Brian Wilcox, an ex-member of the NASA Advisory Council on Planetary Defence, made the horrifying revelation while discussing a report by the space agency.
He said that NASA's "risky" proposals to prevent an eruption could go wrong with possibly disastrous consequences.
One of the methods would be to drill into the bottom of the United States volcano and to use a pressurised spurt of water to release heat from the magma chamber.
But Mr Wilcox said there is a chance this would not be successful.
"If you drill into the top of the magma chamber and try and cool it from there, this would be very risky," he said.
"This could make the cap over the magma chamber more brittle and prone to fracture. And you might trigger the release of harmful volatile gases in the magma at the top of the chamber which would otherwise not be released."
But Mr Wilcox warned it is clear something must be done about Yellowstone.
He told the BBC: "I was a member of the NASA Advisory Council on Planetary Defence which studied ways for NASA to defend the planet from asteroids and comets.
"I came to the conclusion during that study that the supervolcano threat is substantially greater than the asteroid or comet threat."
Earth has 20 known supervolcanoes, which if they erupted, would trigger planet-changing effects.