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Debris Mitigation Issues are Highlighted By the Impending Reentry of a European Satellite

Even as organizations strive to slow the creation of orbital debris, the approaching reentry of a defunct European satellite serves as a reminder of the risks posed by currently in-orbit items.

After spending over 20 years in space, the European Remote Sensing (ERS) 2 satellite from the European Space Agency is scheduled to reenter on February 21. The satellite is expected to return around 11:32 a.m. Eastern, according to the most recent ESA statement on February 20. There is a 4.61-hour margin of error.

The spacecraft, which weighed close to 2,300 kg, was launched in 1995 with a suite of equipment, including a synthetic aperture radar (SAR) mapper and a radar altimeter, to provide Earth scientific data. Although the spacecraft was shut down in 2011, it was not equipped with enough propellant to make a safe return.

At a briefing on February 13, Mirko Albani, the manager of missions and heritage space program for ESA’s Earth Observation program, stated that certain parts of ERS-2 will survive reentry. Together with a few interior panels, that comprises four gasoline tanks. The 52-kg SAR antenna is the largest single component that should survive reentry.

Although the ESA did not assess the dangers associated with this particular reentry, there is a very minimal chance that someone may be injured by falling debris. Albani went on to say that there are no radioactive or hazardous chemicals in any of the ERS-2 debris.

In order to avert an explosion that would have created debris, ESA passivated onboard equipment like the batteries when it chose to terminate ERS-2 operations in 2011. The spacecraft’s orbit was lowered from 785 kilometers to 573 kilometers using the remaining propellant. Tim Flohrer, head of ESA’s Space Debris Office, stated that although the spacecraft had limitations in its design that would have barred it from functioning below roughly 560 kilometers, it lacked the propellant to go lower.

The earlier debris mitigation criteria, which stipulate that satellites must deorbit within 25 years of their end of life, are adhered to by ERS-2. However, the ESA released a revised orbital debris mitigation guideline in November that shortens the post-mission disposal period from 25 to 5 years, among other things.

According to Francesca Letizia, an ESA space debris mitigation and reentry safety engineer, “it is part of ESA’s zero-debris vision.” In addition to the shortened deorbiting period, the policy mandates that spacecraft that are not classified as “low risk” be equipped with an interface that would enable them to be grappled by an active debris removal mission.

She stated that the new policy’s implementation is still in its transitional period. “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 rather doing so gradually until the end of the decade.

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

Additionally, spacecraft that are already in orbit are not entirely covered by the new regulation. “The applicability of these new rules is not 100%,”  Albani stated. To evaluate how the regulations should be applied to those older missions on an individual basis, ESA will set up a space debris mitigation assessment board. “For future missions, the target is to achieve zero-debris by 2030 onwards.”

The lessons learned from past failed missions also form the basis of the policy. Nearly identical to ERS-2, ERS-1 malfunctioned in orbit in 2000, leaving it stranded at a height of about 800 kilometers. According to Albani, ERS-1 will probably stay in orbit for a minimum of 100 years.

An even larger ESA Earth scientific project, Envisat, was rendered inoperable in 2011 by a similar on-orbit problem, placing it in a comparable orbit. It is anticipated that the eight-ton spacecraft will stay in orbit for at least a century. It is also commonly mentioned as one of the most dangerous space trash objects besides rocket bodies.

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

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