Atlantis blasts off on the final shuttle mission. Credit: NASA.

Space Shuttle Atlantis lifted off today on its final voyage, bringing NASA's 30-year-old space shuttle program to an end. With the retirement of the shuttles, and no U.S. rocket capable of taking astronauts into space, experts are buzzing with one question: what's next for NASA? Here are a few important issues surrounding the agency and its plans going forward.

Relying on the Russians to send U.S. astronauts to space is the first piece of NASA's future plan. For many at the agency, this is a bitter pill to swallow. According to SpaceflightNow.com,

In the near term, "we're going to have a reverse brain drain," [Michael Griffin, former NASA administrator and architect of the Bush administrations moon program], told CBS News. "It used to be that people came from other places and other industries to work in the space program because of what it meant and what it was. And as it goes away, we're going to lose those people because talented folks go where there are tough problems. And that's not going to be good for the country."

Griffin's concerns are echoed by many critics of the Obama administrations' plan to retire the shuttles. The administration has also canceled the Constellation program (the moon program devised by the Bush administration), tasked the commercial space industry with developing new low Earth orbit transportation systems, and asked NASA to build a heavy-lift rocket for trips to an asteroid, the moon, and eventually Mars.

Former astronauts, Neil Armstrong, John Glenn, and Jim Lovell, believe NASA should keep flying the shuttles—an agency without a backup system for getting into space and bringing astronauts home is a violation of one of NASA's critical design criteria, these astronauts told the Associated Press. Armstrong and Lovell wrote a letter to the Obama administration saying an end to the shuttle program was a mistake,

Glenn said he doesn't disagree with Obama's plans, although he said he believes private spaceflight will take years longer than [NASA administrator Charles] Bolden predicts. What Glenn objects to is the gap between the shuttle and a future spacecraft. While the Soyuz is reliable, Glenn said NASA should always want an alternative in case of a "hiccup" in the Soyuz plans.

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"Throughout the history of the manned spaceflight program we've always had another program to transition into [...] we had that and it got canceled and we don't have anything," launch manager Leinbach told his fellow workers at Kennedy Space Center. "Frankly as a senior NASA manager I would like to apologize that we don't have that."

The future is not all bleak, though, according to NASA administrator Bolden. During a press conference at Kennedy Space Center held on July 7, according to Space.com,

Private spaceflight firms will pick up NASA's slack before too long, ferrying humans to low-Earth orbit and back relatively cheaply and efficiently, [said Bolden].

And handing off that taxi service to commercial companies, Bolden added, will free the space agency up to do what it was meant to do: explore further afield in our solar system. So the nation is not abandoning human spaceflight, despite a pervasive public perception to the contrary, he said.

NASA is pouring money into the commercial industry while still hammering out a design for a heavy-lift rocket. David Mindell, professor of aeronautics and astronautics at MIT, told me in an interview last week that there is a lot of anxiety about the ending of the shuttle program and part of the problem is that NASA's leadership is not doing a good job at articulating clearly to congress and the public what they are trying to do.

Mindell believes that it is time for the shuttles to retire because "we can't afford two space programs." He adds that the shuttle became an unambitious program, while a lot of other exciting things are happening in commercial spaceflight. "SpaceX is doing things NASA tried to do at one-third the cost; private space will deliver something," he said.

A detailed plan for NASA's future will become clearer when Congress releases the agency's 2012 budget. According to Mindell, there is still one fundamental question that NASA needs to clearly answer, "why send people at all?"

A view from above Spirit on Mars. This mosaic image was taken by the navigation camera on the rover and is an overhead view of the rover on the surface. The front of the rover is up, and retracted airbags are visible underneath the spacecraft.Credit: NASA/JPL-Caltech/Cornell

Today, NASA is ending its attempts to contact the Mars exploration rover Spirit, which last communicated with the agency on March 22, 2010. The cause of its silence: the Martian winter.

Spirit relies on solar energy to run heaters that keep its internal components warm. This past Martian winter was particularly sunless, and the rover most likely endured colder internal temperatures than any of its prior six years on Mars, leaving many critical components and connections susceptible to damage from the cold, according to NASA.

Spirit landed on Mars in January 2004 (its sister rover Opportunity landed a few weeks later on the opposite side of the planet) for what was supposed to be a six-month visit. It ultimately had a successful and turbulent life lasting six years. It was loaded with tools, science instruments, and systems to investigate the composition of soil and rocks, study tiny wobbles in the rotation of Mars to gain insight about the planet's core, and monitor the atmosphere. Most notably, it discovered deposits of salts and minerals such as sulfur and silica that only form in the presence of water.

Spirit survived dust storms, sand traps, and three freezing winters (each Mars years is about two Earth years). Its days as a mobile explorer ended in January 2010, when, after several months of trying to free the rover from a sand trap, NASA decided the rover would remain a stationary science platform.

But now engineers have concluded that even this final stage of Spirit's mission is over. But a new, more capable rover is already waiting in the wings: Curiosity, scheduled to launch in November, is designed to help determine whether there was—or is—life on Mars.

"However, while we no longer believe there is a realistic probability of hearing from Spirit, the Deep Space Network may occasionally listen for any faint signals when the schedule permits," said Dave Lavery, NASA's program executive for solar system exploration, in the press release.

Two panoramas of the Martian surface from Spirit captured in 2005. Credit: NASA/JPL-Caltech/Cornell

Spirit moves its robotic arm. The image was taken by a camera on the rover. Credit: NASA/JPL-Caltech/Cornell

A mosaic of images shows the soil in front of Spirit after a series of short backward drives during attempts to extricate the rover from a sand trap in January and February 2010. Credit: NASA/JPL-Caltech/Cornell University

SpaceX engineering model of its Dragon capsule. Credit: Misha Gravenor

In a much anticipated announcement, the U.S. space agency finally awarded four commercial companies funds to develop spacecraft that can carry astronauts to space. The awards are part of NASA's commercial crew development program, initiated to utilize the private sector to reduce the gap in American human spaceflight capabilities as the space shuttles are scheduled to retire this year, and the Obama administration has canceled the agency's Constellation program.

"The next American-flagged vehicle to carry our astronauts into space is going to be a U.S. commercial provider," said Ed Mango, NASA's Commercial Crew Program manager, in a press release.

The four companies selected: Space Exploration Technologies (SpaceX), which currently has a contract to carry cargo to the International Space Station, will receive $75 million to make its Falcon 9 rocket and Dragon space capsule ready for humans; Sierra Nevada Corporation will receive $80 million for its space plane design; Boeing will receive $92.3 million for a capsule design; and Blue Origin will receive $22 million to develop its capsule design.

"We're committed to safely transporting U.S. astronauts on American-made spacecraft and ending the outsourcing of this work to foreign governments," said NASA Administrator Charles Bolden, in the press release. Until commercial spacecraft are ready the U.S. will have to rely on the Russians to carry astronauts to space at a cost of more than $50 million per seat. SpaceX's vice president of astronaut safety, Ken Bowersox, told me that the company estimates its seats at $20 million each with a crew of seven, or $140 million per mission.

"These agreements are significant milestones in NASA's plans to take advantage of American ingenuity to get to low-Earth orbit, so we can concentrate our resources on deep space exploration," said Bolden.