The James Webb Space Telescope's ISIM structure recently endured a "gravity sag test" as it was rotated in what looked like giant cube in a NASA clean room.

The Integrated Science Instrument Module (ISIM) that will fly on the Webb telescope was rotated upside down inside a cube-like structure in the cleanroom at NASA's Goddard Space Flight Center in Greenbelt, Maryland. The purpose of "cubing" the ISIM was to test it for "gravity sag," which is to see how much the structure changes under its own weight due to gravity.

The Integrated Science Instrument Module (ISIM) is one of three major elements that comprise the Webb Observatory flight system. The others are the Optical Telescope Element (OTE) and the Spacecraft Element (Spacecraft Bus and Sunshield).

Engineers need to see how gravity affects the alignment of the instruments within the module when ISIM is in different orientations on the ground.

Engineers use measurements from this sag test to verify what the instrument alignments will really be in space, when there is no preferred direction from gravity, and take account of sag during assembly with the telescope.

The cube with the ISIM inside it was rotated using a massive machine called an Aronson table; the rotation was just the dramatic start to the test. The cube and ISIM were then lifted off of the rotation table while inverted and kept upside down for several days while lasers made precise position measurements.

The James Webb Space Telescope is the scientific successor to NASA's Hubble Space Telescope. It will be the most powerful space telescope ever built. Webb is an international project led by NASA with its partners, the European Space Agency and the Canadian Space Agency.