The more massive the satellite, the more massive the reaction wheels must be. Kepler weighs in at just over one metric ton. Honeywell and only a few other companies make reaction wheels massive enough for that job, says Cooper. MSCI focuses on small satellites, so its largest wheel can handle a craft only half as massive. “But the principle is the same,” he says. In fact Kepler’s mass can be an asset, because once the satellite is pointing in the right direction it’s harder to make it drift.