PARIS — The world’s space agencies will need to agree to finance missions to remove five to 10 satellites and rocket upper stages per year from low Earth orbit to avoid the potential shutdown of space exploration in the coming decades, an international gathering of space debris experts concluded.

But even the agencies most cognizant of the dangers of space debris continue to launch satellites and rockets that will remain in Earth orbit after their missions end for far longer than the 25-year limit the agencies have promised to respect.

“The rule is there, but it is not so well respected,” said Luisa Innocenti, head of the European Space Agency’s (ESA) Cleanspace program, which is reviewing technologies to remove satellites from low Earth orbit at the end of their missions.

Addressing the 6th European Conference on Space Debris, held April 22-25 at ESA’s European Space Operations Centre in Darmstadt, Germany, Innocenti said owners of Earth observation, science and technology satellites in low Earth orbit are loath to use fuel that could extend the satellite’s mission to place it into an orbit that assures its atmospheric re-entry within 25 years.

“You tend to forget about the rule,” Innocenti said, explaining why even the best-intentioned agencies often violate the 25-year recommendation.

Almost all the major spacefaring nations are members of the Inter-Agency Space Debris Coordination Committee (IADC). Six of these agencies looked at how the debris fields in low Earth orbit — particularly the most-used regions of between 700 and 1,000 kilometers in altitude and in high-inclination polar orbits — will evolve in the next 200 years.

All six agencies came to the same conclusion: Even assuming that 90 percent of satellites and rockets follow the 25-year rule, the debris population will increase. Perhaps optimistically, the studies assumed no future in-orbit explosions in the interim.

Government officials said that today’s situation is far from the 90 percent post-mission disposal rate. About 80 percent of rocket upper stages currently follow the rule, but only 60 percent of the satellites are forced to lower orbit to re-enter within 25 years.

“It’s easier for a rocket builder to design enough fuel to deorbit the stage after the mission,” said Christophe Bonnal, a space debris expert at the French space agency, CNES. “For a satellite owner, the temptation of mission extension is very strong, and for larger satellites the owners would need to leave a lot of fuel and intelligence onboard to direct the spacecraft into a controlled re-entry over the Pacific Ocean. This is difficult.”

In addition to the 25-year rule, the IADC has recommended that if a satellite has a 1-in-10,000 chance of surviving atmospheric re-entry, its re-entry must be controlled so that it occurs over unpopulated areas.

Doing this requires lots more fuel and enough on-board intelligence to change the economics of the satellite’s construction, a prospect that is not welcome to mission planners already struggling to fit their hardware into the assigned budget.

Officials attending the conference agreed that more and more spacefaring nations are “passivating” their rockets and satellites once their missions have ended — draining batteries, emptying fuel tanks and otherwise making the hardware less likely to explode on impact with another piece of debris.

But the debris forecasts say this will not be enough and that if nothing dramatic is done, the risk of sending astronauts and expensive satellites into orbit may increase to a point where it is no longer feasible.

The only answer, the experts said, is active debris removal.

Removing five to 10 large objects per year from low Earth orbit would prevent these objects from colliding with other debris, creating thousands of more pieces of debris in a cascading effect that has long been predicted by experts. The total population of debris could thereby be stabilized.

Multiple technology options, none of them yet mature enough to be launched now, are being investigated. They include satellites equipped with harpoon-type devices to spear a large piece of debris and then guide it into atmospheric re-entry.

Grapple fixtures, nets, lasers used from the ground to slow debris orbits, quickening their re-entry — all are on the table. None has won a consensus among IADC members.

Heiner Klinkrad, head of ESA’s space debris office, said the conference delegates, representing 26 nations with substantial Russian and Chinese presence — because of U.S. budget issues NASA was not there, but NASA data was front and center — said there is an “urgent need to undertake active debris removal.”

As if it was not already complicated, Klinkrad said international law would make it difficult for one agency to remove debris “owned” by another agency without that agency’s permission. He said the simplest mission is one in which an agency attempts to remove its own garbage from low Earth orbit.

For ESA, he said, target No. 1 would be ESA’s Envisat satellite, which at 8 meters in length and weighing 8,200 kilograms is too big to be nudged into an uncontrolled re-entry.

Klinkrad said it is not known how much an active debris-removal mission would cost, but he said 200 million euros ($270 million) could be in the neighborhood.

Innocenti said her initiative’s budget totals about 15 million euros per year through 2016.

Peter B. de Selding was the Paris Bureau Chief for SpaceNews.