The private group advocating a 2018 manned flyby of Mars revealed details of its proposed mission, including a surprise request for hundreds of millions of dollars in federal help.

Multimillionaire Dennis Tito, the world's first space tourist and the main proponent of the nonprofit entity behind the project, spelled out his vision for the effort Wednesday during a news briefing and in material prepared for testimony before a House of Representatives science subcommittee. But even before specifics could be analyzed, he called the proposal a "long shot" and acknowledged his team already is considering a fallback option of launching in 2021.

Mr. Tito's organization didn't provide a comprehensive price tag, or say how much of the National Aeronautics and Space Administration's current spending already is going toward technology that could be used for the Mars mission, though it provided new technical details and political arguments. Calling it a "grand and worthy goal" aimed at "winning back the world's attention," the organization also described the proposed mission as a model for "daring, high-yield" private-public partnerships among various private groups, industry, scientists and NASA.

In his first detailed projections of the overall cost of the proposed 800-million-mile Mars mission, Mr. Tito told the subcommittee it is likely to take "less than $1 billion" above current NASA spending levels. He testified that once NASA is fully on board and planning is far enough along that "we actually have a mission on the manifest" the foundation hopes to raise about $300 million in philanthropic donations over roughly five years.

But the document also includes an unexpected plea for direct federal funding. Seeking to justify taxpayer investment of several hundreds of million dollars in new federal appropriations—which the report describes as "not much" for such an ambitious endeavor—the foundation said the mission would complement many other NASA projects, "reward years of effort and put to use all that we have learned."