An asteroid more 400m wide will pass close to Earth, zooming by at a distance of just over 1.8 million km, but with no chance of impact, according to NASA scientists.
Asteroid 2014 JO25 will pass by at less than five times the distance to the Moon.
It will be the asteroid's closest approach to Earth for at least the next 500 years.
The peanut-shaped asteroid will not be visible to the naked eye, but sky watchers should be able to view it with home telescopes for one or two nights, starting tonight.
Smaller asteroids routinely make closer passes to Earth, but 2014 J025 will be the largest asteroid to come this near to the planet since 2004, when the 5km-wide asteroid Toutatis passed about four lunar distances, or 1.6 million km, from Earth.
Frances McCarthy from Cork Institute of Technology's Blackrock Castle Observatory has said the asteroid will be visible tonight.
Speaking on RTÉ's Morning Ireland, she said the asteroid's closest approach will be this afternoon, but people should be able to see it at around 9pm.
Ms McCarthy said the asteroid was discovered three years ago.
