SAN FRANCISCO (Reuters) - An asteroid may come uncomfortably close to Earth in 2036 and the United Nations should assume responsibility for a space mission to deflect it, a group of astronauts, engineers and scientists said on Saturday.

Astronomers are monitoring an asteroid named Apophis, which has a 1 in 45,000 chance of striking Earth on April 13, 2036.

Although the odds of an impact by this particular asteroid are low, a recent congressional mandate for NASA to upgrade its tracking of near-Earth asteroids is expected to uncover hundreds, if not thousands of threatening space rocks in the near future, former astronaut Rusty Schweickart said.

“It’s not just Apophis we’re looking at. Every country is at risk. We need a set of general principles to deal with this issue,” Schweickart, a member of the Apollo 9 crew that orbited the earth in March 1969, told an American Association for the Advancement of Science conference in San Francisco.

Schweickart plans to present an update next week to the U.N. Committee on Peaceful Uses of Outer Space on plans to develop a blueprint for a global response to an asteroid threat.

The Association of Space Explorers, a group of former astronauts and cosmonauts, intends to host a series of high-level workshops this year to flesh out the plan and will make a formal proposal to the U.N. in 2009, he said.

Schweickart wants to see the United Nations adopt procedures for assessing asteroid threats and deciding if and when to take action.

The favored approach to dealing with a potentially deadly space rock is to dispatch a spacecraft that would use gravity to alter the asteroid’s course so it no longer threatens Earth, said astronaut Ed Lu, a veteran of the International Space Station.

The so-called Gravity Tractor could maintain a position near the threatening asteroid, exerting a gentle tug that, over time, would deflect the asteroid.

An asteroid the size of Apophis, which is about 460 feet

long, would take about 12 days of gravity-tugging, Lu added.

Mission costs are estimated at $300 million.

Launching an asteroid deflection mission early would reduce the amount of energy needed to alter its course and increase the chances of a successful outcome, Schweickart said.

NASA says the precise effect of a 460-foot (140-meter) object hitting the Earth would depend on what the asteroid was made of and the angle of impact.

Paul Slovic, president of Oregon-based Decision Research, which studies judgment, decision-making and risk analysis, said the asteroid could take out an entire city or region.