By Paul Rincon
Science reporter, BBC News, Houston
The head of Nasa has been outlining its new strategy at a major conference in Houston, Texas.
Phoenix arrives at the Red Planet in just few weeks' time
The US space agency will concentrate more on exploring the outer Solar System and a little less on exploring Mars.
Dr Mike Griffin tried to reassure the community of scientists who study the Red Planet, but also warned them to be prepared for change.
Nasa's 2009 budget request called for significant funding cuts for its Mars
programme over the next five years.
The average annual budget plan for Mars exploration from 2009-2012 stands at about $343m, compared to an average of $620m for the same period in last year's budget estimates.
But Dr Griffin pointed out that Nasa would simply be returning Mars funding to its historical average over a 25-year period: "That's not a bad deal," he told the audience.
The Nasa administrator was addressing scientists this past week at the 39th Lunar and Planetary Science Conference.
Nasa's Mars programme has provided some of the most exciting science results of the last 10 years.
Global Surveyor, Odyssey and Reconnaissance Orbiter, not to mention the Mars Exploration Rovers, have been hugely successful and popular.
New, more powerful rockets shorten journey times to distant planets
A lander, Phoenix, is scheduled to touch down in May to examine Mars' "Arctic" terrain. And in 2009, the agency will send an 800kg rover to the planet to search for signs of past or present life.
But Mike Griffin said it was now time to re-balance the agency's science priorities. Among the new ventures announced in the latest budget was a major robotic mission to explore the outer planets.
He referred to a recent grading of Nasa's planetary exploration portfolio by the US National Research Council (NRC) which gave the space agency an "A" for its Mars programme, but also a "D" for outer planets and a "C" for research and analysis.
"We've rebalanced our planetary science portfolio accordingly," Dr Griffin told the conference.
"As I discussed elsewhere, we've learned more, and had more questions to answer, about the many other planets and moons in our Solar System.
"So after Mars Science Lab - the current planetary sciences flagship - we are now planning in earnest for an outer planets flagship to Europa, Titan or Ganymede."
During a question and answer session following his speech, Dr Griffin was challenged over the agency's commitment to the international Mars Sample Return mission in light of "massive" cuts to its Mars exploration programme.
Nasa, Esa and international partners have begun work on a strategy for this ambitious mission to return samples of Martian soil to Earth.
Neptune has yet to be visited by a dedicated mission
But the audience member said scientists in Europe now faced a challenge in selling the project to their ministers because a perception had been created that Nasa was no longer serious about Mars exploration.
Dr Griffin responded: "The Mars programme today is at the level it is to support the Mars Science Lab, which is the planetary sciences flagship. It cannot be an entitlement that the Mars programme gets a flagship and then retains, for all future time, a flagship level of funding."
He added: "Now, if as an entry-level requirement to get co-operation on international missions, I am required to keep the funding of any one community at a historically high level, then I can't meet that requirement."
But he reiterated Nasa's commitment to the mission, saying the agency's associate administrator for science, Dr Alan Stern, was dedicated to getting the long-mooted venture off the ground.
The administrator was also asked to address the effect of the agency's "oscillating" priorities on the next generation of young scientists and engineers building up expertise in their chosen field - professionals who would be contributing to the bold vision Dr Griffin said he wanted for Nasa.
The Maryland-born engineer and physicist replied that most scientists' careers were not in danger, but added: "The second comment I would make is: don't specialise. Specialisation is for insects. Don't specialise. It's not professionally smart."
Nasa plans to go back to the Moon with its Orion ship
He added: "I think a bold vision has to be flexible and adaptable."
Mike Griffin joined the space agency in the aftermath of the Columbia shuttle disaster, which killed seven space shuttle astronauts, and has presided over the implementation of President Bush's Vision for Space Exploration, which aims to return astronauts to the Moon by 2020.
Questioned about the biggest achievement and disappointment of his tenure, Dr Griffin replied: "I can't grade my own paper - I have a deep ethical aversion to self-assessment."
But he continued: "I would like people to say that I repopulated Nasa headquarters with people who were at the top of the business, rather than people whose first job in the space business was at the top."
On his biggest regret, Dr Griffin said: "I regret that I have been unable to make an appropriate case at policy level for having a smooth transition between the shuttle and its replacement systems. I yield to no one in my desire to see the shuttle retired by the end of 2010."
The space shuttle's successor, the Orion capsule, is due to make its maiden flight in about 2015. Dr Griffin said he had been unable to make the case to politicians that it should be brought online sooner.