The Trump administration's proposed $19.9 billion NASA budget for 2019 seeks to refocus the agency on exploration with the near-term goal of returning astronauts to the moon, aims to end federal support of the International Space Station in 2025 and kills off a high-priority space telescope known as WFIRST, the space agency reported Monday.

Nearly half of the proposed budget -- $10.5 billion -- is earmarked for "an innovative and sustainable campaign of exploration" leading to "the return of humans to the moon for long-term exploration and utilization followed by human missions to Mars and other destinations," according to a NASA overview.

"It really reflects the administration's confidence that America will lead the way back to the moon and take that next giant leap from where we made the first small step for humanity some 50 years ago," Robert Lightfoot, NASA's acting administrator, told agency workers in a televised address. "This is a pretty exciting time for us.

Get Breaking News Delivered to Your Inbox

"The budget focuses NASA on its core exploration mission, it reinforces the many ways we return value to the Amerian taxpayer through knowledge and discoveries, strenghtening our economy and security and deepening partnerships with other nations."

The budget supports continued development of NASA's Space Launch System, or SLS, megarocket and Orion deep space crew ship, which the agency had been hoping to launch on an initial unpiloted test flight by late 2019. The budget proposal shows that flight in 2020 with a second mission to send NASA astronauts around the moon in 2023.

The plan calls for establishing a Lunar Orbital Platform in orbit around the moon to serve as an so-called gateway and testbed for advanced technologies needed for deep space exploration and launching a power and propulsion "space tug" in 2022 as the LOP's first major component. Some $200 million will go to initial studies of small lunar landers with follow-on plans for larger vehicles.

"This will give us a strategic presence in the lunar vicinity, and it will drive our activity with commercial and international partners and help us further explore the moon and its resources and translate that experience toward human missions to Mars," Lightfoot said.

Robert Lightfoot, NASA's acting administrator, discusses the Trump administration's 2019 budget request with agency workers at the Marshall Space Flight Center in Huntsville, Ala. NASA

Closer to home, the International Space Station currently is approved for operations through 2024 and while many at NASA believe the lab could be operated at least through 2028, the Trump administration plans to stop direct federal funding in 2025.

Instead, the 2019 budget includes $150 million to begin exploring how to make the transition from a fully government-funded outpost in low-Earth orbit to one that is more tightly focused on commercial activity that would be funded in large part by the private sector.

The administration is holding open the possibility of using at least some components of the International Space Station in a follow-on outpost and says NASA would utilize whatever is available to carry out medical research needed before eventual flights to Mars and to develop the life support and other technologies such missions would require.

But the emphasis clearly is on helping private industry move into and utilize low-Earth orbit on a for-profit bases.

The $150 million included in the 2019 budget proposal, along with additional downstream funding, will enable "the development and maturation of commercial entities and capabilities which will ensure that commercial successors to the ISS -- potentially including elements of the ISS -- are operational when they are needed," according to an internal NASA overview.

"It is the intent of NASA and the administration to maintain seamless access to a human platform in LEO that meets NASA's and the nation's goals."

Sen. Bill Nelson, D-FL, a long-time space advocate who orbited Earth aboard a shuttle in 1986, called the budget proposal a "nonstarter."

"If we're ever going to get to Mars with humans on board and return them safely, then we need a larger funding increase for NASA," he said. "The proposal would also end support for the International Space Station in 2025 and make deep cuts to popular education and science programs.

"Turning off the lights and walking away from our sole outpost in space at a time when we're pushing the frontiers of exploration makes no sense," he said.

The budget proposal includes $2.1 billion for space transportation, including continued support for commercially procured space station cargo missions by SpaceX, Orbital ATK and, eventually, Sierra Nevada, along with development of new commercial crew ships being built by SpaceX and Boeing to ferry astronauts to and from the space station. Initial test flights are expected late this year or early next.

