Space and Physics
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The Hubble Space Telescope (HST) has been pivotal to progress in the last three decades of astronomy and cosmology. Traveling around the Earth in low orbit at 28,163 kilometers per hour (17,500 miles per hour), the mirror-based optical telescope has made over 1.3 million observations since its launch in 1990.
The telescope, with its precise observations of distant Type Ia supernovae and cepheid variable stars, has been pivotal in pinning down the expansion rate of the universe and the discovery of dark energy. Before the telescope was launched, we didn't know how old the universe was, with estimates ranging from 10 to 20 billion years. The telescope's observations of cepheid variables helped us refine this to our current estimate of 13.8 billion years old. In between the groundbreaking discoveries, it also provided us with some of the most spectacular images of the cosmos humanity has ever taken.
But all good things must come to an end. Hubble was not designed to orbit the planet forever, and atmospheric drag as the telescope travels through the sparse, outer-regions of our atmosphere (fun fact: the Earth's atmosphere extends beyond the Moon) brings it ever closer to the time when it can no longer continue to orbit, and crash back down to Earth.
A new study has taken a look at when and how the telescope will end its remarkable life, and the risk it may pose to the planet it will smash into.
During its long mission, the telescope has been pushed into higher orbit by astronauts sent to service it.
"Once the astronauts completed all servicing tasks via a three- to five-day series of spacewalks, the STOCC controllers and Johnson Mission Control prepared the telescope for release," NASA explains. "Often this also involved using the shuttle’s thrusters to carry Hubble into a slightly higher orbit, a step that prolonged Hubble's life by keeping it from naturally deorbiting due to atmospheric drag."
This particular telescope-saving scheme is no longer an option, and the original plan for retrieving the space telescope has gone too.
"The original deorbit plan for HST was to use the space shuttle to retrieve it at [the] end of its life," a paper on the prospect of HST deorbit explains, "however, no one foresaw that it would outlive the Space Shuttle Program."
Now, the space telescope's orbit is expected to slowly decay in the coming years, before it has an uncontrolled reentry.
"Accurately predicting when this reentry will occur and assessing potential risks to the public are critical concerns for NASA. This assessment evaluated and independently checked HST decay prediction methodologies, analyzed potential hazards upon reentry, and provided recommendations for managing associated risks," the authors of a new orbit decay study explain, adding that solar activity can affect satellite orbits too.
"Additionally, as HST was not designed for controlled reentry, there are concerns regarding surviving debris reaching Earth’s surface and the associated casualty risks."
The study found that in the best-case scenario, the telescope could remain in orbit until 2040, but in the worst-case scenario could enter as early as 2029. The most likely scenario took place 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 orbit decay study explains.
"While the exact location of reentry and footprint were not estimated, the probability of casualty ranges from and 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 from the two simulations conducted."
Though the risks are still low and relatively distant, the team points out that they are technically unacceptable by NASA standards.
"For an orbital decay reentry in the year 2033 with a 28.5° inclination, the casualty risk is 1:330. This is significantly higher than the NASA-STD-8719.14C requirement that [the risk] for the public be less than or equal to 1:10,000," they explain.
"The worst-case scenario 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 that further studies should be undertaken using more precise solar cycle predictions as well geomagnetic conditions, and that a population risk analysis should also be conducted, using projected populations in at risk areas, ahead of the telescope's eventual return to Earth. Even a telescope as iconic as Hubble would be tainted somewhat if its last act was to splat civilians on its return.
The paper is posted to NASA's Technical Reports Server.
