Hubble Space Telescope: A Window to the Universe

Hubble Space Telescope

Hubble Space Telescope (HST) was the first in the family of great observatories to witness the glamour of the universe. It has been orbiting around the Earth at a distance of about 540 km since its launch and deployment by the space shuttle Discovery (STS – 31) on 24th – 25th April 1990. “A telescope in space” was an innovative concept back in the era. It was initiated to overcome the hindrance caused for ground-based telescopes by Earth’s atmosphere. That concept eventually became a reality as the Hubble Space Telescope, named in honor of legendary astronomer Edwin P. Hubble. Originally, the objectives of HST were to investigate the celestial objects, stellar and galactic formation, and evolution of the universe. Scientists expected it to confirm the universality of physics laws and contribute to optical astronomy persistently for 15 years.

Hubble Space Telescope
Figure 01: Hubble Space Telescope

The design of HST is in favor of on-orbit servicing. Four servicing missions in five visits have been completed to this date, ensuring a smooth and extended life. HST is also capable of partnering up with other space or ground observatories to accomplish better detection and capturing. One such instance was when the Hubble studied Whirlpool Galaxy (a spiral galaxy) together with the Spitzer Space Telescope and Chandra X-ray Observatory to compare different views of visible, infrared, and x-ray observations.

Hubble Space Telescope - Mission 4
Figure 02: In May 2009 astronauts perform work on the Hubble Space Telescope, locked down in the cargo bay of the Earth-orbiting Space Shuttle Atlantis. Servicing Mission 4 was one of the most active and intense of the Hubble missions.

Science of Hubble Space Telescope

Hubble is a Ritchie-Chrétien Cassegrain reflector telescope. When light from a celestial body arrives through the tube, HST collects it using an inwardly curved primary mirror allowing the light to reflect towards an outwardly curved secondary mirror. This secondary mirror bounces back the beam to the primary mirror by a hole in its center. Various scientific instruments placed in HST finally pick up this light focused on a focal plane.

Technicality of Hubble Space Telescope

Figure 03: Hubble’s 1,825 pound, 7.8-foot (2.4-meter) diameter primary mirror collects light from its astronomical target and reflects it to a 12-inch (0.3-meter) diameter secondary mirror located in the optical tube. This secondary mirror then reflects the light.

Hubble Space Telescope is sensitive to a significantly broad region of wavelengths of the electromagnetic spectrum: from ultraviolet (UV) through visible to infrared (IR) (115 -1700 nm). Celestial bodies emit energy that discloses divergent facts concerning the universe.  There are plenty of stars that emit visible light. However, as the stars get hotter they tend to release rays that belong to lower wavelengths such as UV. Thus, these high energy emitting hotter stars are blue whereas the cooler stars are red. Hence, IR is capable of detecting the cooler stars. Furthermore, the atmospheres of planets and stars comprised of gases and dust can be studied using IR.

Features of HST

HST uses three out of the six high-precision gyroscopes it has at the typical operating mode. Gyros detect the rate and the direction of motion while the reaction wheels reorient the observatory. Moreover, HST owns Fine Guidance Sensors (FGS) which are interferometers, to remain motionless while observing. Targeting cameras lock onto guide stars and maintain the right direction for the Hubble.

Two solar panels store electricity in gallium-arsenide photovoltaic cells for all the instruments of HST to work simultaneously even under the shadowed conditions. Yet, Hubble has an aperture door to prevent sunlight from damaging the telescope.

Hubble Space Telescope communicates with its control center using two high-gain antennas (HGAs) that communicate through NASA’s Tracking Data and Relay Satellite System in geosynchronous orbit.

Figure 04: This diagram shows the locations of Hubble’s instruments inside the telescope. The instruments are located in containers that make them easy to remove and replace. Credit: NASA, ESA

Cutting-edge Instruments On Board

HST is a combination of a variety of instruments that are of different technicalities. Each design observes the universe uniquely.

There are 2 main varieties of instruments in HST:

01. Cameras

Capture images of the universe. None of the cameras is a natural color camera. These digital Charge-Coupled device cameras use color filters to isolate different colors in images captured as greyscale pixels.

Wide Field Camera 3 (WFC3)

Known as the workhorse camera of HST. WFC3 is sensitive to UV, visible, IR wavelengths of light. Hence, this complements the ACS by capturing UV and IR wavelengths of images. WFC3 snaps an extensive area from nearby stellar nurseries to distant galactic objects.

Advanced Camera for Surveys (ACS)

ACS mainly conducts surveys in the universe with its three cameras known as channels. The wide field of view and high resolution are the specialties of this camera. Thus, ACS undertake mapping the distribution of dark matter, distinguishing distant galactic objects, detecting massive planets, and shedding light on the evolution of clusters of galaxies.

02. Spectrographs

Spectrographs split the light coming from celestial objects into their component colors for analysis. As a result, scientists have the opportunity to discover the properties related to the relevant heavenly bodies by comparing them with the spectra of known elements.

Figure 05: A spectrograph passes light coming into the telescope through a tiny hole or slit in a metal plate to isolate light from a single area or object. This light is bounced off a special grating, which splits the light into its different wavelengths (just like a prism makes rainbows). The split light lands on a detector, which records the spectrum that is formed.
Cosmic Origins Spectrograph (COS)

With its ability to break UV into components, COS analyses the evolution of the galaxy, the formation of planets, and the emergence of the essential elements. COS is an expert in photographing stars and other points of light.

Space Telescope Imaging Spectrograph (STIS)

STIS, the fingerprint scanner of Hubble combines a camera and a spectrometer. This has access to UV, visible to near IR. STIS skilfully detects the temperature, chemical composition, density and motion of celestial objects. Being known for its high-contrast imaging, this also assists in unveiling the mysteries of the evolving universe such as black holes, monster stars, and the intergalactic medium lying between galaxies. Further, STIS inspects the atmospheres of planets around stars other than our solar system.

COS and STIS work together to provide complete photographs for research purposes.

Near Infrared Camera and Multi-Object Spectrometer (NICMOS)

This IR-sensitive instrument is capable of disclosing heavenly bodies that are not available to visible light but IR. Thus, NICMOS catches sight of stellar formation sites and other components of the universe hidden in interstellar dust. Additionally, this possesses three cameras with different fields of view.

Range of Hubble's instruments
Figure 06: Hubble’s instruments collectively observe wavelengths (measured in nanometers) from ultraviolet through infrared. Each instrument was designed to operate in a particular wavelength range and function as an imaging camera or a spectrometer, though some instruments do both. The Fine Guidance Sensors (FGSs) not only help the telescope stay locked on target, but can be used as science instruments to accurately determine the relative position of stars.

A Meritorious Service of Over 3 Decades

Hubble Space Telescope is not a mere optical instrument. It was obvious since its sharper first-light image of a star cluster about 1300 light-years away from earth. Thenceforth, Hubble has not failed to surprise the earthlings with its groundbreaking discoveries. Uncovering the first evidence of water vapor at Jupiter’s moon Ganymede is one of the latest. HST has enhanced our vision of astronomy with more than 1.4 million observations up to date. These observations have made way to publish more than 18,000 peer-reviewed scientific publications on numerous themes. Hubble has been keeping a watchful eye from the Earth’s cosmic backyard to the high-redshift universe for 31 years. It is one of the longest-lived and most valuable space observatories due to its upgradeable design. Hopefully, by joining forces with the next generation space telescopes such as James Webb, Hubble Space Telescope will be unstoppable for years to come.

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