A Canadian astronomer's 20-year search has finally been rewarded with the discovery of a so-called Trojan asteroid -- a rock the size of several football fields that is effectively riding in tandem with Earth as it orbits the sun.

Dr. Martin Connors, who holds a Canada Research Chair in Space Science at Athabasca University, led the team that discovered the asteroid using an orbiting NASA telescope.

While there are many types of asteroids orbiting the sun, most are travelling at their own speeds.

"This one is going at the same speed as the Earth and it stays pretty equidistant from the Earth although it's moving in our own orbit, so that's what makes it special," Connors told CTV's Canada AM.

The rock, dubbed 2010 TK7, is estimated to be between 200 and 300 metres in diameter, and sits in a gravitational sweet spot just ahead of Earth -- effectively locked in a gravitational balancing act between the planet and the sun.

As a result, it poses no danger to Earth, he said.

Astronomers are excited about Trojans because they represent one of the best opportunities to land an astronaut on an asteroid -- one of NASA's stated future goals now that the shuttle program has been mothballed.

Unlike other asteroids, Trojans are stable and predictable, and in relatively close proximity to Earth. Other Trojans have been discovered near Jupiter, Neptune and Mars.

Connors said 2010 TK7 is tilted, and therefore isn't likely a candidate for a visit from an astronaut. But its discovery makes it highly likely there are other Trojans also sharing Earth's orbit, which could make better candidates for a landing.

"If the U.S. economy doesn't collapse, potentially we'll have space travel in the future and it is a stated goal to send astronauts to an asteroid and this type of asteroid would actually be a pretty good type to go to," he said.

A successful landing, he said, could help solve mysteries about the Earth's origin and answer questions about whether asteroids could be used as a source of important minerals.

Because Trojans are caught between their host planet and the sun, they are only visible for a brief window each evening, and in the early morning hours. As a result, a space-based telescope such as NASA's Wide-field Infrared Survey Explorer is typically needed to spot them.

2010 TK7 currently sits about 80 million kilometres from Earth.