The United States said Tuesday that Secretary of State Hillary Clinton will head to Rio de Janeiro for the Earth Summit\'s 20-year follow-up, which is drawing wide global participation. Clinton will lead the US delegation to the June 20-22 conference and also meet with world leaders on the sidelines, State Department spokeswoman Victoria Nuland said. Other US officials heading to Rio will include Environmental Protection Agency chief Lisa Jackson and Todd Stern, the US special envoy on climate change who will serve as the chief US negotiator in Rio. The landmark 1992 summit, where then US senator Al Gore played a high-profile role, set the ball rolling in UN-led efforts to tackle problems facing the planet including climate change and species loss. As many as 50,000 activists, policymakers and business executives are expected for the 20-year follow-up. But expectations for major achievements are low, with much of the world more focused on present economic woes. The conference also comes as President Barack Obama pursues a tough re-election battle. The 1992 Earth Summit and the blueprint it produced, Agenda 21, have been longstanding targets of US right-wing activists and conspiracy theorists.