NEW YORK — While NASA may appear to have its pick of thousands of known asteroids for a manned mission, only two are good targets within the next 20 years.

An asteroid mission requires a large-enough destination that astronauts could reach within a few months of launch from Earth, says Lindley Johnson, head of NASA’s Near-Earth Object program in Washington. Other limits to such an ambitious undertaking include the viewing range of ground-based telescopes.

“They don’t come all that close all that often,” Johnson said at a near-Earth object (NEO) workshop NASA held in August.

While NASA admits more knowledge about objects that pass within 45 million kilometers of Earth could increase the number of possible destinations, only two currently meet the guidelines set out by the space agency in its attempt to send a manned mission to an asteroid by 2025, a goal set by President Barack Obama. One of the asteroids could be reached in 2020 and the other in 2025.

A third candidate would not be within range until 2045.

Experts attending the NASA workshop discussed how, where and when to launch an asteroid-bound mission. Johnson later told Space News, “In our limited understanding of the smaller NEO population, we believe the known catalog represents less than 5 percent of the projected total population of accessible objects. So yes, improved NEO search assets should find additional targets.”

NASA has not yet chosen any specific target for astronauts, Johnson said. It has simply begun examining its options under certain human spaceflight scenarios.

The earliest NASA could expect to reach an asteroid would be in 2020, when ground-based telescopes are able to spot the return of a 60-meter rock known as 2009 OS5.

Under one scenario, astronauts would launch March 11, 2020. The space explorers might even fly aboard part of the international space station, a node that could be converted and detached, according to a concept outlined in a separate NASA presentation.

The round trip would take 170 days, which fits within NASA planners’ ideal guidelines of an asteroid-bound round trip that lasts no longer than 180 days. Another guideline is that the asteroids have a size of at least 50 meters.

The March 2020 launch would come just shortly before Earth-based telescopes could survey the returning asteroid in April 2020. If NASA missed that launch window, it could theoretically try again for the same space rock in 2036, agency officials said.

A launch opportunity to visit a different asteroid would arise Sept. 19, 2025, with astronauts aiming for another 20-meter space rock, called 1999 AO10. But the viewing opportunity for Earth-based telescopes to spot that target would come several months later, in January 2026.

A later effort could target the 100-meter object called 2003 SM84 in 2045.

The three possible candidates for the 2020-2050 time frame came from a larger list of 44 known objects in orbits reachable by a human mission.

That assumes the future mission would have a rocket launch capability similar to the heavy-lift Ares 5 rocket being developed under the Constellation program, but that could be canceled if the Obama administration’s 2011 NASA budget proposal is approved by Congress.

But just 17 of the objects reachable by humans are larger than 50 meters, and only 15 are accessible within the next several decades. Of those 15, only three fit within the 180-day round-trip limit.

The list of possible targets reachable by humans could expand with more-complete surveillance of NEOs. Experts agreed that NASA and its international partners need to invest in more ground telescopes, or in a possible space telescope that would roughly follow a Venus orbit.

Better monitoring of space rocks also could provide more information about spin state, composition or possible neighboring objects. Even the three possible targets reachable before 2050 remain largely unknown besides orbit and rough size.