The 2019 budget supports continued development of the Europa Clipper, a "flagship" mission to study Jupiter's icy moon Europa and its subsurface ocean, two Mars landers, Insight and the Mars 2020 rover, and funding to study possible robotic missions to return soil and rock samples from the red planet.

NASA is developing the powerful Space Launch System rocket to launch astronauts, cargo and spacecraft beyond low-Earth orbit for near-term flights to the moon and eventual voyages to Mars. NASA

The $8.8 billion James Webb Space Telescope, an ambitious follow-on to Hubble that is scheduled for launch in 2019, remains a top NASA priority. But development of a follow-on to JWST, the Wide Field Infrared Survey Telescope, or WFIRST, will be canceled.

"Development of the WFIRST space telescope would have required a significant funding increase in 2019 and future years, with a total cost of more than $3 billion," according to an Office of Management and Budget list of "major savings and reforms." "Additionally, a recent independent review concluded that WFIRST was not executable within its previous budget.

"Given competing priorities at NASA, and budget constraints, developing another large space telescope immediately after completing the $8.8 billion James Webb Space Telescope is not a priority for the administration. The budget proposes to terminate WFIRST and redirect existing funds to other priorities of the science community, including completed astrophysics missions and research."

Princeton astrophysicist David Spergel tweeted that "abandoning WFIRST is abandoning US leadership in dark energy and exoplanets."

"What is driving the acceleration of the universe? What are the properties of exoplanet atmospheres? How did our galaxy and its neighbors form and evolve? What determines the architecture of exoplanets?" Spergel tweeted. "US should be leading the world in addressing these big questions."

Scheduled for launch in the mid 2020s, WFIRST was to be built around a large Hubble-class 7.9-foot-wide mirror originally developed for the National Reconnaissance Office and re-purposed to probe the nature of dark energy, the mysterious force causing the expansion of the universe to speed up, and to search for and directly photograph planets orbiting other stars.

The National Research Council's decadal survey committee recommended WFIRST as the top priority for the next decade in astronomy and NASA formally made it an approved mission in February 2016.

Astrophysicist Scott Trager called the decision to cancel WFIRST "a disaster for US and world astronomy."

In the Earth science arena, the Trump administration proposes $1.8 billion in spending, but continues its push to eliminate five climate science missions or instruments.

"The missions proposed for termination are lower-priority science missions that cannot be accommodated under constrained budgets," the OMB says. "The proposed termination of these five missions realigns the NASA Earth science portfolio to focus on the highest-priority missions for the science and applications communities within a balanced, comprehensive Earth science program."

The Trump administration wants NASA to develop a more commercially focused follow-on to the International Space Station, proposing to stop government funding of the outpost in 2025. NASA

The Trump administration also followed through on its vow to eliminate NASA's education office, saying the agency would "continue to support other education activities, such as fellowships and the Science Activation Program within the Science Mission Directorate that are funded outside the Office of Education."

Some $2.2 billion is earmarked for planetary science, including creation of a Lunar Discovery and Exploration program that will support increased public-private partnerships.

Along with supporting continued development of the Europa Clipper and Mars 2020 programs, the budget calls for establishment of a Planetary Defense program, including an asteroid redirection test, studies of a potential Mars sample return mission and development of two asteroid probes known as Lucy and Psyche.

In the realm of astrophysics, the budget supports a spacecraft known as TESS that will search for exoplanets that is scheduled for launch later this year and the James Webb Space Telescope, scheduled for launch in 2019. All astrophysics missions operating beyond their originally projected prime missions will be subject to review.

The Trump administration wants to make its "Exploration Campaign" NASA's central focus, providing $4.6 billion for deep space exploration systems, $1 billion for exploration research and technology and $4.6 billion for spaceflight operations in low-Earth orbit. Those totals would stay roughly the same through 2023.

The Exploration Campaign prioritizes human exploration, provides the funding to begin the transition to a more commercial outpost in low-Earth orbit and establishes a cislunar strategy to enable "U.S. preeminence to, around, and on the moon, including commercial partnerships and innovative approaches to achieve human and science exploration goals."