The first flight in the National Aeronautics and Space Administration's program to build a next-generation rocket to replace the space shuttle could also be the swan song.

The launching of the Ares I-X, a prototype, is scheduled for 8 a.m. on Tuesday. It comes less than a week after a blue-ribbon panel reviewing NASA's human spaceflight program released its final report, which offers a less-than-ringing endorsement of the Ares I rocket and brings the entire program into question. Most of the options explored by the panel look to alternatives from commercial companies to carry astronauts into Earth orbit.

The $450 million Ares I-X is almost the same size and shape as the planned Ares I, but with a less powerful first stage and a dummy second stage. The test flight will carry 700 sensors to measure stress, temperature, vibrations and other data that will give engineers a chance to validate and tweak their designs.

Critics of the Ares I, which is part of NASA's Constellation program intended to return astronauts to the Moon by 2020, have described it as too expensive, underpowered and technically flawed.

However, the biggest problem with the Ares I is that it might be too late.

NASA had originally hoped that it might begin carrying astronauts to the International Space Station in 2012. The current NASA schedule calls for the first flight in 2015, but the review panel concluded that with the agency's financial constraints, the Ares I was not likely to make its first manned flight until 2017.

The panel said the Ares I program did not suffer from any showstopping technical issues, but wondered whether NASA should instead focus on building a heavy-lift rocket that would be needed for missions beyond Earth orbit.

Despite questions about the fate of the Ares I, the Ares I-X test will proceed as planned.

“;We do think it's appropriate to fly the Ares I-X,”; said Norman R. Augustine, the panel's chairman. “;We think there are important things to be learned that will help the program.”;

The skinny rocket, towering 327 feet, is the tallest to sit on a launching pad at the Kennedy Space Center in Florida since the 363-foot Saturn V rockets used for the moon landings in the 1960s and 1970s.

“;The vehicle is in great shape,”; Jeffrey G. Spaulding, the test director, said at a news conference on Sunday.

The flight will be a short one, never reaching orbit. The first stage, which essentially consists of a spare solid rocket booster from the space shuttle program, will burn for about two minutes, lifting the 1.8-million-pound Ares I-X eastward to an altitude of 25 miles. Then the first stage will separate and descend by parachute, while the dummy second stage will continue along its parabolic arc and dive into the Atlantic.

Stormy weather, however, might keep the Ares I-X on the ground. Although the flight needs only about 15 minutes of good weather in the four-hour launching window that stretches from 8 a.m. to noon, Kathy Winters, the weather officer for the launching, said there was only a 40 percent chance of favorable weather on Tuesday. Wednesday is more promising, with a 60 percent chance of favorable weather during the 8 a.m. to noon window.

Because of conflicts with launchings from the neighboring Cape Canaveral Air Force Station, if Ares I-X cannot get off the pad on Wednesday, the test may be delayed to November.

The Ares I is to have a five-segment solid rocket motor, compared with the four-segment motors used for the shuttle boosters and the Ares I-X. The first ground test of the five-segment motor was successfully conducted in September in Utah.

Despite the differences between the two rockets, engineers designed the weight of the second stage and the flight path so that the Ares I-X will experience forces similar to those the Ares I would experience on its way to orbit.