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Hubble's orbit is decreasing, and in about a decade it will re-enter Earth's atmosphere and burn up.

The Hubble Space Telescope must rank as one of the most impressive and successful projects ever undertaken by mankind.

Since its launch in 1990, it has made several major discoveries, including discovering Pluto's moons Nix and Hydra; watching comet Shoemaker-Levi 9 impact Jupiter; calculating the age of the universe; probing exoplanet atmospheres; locating dark matter; and even figuring out how galaxies evolved in the first place.

Unfortunately, although the 32-year-old satellite is still fully functional, its lifespan will soon be shortened. Hubble's orbit is decreasing, and in about a decade it will re-enter Earth's atmosphere and burn up.

SpaceX, however, recently proposed to NASA a solution to save Hubble. But how? Will this plan really work?

Hubble is falling.

Hubble never had any ability to move its orbit, which degraded rather rapidly.

Hubble originally operated 600 kilometers above the Earth's surface, hence the name Space Telescope. Normally, we think of the 100 km altitude as the boundary between the atmosphere and space, the so-called Karman line.

However, this number is only artificially defined. The atmosphere is indeed quite thin at this altitude, but it is by no means zero. Even at Hubble's altitude, there is still an extremely thin atmosphere that provides resistance to Hubble.

If you remember circular motion in high school physics, a decrease in linear velocity is accompanied by a decrease in orbit. In the case of our space station, for example, at an altitude of 400 kilometers, it is also necessary to raise its orbit on time to prevent it from falling into the atmosphere. Hubble, however, has no such function. It has no onboard thrusters, which causes it to "drift" and gradually decay its orbit over time. It is currently about 540 kilometers above the surface.

To solve this problem, Hubble was designed so that the space shuttle could dock with it. This way, the crew can perform routine maintenance and upgrade necessary components, and the shuttle can use its thrusters to assist in lifting Hubble back to its original altitude.

But the last of these space maintenances occurred in 2009, shortly after the shuttle was retired in 2011 due to repeated accidents. Since then, no spacecraft has been able to service Hubble or restore its orbit, and Hubble has been left to decay its orbit.

Obstacles to Hubble's orbit recovery

So how would Musk accomplish such a feat? The answer is that he plans to use a modified version of the manned dragon spacecraft.

Crewed Dragon, which can ferry people and cargo to the International Space Station, is currently the only way to get there from the mainland. It replaced the nose cone of the Falcon 9 rocket and served as the final stage of the rocket.

This means that the manned dragon spacecraft can dock with satellites and have its own thrusters. SpaceX therefore proposes to use the Crewed Dragon spacecraft to dock with Hubble and then use its thrusters to propel its orbit, just as the space shuttle did years ago.

You might think that the space shuttle was used to dock the international space station with the hubble telescope, but now the manned dragon spacecraft can replace the space shuttle and dock with the space station, so docking with hubble is naturally easy.

This is not the case, however, because the docking mechanism is different. The ISS's closed-channel docking mechanism is shared between the shuttle and the manned Dragon spacecraft, but when docking with Hubble, the shuttle actually uses a huge robotic arm to grasp Hubble instead of docking with the ISS. That means astronauts will need to take a spacewalk to use the telescope.

The manned dragon spacecraft has neither a robotic arm nor the ability to allow astronauts to walk in space. That means Musk will have to modify the capsule to have a similar robotic arm to grab the Hubble telescope. Such modifications are feasible, but would take up almost all of the interior space, leaving little room for astronauts.

While Dragon could be modified to have the articulated robotic arms needed for docking, the spacecraft still lacked the space needed for the airlock for astronauts to perform spacewalks.

So that's the biggest challenge SpaceX faces in restoring Hubble's orbit, and once they succeed, they can extend Hubble's life by another 15 to 20 years. Meanwhile, sensor and computing technology has come a long way since 2011, and several upgrades will make Hubble even more powerful.

Why extend Hubble's lifespan?

NASA has not yet approved the project, but has agreed to conduct a feasibility study for the mission, which will take about six months. At this stage, NASA will not have to spend money, but if it does perform maintenance missions in the future, it may have to pay.

After all, Hubble is still doing a lot of excellent science and remains one of humanity's most powerful telescopes. You might think that the James Webb Space Telescope (JWST) has replaced Hubble because it is so much more powerful. But in reality, the spectra of the two telescope images are different. JWST images are infrared images, while Hubble images are invisible near-infrared images. This means that JWST cannot directly replace Hubble. So it makes sense to keep Hubble moving forward.

However, in 2026 SpaceX will launch a new NASA space telescope called Nancy Grace Roman, which could directly replace Hubble. It has the same 0.1 arc-second resolution as Hubble (each pixel fills 0.1 arc-second of the sky) and has images of the same visible and near-infrared spectra, but with a field of view 100 times larger! This means that Rome can perform exactly the same task as Hubble, but much faster because Hubble needs to take 100 images to match one Rome image.

With such an opportunity to replace Hubble outright, why would NASA still consider spending money on saving Hubble?

Because the cost of the roman telescope is a staggering 3.2 billion dollars! Falcon 9 costs just $67 million to launch, and retrofitting the manned Dragon spacecraft could cost just a few million dollars. This means that Hubble's images cost significantly less than Rome's.

Let's assume the SpaceX Hubble mission costs $100 million (although it could be less). Hubble currently has a 290 terabyte data archive and has taken more than 1 million images since 1990, each up to 290 KB in size. Hubble currently produces 10 terabytes of data a year, equivalent to 34,483 images. Assuming the SpaceX mission keeps Hubble alive 15 years longer, that's an extra 517,245 images. That would cost $193.33 per image.

If the rome telescope could take as many pictures a year as hubble, it would have the same condensing power because they were about the same size. Roman telescopes have a life expectancy of only five years. This means that the telescope was able to take about 172,415 images. But these images are 100 times larger than Hubble images, so to make them comparable to Hubble images, let's multiply them by 100. This means that the Rome telescope will take the equivalent of 17,241,500 Hubble images in total, at a cost of $3.2 billion. That's an average cost per image of $185.60.

This is a very rough calculation. It doesn't consider image stacking or data quality and is based on some very loose assumptions. But that explains why NASA is interested in extending the Hubble telescope's life, even though it will soon have a replacement. This is simple economics.

But Roman telescopes are also designed for extended service missions. Because it is so much more powerful, such a mission would offer a lower cost per image than SpaceX Hubble. NASA may therefore also view SpaceX's proposal as a test run to see if SpaceX can also extend the life of the Roman telescope. Extending the life of Roman telescopes is a riskier task. Since telescopes are more expensive and younger, the stakes are much higher, so it makes sense to give SpaceX something to try first.

So, that's how SpaceX could save Hubble, and why NASA might accept their offer. It is amazing that a telescope with the technology of the 1980s can still be an efficient and excellent instrument for cutting-edge science.

Personally, I still hope NASA accepts SpaceX's offer and that Hubble will continue to exist for many more years. Somehow, space exploration would not be the same without it. In any case, it is still amazing to see how bright the future of astronomy is.

This article comes from Weixin Official Accounts: New Research (ID: chuxinyanjiu), Author: Will Lockett, Source: Medium, Compiled by: Liu Tangshi

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