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James Webb Space Telescope, A Long Awaited Launch

Set for launch tomorrow at 7:20am EST (12:20 GMT) is NASA’s long-awaited James Webb Space Telescope (JWST). A telescope which began its development in 1996 & after 25 years of development, it will enable scientists to peer further back in the universe’s history than ever before.

But the James Webb Space Telescope is only the next in line to a long history of space telescopes.

When someone mentions space telescopes, the one that most often comes to mind is likely NASA’s Hubble Space Telescope, the largest space telescope launched from the Space Shuttle Discovery in 1990 during STS-31, however, Hubble was not the first space telescope launched.

The Soviet Union launched 4 satellites from 1965 to 1968. The satellites were Proton 1, 2, 3 & 4 (sharing the same name as the rocket that launched them). These satellites measured cosmic radiation levels but did not yet contain traditional image-taking apertures as we’re accustomed to seeing.

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In 1989, the European Space Agency launched the Hipparcos satellite, the first space telescope which recorded views of stars in the visible wavelength of light humans can see. This telescope, however, did not capture images. Rather it was used to record the proper motions of stars along with parallaxes of those stars.

The 1990 launch of the Hubble Space Telescope introduced the clearest views of deep space yet. Enabling astronomers to see the universe with incredibly clear detail as the telescope would not be impeded by the dense atmosphere of Earth below it. After it was launched, Hubble spent 3 years unable to complete its primary scientific mission due to a flaw in its primary mirror. A flaw that was fixed in 1993 when the STS-61 crew undertook 5 extravehicular activities to replace & upgrade many of Hubble’s systems & components.

James Webb Compared To Hubble

Given the popularity of the Hubble Space Telescope, a telescope so popular its name outshines many others, it is quite common for people to be curious about the two vehicles, especially how they stack up against one another.

Firstly, Hubble masses in at a hefty 24,490lb (11,110kg) whereas the James Webb Space Telescope weighs in at a more modest 14,300lb (6,500kg). Of note, is the optical mirrors that will support both spacecrafts. Hubble’s measure’s in at 7.87ft (2.4m) compared to the approximately 21.3ft (6.5m) segmented hexagonal mirrors that the James Webb Space Telescope will use.

credit: NASA

Inside of its fairing, the James Webb Space Telescope measures just 14.67ft (4.472m) in diameter, but once its sun shield deploys, it will expand to become 69.5ft long (21.1m) & 46.5ft wide (14.1m). In comparison, Hubble is 43.5ft (13.2m) long with a diameter of 14ft (4.2m).

Future For James Webb Space Telescope & Hubble

James Webb Space Telescope’s Limited Lifespan

Given its 25 years spent being designed & developed, one might hope that the operational lifespan of this vehicle would be able to operate long enough to make the two and a half decades of work worthwhile. Especially since the Hubble Space Telescope has been in operation for more than 30 years, one might hope that the James Webb Space Telescope would have an equally long life, but that is not the case.

Unlike the Hubble Space Telescope, the James Webb Space Telescope has a hard limit on how long it can operate as it will exist in a special point in space where gravity between the Earth & Sun are equal, known as a Lagrange Point, specifically L2, beyond the orbit of the moon. This point in space however is not stable & James Webb Space Telescope will eventually drift from this location.

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credit: NASA
credit: NASA

The James Webb Space Telescope does have its own thrusters to allow it to keep its position around L2, but with a ∆v budget of 150m/s, this fuel supply will eventually run out leading to the telescope drifting from its position.

The choice to put it in a halo orbit around the L2 point was to ensure that the Sun, Earth & moon all remained on the same side, as well as to prevent the shadows of the Earth & Moon from crossing the spacecraft. It is important that the James Webb Space Telescope maintain a constant temperature so that the telescope’s mirror segments remain in constant alignment.

The massive solar shield will help protect the optical equipment from more minor temperature changes that occur from the sun however more drastic changes, such as the shadow of the moon crossing over the telescope which could cause the mirror segments to shift out of alignment.

These issues give James Webb Space Telescope a nominal mission duration of 10 years. It may be possible that NASA may devise ways to utilize the telescope once its fuel depletes but given its precise location in space, repair or rescue missions are not possible. So once its fuel depletes, or a critical issue arises, NASA can only use tools on the spacecraft itself to solve any problems. Flaws as we saw with the Hubble Space Telescope in 1990 cannot be fixed once the James Webb Space Telescope has been launched.

Hubble Demise

Despite the Hubble Space Telescope not being limited by fuel supplies like the James Webb Space Telescope, Hubble too has a limit on its lifespan. Without another boost to its orbit, the Hubble Space Telescope could fall from orbit somewhere between 2030 & 2040, anywhere from 9 to 19 years from now. Exactly when depends on what’s bringing it back to Earth; namely atmospheric drag.

Most people assume that the “space” we see astronauts performing routine spacewalks on the International Space Station is in a complete vacuum, but that’s not true. Earth’s atmosphere stretches out thousands of miles. The altitude that the International Space Station operates in is called the thermosphere. The atmosphere at this altitude is around 1 trillionth of a kilopascal (0.000000000001). By comparison, at sea level, we experience approximately 101 kilopascals. Despite being thin, there’s still enough drag that it is gradually slowing the Hubble Space Telescope. Without a boost, its orbital will drop into the Earth’s atmosphere where it will burn up.

The good news is, during Hubble’s final servicing mission (STS-125 in 2009), a Soft Capture Mechanism was installed. Meaning a future craft could dock to the telescope & give it the needed boost to keep operating.

The bad news is that even if we kept boosting Hubble into space, its computers & gyros are on a gradual decline as hardware failures mount over the last 3 years. With a 9 day pause in January of 2019, another in June of 2020 & and a recent fault which brought Hubble’s primary science operations to a halt starting in October, but was recently solved on December 7th.

Another servicing mission could potentially be mounted using new commercial spacecraft like Sierra Nevada’s Dreamchaser, SpaceX’s Crew Dragon, or even NASA’s own Orion spacecraft in place of the Space Shuttle, however, no such plan has currently been fully developed.

James Webb Space Telescope’s Launch

If all goes well, we will see the long-awaited James Webb Space Telescope lift off atop its European Ariane V rocket. After 140 seconds, the two P241 solid rocket boosters will fall away, leaving the liquid hydrogen & liquid oxygen core stage to continue burning for another 465 seconds before MECO, or Main Engine Cut Off. After which, the ECA upper stage powered by a similarly fueled HM7B will also ignite to ferry the James Webb Space Telescope on its voyage amongst the proverbial stars. A voyage that will take 30 days for the telescope to reach its position at L2, and another 6 months before it will begin full science operations.

credit: NASA

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