My Science School

A mission conceived as one for preventive maintenance turned out to be more urgent after four of the six gyroscopes on board the Hubble space telescope failed.

The Hubble space telescope has changed our understanding of the universe A telescope that was launched into low Earth orbit in 1990, Hubble still remains operational and continues to be a vital research tool. Following NASA's most recent review of Hubble's operations, it has been announced that NASA would support the observatory through June 2026, with estimates suggesting that it might be able to continue operations until the mid-2030s and even beyond.

Designed to be visited

While the longevity of the telescope is testament to the vision of those who conceived the mission, there's another crucial factor that has made it possible. Hubble was the first telescope that was designed to be visited in space. This meant that astronauts could not only perform repairs and replace parts, but also upgrade its tech with newer instruments. There have been five such missions that have taken place from 1993 to 2009. One such servicing mission in December 1999 turned out to be a life saver for the telescope.

After the first servicing mission in 1993 and the second one in 1997, the third to carry out preventive repairs was scheduled for June 2000. Since Servicing Mission 2 in February 1997, however, three of the six gyroscopes aboard Hubble had failed. With at least three working gyroscopes necessary for Hubble's operation, it prompted the managers to split Servicing Mission 3 (SM3) into two parts, SM3A and SM3B, with the former scheduled for December 1999.

An unexpected failure

 On November 13, 1999, a fourth gyroscope failed unexpectedly. With SM3A planned for the following month, this triggered NASA to place Hubble into safe mode. The safe mode was a sort of protective hibernation that prevented the telescope from making any observation. Hubble was in this state for over a month, waiting for the crew of SM3A to make their way.

With servicing mission veterans Steven Smith and Michael Foale at the helm, the seven-member crew aboard the Discovery Space Shuttle (STS-103) left for space on December 19. Within a couple of days, they manoeuvred close enough to Hubble such that it could be grappled with Discovery's robotic arm.

All six gyroscopes replaced

 The first of the three scheduled spacewalks took place over 8 hours and 15 minutes spanning December 22-23 (Central European Time, CET). Smith, along with fellow crew member John Grunsfeld, managed to replace all six of Hubble's gyroscopes. The entire astronomical community heaved a collective sigh of relief on receiving this news. The duo also replaced kits to prevent Hubble's batteries from overcharging.

While Hubble's main computer was changed in the second spacewalk that spanned 8 hours 10 minutes through December 23-24 (CET), the final spacewalk spanning 8 hours 8 minutes through December 24-25 (CET) saw a faulty transmitter and data tape recorder being replaced. Preliminary tests were then conducted to ensure that all of Hubble's systems, be it old or new, were performing satisfactorily.

Backs away slowly

Minutes into December 26 (CET), the Hubble telescope was released. Discovery then backed away from Hubble slowly. Having successfully performed the major objectives of the mission, the astronauts on board used the time remaining to stow away equipment, apart from making preparations for landing.

After orbiting the Earth 119 times and travelling more than 5 million km, Discovery made its way back. It performed a smooth night-time landing, touching down on the runway at the Kennedy Space Center in the U.S. on December 27. Hubble was successfully back in operation, and has been for over two decades since then.

Picture Credit : google 

Discovered first on December 20, 1900, comet 21P/Giacobini-Zinner gets its name from two astronomers. From being one of the last comets to be discovered in the 19th Century, this comet is now best known for having the first encounter with a spacecraft.

Comets are popular for different reasons. There's Halley's comet, which is the most famous of them all. Regularly visible to the naked eye from the Earth, Halley's comet has been observed and recorded by astronomers for over 2,000 years. Then, there is comet Hyakutake. Discovered only in 1996, this comet's passage near the Earth in the same year was one of the closest cometary approaches in nearly 200 years. We will be turning our attention to comet 21P/Giacobini-Zinner, whose claim to fame now includes being the first comet to encounter a spacecraft. This comet was first discovered on December 20, 1900, making it one of the last comets to be discovered in the 19th Century. A discoverer of a number of comets, French astronomer Michel Giacobini found this comet while skygazing from Nice Observatory. It was followed for two months and orbital calculations revealed that the comet was a periodic object with an orbital period less than seven years.

Recovered in 1913

It wasn't recovered in 1907, when it was not placed favourably for viewing. Even though the comet was expected to be unfavourably placed in 1914 as well, German astronomer and renowned science historian Ernst Zinner accidentally rediscovered it on October 23, 1913.

Since both Giacobini and Zinner discovered and recovered this comet, it is named after them and is called comet 21P/Giacobini-Zinner. The letter "p" indicates that it is a periodic comet, which are comets with orbital periods less than 200 years. When orbital calculations were revised when the comet was recovered in 1913, its orbital period was found to be close to 6.6 years, and the comet has been observed on almost every return since then.

Draconid meteor shower

This comet had favourable returns in 1959, 1985, and 2018, when it was well observed as its perihelion (closest approach to sun) allowed it to pass close to the Earth. The nucleus of the Giacobini-Zinner sprays ice and rock into space every time it returns to the inner solar system. This makes the comet the parent comet of the Draconid meteor shower, which takes place in early October each year.

While this meteor shower is quite weak in most years, there have been Draconic meteor storms on record, meaning that over 1,000 meteors were seen per hour at the location of the observer. The 1933 and 1946 Draconid storms were particularly intense, with over 500 meteors observed per minute in Europe during the former and 50-100 per minute seen in the U.S. during the latter.

Farquhar's idea

Comet Giacobini-Zinner's current claim to fame was a result of its favourable return in 1985. When funding for a spacecraft mission to comet 1P/Halley, which was enroute to its 1986 perihelion passage, didn't materialise, planetary scientist Robert Farquhar came up with an idea. He suggested that the already existing International Sun-Earth Explorer 3 (ISEE-3) be placed on an alternate path that would take it towards Giacobini-Zinner.

Once the idea was approved, ISEE-3 was sent on a series of lunar flybys that would take it towards Giacobini-Zinner. Following the final lunar flyby in December 1983, ISEE-3 was renamed the International Cometary Explorer (ICE).

On September 11, 1985, ICE passed through the ion tail of Giacobini-Zinner, thereby completing the first encounter between a comet and a spacecraft. While ICE lacked cameras, it did carry scientific instruments that enabled it to record measurements of the electric environment around the comet and also as to how the comet interacted with the solar wind.

Even though an international fleet of spacecraft, including ICE, met Halley in 1986 from a number of vantage points for a study like never before, Giacobini-Zinner will forever hold the title of being the first comet to encounter a spacecraft. While its most recent return in 2018 might be comet 21P's most favourable return in the 21st Century, you can still look forward to its approach once in less than seven years, and maybe even try and track it.

Picture Credit : Google