New propulsion systems for rocket engines would use mercury as a fuel, which could run the risk of spreading toxic chemicals through Earth’s atmosphere.

NASA experimented with mercury in the 1960s because it’s a low-cost, high-power option for ion engines. Startup Apollo Fusion has discovered a new approach to using mercury—but again, there’s a catch. Mercury is heavier than the xenon and krypton powering other ion engines in use today.

What customers might save on costs could pollute the atmosphere in potentially harmful ways. While the U.S. government has tried to reduce our mercury emissions since the 1990s, longstanding regulations do not specifically cover spacecraft hovering above us.

The Federal Aviation Administration requires companies to disclose hazardous materials, but this doesn’t include satellites. It’s an area where, yet again, technology has leapt beyond our governing agencies and the policy they write.