The Hubble Space Telescope and a mission to explore Jupiter's moons look to be the biggest casualties in Nasa's 2006 budget plans outlined on Monday.
Nasa would send a mission to bring Hubble down safely
Under the proposals, a mission to service Hubble would be scrapped and the observatory brought back to Earth.
Nasa's total budget would rise 2.4% over 2005 to about $16.5bn (£9bn), but only $93m would be spent on Hubble.
About $75m (£40m) of that would be used to develop a robot mission to steer it into the ocean at the end of its life.
The US space agency (Nasa) has fared better than many government agencies in President George Bush's 2006 budget request.
But the White House is not seeking as much money for the space agency as had previously been planned - and that is bad news for Hubble.
Chance of reprieve?
Nasa comptroller Steve Isakowitz said Nasa planned a robotic mission to prepare the satellite for decommissioning and a safe descent into the ocean.
"Hubble is a spacecraft that is dying," he said at a briefing in advance of the budget's release.
"We have decided that the risks associated with the Hubble servicing at this time don't merit going forward."
This will infuriate Hubble supporters who believe the telescope still has good years of science observation ahead of it - provided it is serviced.
Sean O'Keefe answered questions during a briefing at Nasa HQ
They will hope Congress, which has to approve the budget, will insist on money being found to save the orbiting observatory.
"There is a long history of Congress putting into the Nasa budget what Nasa or the executive branch don't like and have taken out," said Rodger Thompson, a University of Arizona astronomer and principal investigator of the Near-Infrared Camera and Multi-Object Spectrometer (Nicmos), which astronauts placed on Hubble in 1997.
"The Gravity Probe-B was cancelled many times and each time was put back in the budget by Congress.
"There is going to be a large debate about this and there is a significant chance that funding for Hubble will be returned to the budget," he told the BBC News website.
In December, a panel of experts concluded that astronauts would do the best job of servicing Hubble and rejected proposals for a robotic rescue mission.
Outgoing Nasa administrator, Sean O'Keefe, said the decision over Hubble had been influenced by this assessment and the commitment to meeting the safety recommendations of the Columbia Accident Investigation Board.
"Even if we could do [a robotic mission], we probably could not deploy that capability in enough time," administrator O'Keefe told reporters at Nasa HQ in Washington DC.
Jimo is part of the ambitious Prometheus propulsion technologies programme
"It is difficult to see how any manned mission to service Hubble would meet the recommendations of the Columbia accident investigation board."
He reaffirmed that the space agency continued to be guided by President Bush's vision for space exploration, announced in January 2004.
This has included a major shift in emphasis towards human exploration, with the intention of returning astronauts to the Moon and, possibly, taking them on to Mars.
This comprises the development of a reusable manned spacecraft called the Crew Exploration Vehicle (CEV) as a replacement for the shuttle.
The multi-billion dollar Jupiter Icy Moons Orbiter (Jimo) mission was to have been launched in about 2015 as a demonstration for the Project Prometheus nuclear power and propulsion initiative.
It would have gone into orbit around the giant planet and its moons, possibly putting landers on their surfaces in much the same way as Cassini has done with Huygens on Titan.
Nasa officials now say Jimo is too ambitious an undertaking for an initial demonstration project, and a search for an alternative mission is underway.
"These big missions always have ups and downs," commented Professor Fred Taylor, from Oxford University, UK, and a scientist on the Galileo mission to Jupiter in the 1990s.
"At this stage it was always just a study - and when approved missions get cancelled, then one should really get upset.
"If the alternative is a cheaper mission that would go more quickly, we might get more science faster. If the current study is uncovering a serious viability problem then we might be better off backing off and looking for other solutions," he told BBC News.