Four industry insiders ultimately confirmed that Apollo Fusion was building thrusters that utilized mercury propellant. Apollo Fusion, which was acquired by rocket manufacturing startup Astra in June 2021, insisted that the composition of its propellant mixture should be considered confidential information. The company withdrew its plans for a mercury propellant in April 2021. Astra declined to respond to a request for comment for this story.
In 2018, an employee at Apollo Fusion approached the Public Employees for Environmental Responsibility (PEER), a nonprofit that investigates environmental misconduct in the United States. The employee—who has remained anonymous—alleged that the Mountain View, Calif.–based space startup was planning to build and sell thrusters that used mercury propellant to multiple companies building low Earth orbit (LEO) satellite constellations.