a vast expanse of the map ... but at the far side was another star symbol, this one a double star with four planetary bodies.
They stared at the glowing map, speechless. There could be no mistaking the meaning of the thing that lay before them, marked in symbols that could mean only one thing to any intelligence that could recognize stars and planets.
But in the center of the sheet was another symbol. It lay halfway between the two Solar Systems, in the depths of interstellar space. It was a tiny picture, a silvery sliver of light, but it too was unmistakeable.
It could be nothing else but a Starship.