WASHINGTON — The impending reentry of a defunct European satellite serves as a reminder of the hazards posed by existing objects even as agencies work to mitigate the growth of orbital debris.

The European Space Agency’s European Remote Sensing (ERS) 2 satellite is expected to reenter on Feb. 21 after nearly two decades in orbit. The latest update by ESA Feb. 20 predicted the satellite will reenter at 11:32 a.m. Eastern, with a margin of error of plus or minus 4.61 hours.

The spacecraft, weighing nearly 2,300 kilograms, launched in 1995 to provide Earth science data with a set of instruments that included a radar altimeter and synthetic aperture radar (SAR) mapper. The spacecraft was shut down in 2011 but lacked the propellant to perform a controlled reentry.

Some components of ERS-2 will survive reentry, said Mirko Albani, heritage space program and missions manager for ESA’s Earth Observation program, at a Feb. 13 briefing. That includes four fuel tanks and some internal panels. The largest single component expected to survive reentry is the SAR antenna, weighing 52 kilograms.

The risk of falling debris injuring someone is extremely low, although ESA did not quantify the risks from this specific reentry. Albani added that none of the debris from ERS-2 contains toxic or radioactive materials.

When ESA decided to end ERS-2 operations in 2011, it used the remaining propellant to lower the spacecraft’s orbit from 785 to 573 kilometers, and then passivated onboard systems like batteries to prevent a debris-generating explosion. However, the spacecraft lacked the propellant to go lower, and constraints in the spacecraft’s design would have prevented it from operating below about 560 kilometers, said Tim Flohrer, director of ESA’s Space Debris Office.

ERS-2 complies with earlier debris mitigation guidelines, which call for deorbiting satellites within 25 years of the end of their lives. However, ESA published a new orbital debris mitigation policy in November that, among other measures, reduces the post-mission disposal timeframe from 25 to 5 years.

“It is part of ESA’s zero-debris vision,” said Francesca Letizia, ESA space debris mitigation and reentry safety engineer. Besides the reduced deorbiting timeline, the policy requires spacecraft not deemed to be “low risk” to be prepared for removal through the addition of an interface that would allow an active debris removal mission to grapple it.

The implementation of the new policy is still in a transition phase, she said. “We don’t expect that we will fully implement all the measures that are requested for zero-debris for the missions that are launched now,” but instead progressively implemented through the end of the decade.

“It is a journey that we have just started now, and we will see how this works,” she added.

The new policy also doesn’t apply fully to spacecraft already in orbit. “The applicability of these new rules is not 100%,” said Albani. ESA will establish a space debris mitigation assessment board to review on a case-by-case basis how the rules should apply to those older missions. “For future missions, the target is to achieve zero-debris by 2030 onwards.”

The policy is also based on the experience of other failed missions. ERS-1, a spacecraft nearly identical to ERS-2, malfunctioned in orbit in 2000, stranding it at an altitude of nearly 800 kilometers. Albani said ERS-1 will likely remain in orbit for at least 100 years.

A similar on-orbit malfunction crippled an even larger ESA Earth science mission, Envisat, in 2011, leaving it in a similar orbit. The eight-ton satellite, frequently cited as one of the most hazardous space debris objects other than rocket bodies, is also expected to remain in orbit for a century or more.

“ERS was designed in the ’90s, and we are making much more progress today,” Flohrer said. “But the technology was not available when ERS was designed.”

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...