The iconic Hubble Space Telescope (HST) has been in orbit for over 35 years now. The space observatory has been pivotal in reshaping our understanding of the cosmos, having made over 1.7 million observations since its launch in 1990.
Hubble has, however, faced several close calls in recent years, as the observatory’s hardware ages in the harsh environment of space.
Ultimately, the effects of atmospheric drag will bring the observatory down to Earth. A new study, published in the NASA Technical Reports Server (NTRS), shows Hubble could reenter Earth’s atmosphere and crash down into the ocean sooner than expected.
Predicting Hubble’s descent to Earth
The Hubble Space Telescope was lifted to a higher orbit several times by Space Shuttle missions between 1993 and 2009. More recently, Elon Musk claimed that a SpaceX mission could lift the iconic telescope to a higher orbit using the company’s Crew Dragon capsule. That lifespan-extending mission appears to be off the table now.
The original plan for Hubble was that it would be retrieved or deorbited by a Space Shuttle mission at the end of its lifespan. However, as an IFL Science report points out, operators failed to foresee Hubble outliving the Space Shuttle Program.
With no immediate plans to lift Hubble, the space telescope’s orbit is expected to slowly decay over the coming years. Ultimately, it will reenter Earth’s atmosphere and perform an uncontrolled reentry. It will most likely crash into the ocean, though there is a chance it could pose a risk to property and to people on the ground.
“Accurately predicting when this reentry will occur and assessing potential risks to the public are critical concerns for NASA,” the NTRS study explained. “This assessment evaluated and independently checked HST decay prediction methodologies, analyzed potential hazards upon reentry, and provided recommendations for managing associated risks.”
Hubble was ‘not designed for controlled reentry’
Importantly, the study highlights the fact that Hubble was originally designed for retrieval. This means that its uncontrolled deorbit could have dangerous consequences.
“As HST was not designed for controlled reentry, there are concerns regarding surviving debris reaching Earth’s surface and the associated casualty risks.”
According to the study, the best-case scenario would see the telescope remain in orbit until 2040. However, the worst-case scenario would see it reenter Earth’s atmosphere as early as 2029. The most likely outcome is that it will reenter in 2033.
“Results of this study conclude that for the nominal predicted solar activity, and average projected surface area, HST is predicted to reenter Earth’s sensible atmosphere in 2033, with a debris footprint that ranges approximately 350 kilometers (km) to 800 km [217 to 497 miles] along the ground track,” the study’s authors wrote.
Is there a risk to human life?
The researchers didn’t predict the reentry location and footprint, though they did use simulations to assess the probability of casualty. This ranges from an average overall risk of 1:330 over the entire inclination region HST crosses to 1:31,000 over the most remotely inhabited region of the South Pacific Ocean.
These risks are low, though, as the research team pointed out in the study, they do not fall within NASA’s acceptable range when it comes to reentry risk factors. The NASA-STD-8719.14C requires that the risk to public be “less than or equal to 1:10,000,” they wrote.
“The worst-case scenario,” they explained, “would occur if the HST debris impacted in Macao, where between two and four casualties would be expected due to its high population density. At least one casualty would also be expected if it impacted in Hong Kong or Singapore.”
The team recommends further studies to determine the risk factor as Hubble nears its reentry date. Other factors should also be considered, such as the impact of geomagnetic storms on the telescope’s orbital decay. Hubble completely altered the scientific landscape, so it would be disastrous for it to be remembered as the telescope that crashed over a populated area.