NASA decided early in the telescope's development to design the observatory for on-orbit servicing. Instruments were designed as modular units, comparable to dresser drawers that could be easily removed and replaced. In addition, designers equipped the telescope with handholds and other special features to make servicing tasks less difficult for astronauts wearing bulky spacesuits.

By periodically upgrading the science instruments, NASA also reasoned that it could provide the science community worldwide with state-of-the-art technology that takes advantage of Hubble's unique position high above Earth's obscuring atmosphere.

This mission boosted Hubble's scientific power and ensured the continued effective operation of the telescope. During Servicing Mission 4, astronauts installed two state-of-the-art science instruments: the Cosmic Origins Spectrograph (COS) and the Wide Field Camera 3 (WFC3). They also added six batteries, six gyroscopes, and a Fine Guidance Sensor.

WFC3 replaced Hubble’s “workhorse” camera, the Wide Field Planetary Camera 2 (WFPC2), which observed celestial objects for 15 years. The new camera greatly improves Hubble’s ability to image large and distant objects, such as galaxies and clusters of galaxies, as well as planets in our solar system.

Astronomers use WFC3 to make wide-field surveys of the distant universe to study how galaxies formed and evolved from about 2 billion years after the Big Bang to the present. WFC3 allows Hubble to see objects that are about five times fainter than Hubble’s landmark deep view of the cosmos, called the Hubble Deep Field, taken in 1996 with WFPC2.

COS examines the ultraviolet light streaming from distant objects. The new instrument allows Hubble to study galaxy formation and the births of stars and planetary systems. COS completes observations much faster than Hubble’s previous spectrograph.

Normally, Hubble’s science instruments run on sunlight collected by its twin solar panels, which make the observatory look like it has wings. The batteries power Hubble’s science instruments when the telescope is in Earth’s shadow. The gyroscopes help keep Hubble steady as it orbits Earth and allow scientists to point the telescope at celestial targets. Hubble has three Fine Guidance Sensors, which also help in pointing the telescope at objects.

When your car needs a tune-up, you take it to your mechanic for servicing. Engineers and scientists designed the Hubble Space Telescope with the same principle in mind. When a component breaks or a more technologically advanced instrument becomes available, astronauts install the new equipment using wrenches, screwdrivers, and power tools during periodic servicing missions.

Astronauts have serviced Hubble five times — in 1993, 1997, 1999, 2002, and 2009 — and have installed many new instruments featuring more advanced technology.