NEW YORK — National emergency plans for natural disasters also can work in the unlikely scenario of an asteroid strike on the United States, according to a letter to Congress by the White House’s top science adviser.

The 10-page letter by John Holdren, director of the White House Office of Science and Technology Policy, adds that the United States has a responsibility to the world as the country most capable of detecting space rocks that threaten Earth. The Oct. 15 letter is addressed to the leaders of the House Science and Technology Committee.

Holdren states that NASA must continue leading efforts to close the gap in detecting and perhaps deflecting near Earth objects (NEOs). The U.S. space agency already has the duty of alerting the rest of the government about any threatening space objects.

Holdren’s letter also lays out the duties of other federal agencies in handling emergency communications and response. It calls for a “senior-level interagency simulation exercise” to test impact response plans before the United States is confronted with an asteroid impact.

“My immediate reaction is that it represents the most detailed consideration of the U.S. government’s response to the NEO threat to date, more clearly delineating communication links and responsibilities than had previously been the case,” said Clark Chapman, space scientist at the Southwest Research Institute in Boulder, Colo.

According to Holdren’s letter, the Federal Emergency Management Agency, under the Department of Homeland Security, has the main responsibility on the ground in the United States. FEMA can rely in part upon the National Warning System, which was designed to alert U.S. citizens to a Cold War nuclear attack.

The Department of Defense would work with NASA on possible mitigation or deflection scenarios that involved military resources.

Meanwhile, the State Department would help coordinate any international warnings or responses in a deep-impact scenario that affects more than just the United States. It has experience notifying other countries about re-entering human-made space objects, including the defunct USA-193 spy satellite that was ultimately destroyed by a U.S. Navy missile.

“The United States is currently the world leader in NEO detection activities and will have a vital role to play in such communications, irrespective of whether the direct risk to the United States or its territories is considered low,” Holdren said.

A NASA advisory council recently suggested that the space agency set up an official Planetary Defense Coordination Office to lead protection efforts against threatening asteroids or comets.

NASA has begun closing in on its congressionally directed goal of finding at least 90 percent of all NEOs with a diameter of 1 kilometer or greater. Search teams had discovered about 903 of an estimated 1,050 NEOs in that size category as of Oct. 1, and the space agency plans to reach its 90 percent detection goal by the end of this year.

Of those discovered objects, 149 have orbits that could possibly bring them into collision with Earth, and none presents an impact threat within the next 100 years. Another 993 objects less than 1 kilometer in diameter also have orbits that could someday pose a threat to our planet.

Yet NASA estimates that the 6,416 known NEOs in the smaller size category, less than 1 kilometer wide, represent just 5 percent of the expected count. In other words, there are probably many more objects out there that represent a possible threat to Earth.

Some of those objects were discovered more recently by NASA’s sky-mapping Wide-field Infrared Survey Explorer mission, which is slated to end in January. But there are possible plans for ground-based telescopes that could join the hunt, such as the Air Force’s Space Surveillance Telescope and the Large Synoptic Survey Telescope.

The National Research Council and NASA also suggested the possibility of a dedicated asteroid hunter that would fly in a Venus-trailing orbit. No firm plans have been made for this.

President Barack Obama’s new National Space Policy and plan for human spaceflight also have targeted a human mission to an asteroid by 2025. That could prove a useful dry run for future efforts that might need to deflect an asteroid away from Earth.

“The planning, required capabilities and ultimate execution of such a mission also would parallel most aspects of a potential robotic asteroid-deflection mission, providing valuable experience in asteroid-rendezvous techniques,” Holdren said.