The phrase "a small asteroid" is correct and usable in written English.
You can use it when describing the size of an asteroid in a scientific context or in discussions about space.
Example: "Astronomers discovered a small asteroid that could potentially pass close to Earth in the coming years."
Alternatives: "a tiny asteroid" or "a minor asteroid."
In 2008, for example, a small asteroid was detected 20 hours before it hit Earth.
A relatively small lump of rock – a small asteroid, perhaps only a few hundred metres across – plunging to Earth would devastate a continent or trigger tsunamis.
The chosen date marks the anniversary of the "Tunguska event", when a small asteroid or comet exploded above Siberia with the force of 1,000 atomic bombs.
One would be to find a small asteroid – so less than 10 metres across – basically encase it in a bag and bring it back with us.
Next week will see a small asteroid pass closer to the earth than TV satellites, but Nasa have assured the world that there is no danger of impact.
Dr. Stuart was certain that he had witnessed a small asteroid hitting the Moon, the flash being the fireball from the event.
Backtracking, they calculated that a single impact by a smaller asteroid could have blasted it all off the asteroid in February or March 2009.
One possibility is that a slow-motion collision with a smaller asteroid or comet once blasted debris into orbit, and the gravitational pull of the largest bits of debris have shepherded the smaller bits into a sharply defined ring.
NASA officials passed on another team's concept to capture a smaller asteroid and house it inside an inflatable structure for the trip back to lunar orbit.
The asteroid that exploded over the Russian city of Chelyabinsk in February 2013 is a sobering reminder of the kind of damage a relatively small asteroid an estimated 65 feet in length can do.
A single small asteroid of the right type contains billions of dollars' worth of platinum alone -- about a year's production on Earth.
