Another particularly challenging campaign followed the flight of Voskhod 1.

"My job is not to set the vision, but to support the museum's mission and its director at a particularly challenging time," she said.

The Sept. 11 anniversary has made this month a particularly challenging time to step into such a position.

But for single mothers, transportation and child care are often particularly challenging obstacles to such jobs.

Fire management through prescribed burns is particularly challenging as these areas often are located near development.

And some research is not done at all because of a lack of funds for particularly challenging problems.

Once a year, usually in the spring, they select a particularly challenging creature to kill.

His anticipated arrival comes at a particularly challenging time in the development organization's history.

Reporting both sides of the political debate in Venezuela is particularly challenging, he told me.

These are particularly challenging times for children who may have had little experience managing strong affects within themselves or in their family.