Former astronaut Neil Armstrong has issued a strongly worded rebuke of President Barack Obama, criticizing the president for proposed revisions to the U.S.' space program.

Armstrong, along with astronauts James Lovell and Eugene Cernan, called the proposal “devastating” in a letter obtained by NBC News. Read below for the full text:

"The United States entered into the challenge of space exploration under President Eisenhower’s first term, however, it was the Soviet Union who excelled in those early years," the letter begins."Under the bold vision of Presidents Kennedy, Johnson, and Nixon, and with the overwhelming approval of the American people, we rapidly closed the gap in the final third of the 20th century, and became the world leader in space exploration. ...

"When President Obama recently released his budget for NASA, he proposed a slight increase in total funding, substantial research and technology development, an extension of the International Space Station operation until 2020, long range planning for a new but undefined heavy lift rocket and significant funding for the development of commercial access to low earth orbit.

"Although some of these proposals have merit, the accompanying decision to cancel the Constellation program, its Ares 1 and Ares V rockets, and the Orion spacecraft, is devastating.

"America’s only path to low Earth orbit and the International Space Station will now be subject to an agreement with Russia to purchase space on their Soyuz (at a price of over 50 million dollars per seat with significant increases expected in the near future) until we have the capacity to provide transportation for ourselves. The availability of a commercial transport to orbit as envisioned in the President’s proposal cannot be predicted with any certainty, but is likely to take substantially longer and be more expensive than we would hope.

"It appears that we will have wasted our current ten plus billion dollar investment in Constellation and, equally importantly, we will have lost the many years required to recreate the equivalent of what we will have discarded.

For The United States, the leading space faring nation for nearly half a century, to be without carriage to low Earth orbit and with no human exploration capability to go beyond Earth orbit for an indeterminate time into the future, destines our nation to become one of second or even third rate stature. While the President's plan envisages humans traveling away from Earth and perhaps toward Mars at some time in the future, the lack of developed rockets and spacecraft will assure that ability will not be available for many years.

Without the skill and experience that actual spacecraft operation provides, the USA is far too likely to be on a long downhill slide to mediocrity. America must decide if it wishes to remain a leader in space. If it does, we should institute a program which will give us the very best chance of achieving that goal.

Neil Armstrong

Commander, Apollo 11

James Lovell

Commander, Apollo 13

Eugene Cernan

Commander, Apollo 17