The aftershocks of the Sept. 11 terrorist attacks in the United States are reaching halfway around the world, rendering uncertain the planning and financing for the World Summit On Sustainable Development, to be held next year in South Africa. The event, which is organized by the United Nations and is better known as the Earth Summit, will mark the 10-year anniversary of the first environmental summit in Rio de Janeiro. The upcoming gathering was originally scheduled to end on Sept. 11, 2002, but conference dates are now up in the air. Financing for the event, which could cost as much as $55.7 million, is also on shaky ground, as potential sponsors worry about global economic slowdown. The conference is expected to draw more than 60,000 delegates, activists, journalists, and heads-of-state, and will focus on establishing timelines and targets for sound environmental management.