Cosmic cloud could obliterate life.
If the solar system hit even a small gas cloud in space, the "bubble" surrounding it, which protects life on our planet, could burst.
As a result, the Earth would be raked by deadly cosmic radiation and suffer unpredictable, destructive climate changes leading to global warming, a great flood, or an ice age.
One especially cloudy region is directly in our path - although it will not be reached for some 50,000 years. However, small knots of gas called the Local Fluff could be encountered far sooner.
The new doomsday prophecy comes in the wake of fears of the end of the world arriving in the form of a giant comet or asteroid.
Mr Gary Zank, of the Bartol Research Institute at the University of Delaware in the United States, based his prediction on computer simulations of what might happen to the cocoon created by the solar wind - charged particles flowing out from the sun.
He said: "We are surrounded by hot gas. As our sun moves through empty or low-density interstellar space, the solar wind produces a protective bubble - the heliosphere around our solar system - which allows life to flourish on Earth.
"Unfortunately, we could bump into a small cloud at any time, and we probably won't see it coming. Without the heliosphere, neutral hydrogen would interact with our atmosphere, possibly producing catastrophic climate changes, while our exposure to deadly cosmic radiation in the form of very high-energy cosmic rays would increase."
Every 66 million years or so, the solar system traces a regular path through the Milky Way galaxy.
Mr Zank pointed out that over the past five million years we have had smooth sailing. The sun had been coasting through calm interstellar "waters" containing on average less than one atom per cubic inch of space.
Currently, the solar system is in a region of space containing between three and four atoms per cubic inch.
But the weather in space could change, Mr Zank warned.
A particularly troublesome cloud zone in a star-forming region near the Aquila Rift, about 80 light years away, was clearly headed our way pushed by galactic wind, and may collide with the Earth's protective bubble within the next 50,000 years.
Some researchers believe smaller clouds containing between ten and 100 atoms per cubic inch of space could be met far sooner. "We won't know that our heliosphere is collapsing until we see highly elevated levels of neutral hydrogen and cosmic rays,"said Mr Zank.
There are suggestions that cosmic clouds may cause periodic extinctions. As long ago as 1939, British cosmologist Sir Fred Hoyle proposed that collisions with cosmic clouds might occasionally obliterate the heliosphere.
If this happened the consequences for life on Earth could be dire. Mr Zank, who presented his findings at the American Geophysical Union's spring meeting in Boston, said: "The protective solar wind would be extinguished, and cosmic radiation might lead t o gene mutations.
"Hydrogen would bombard the Earth, producing increased cloud cover, leading perhaps to global warming, or extreme precipitation or ice ages.