The Gravity Probe B mission provided two new quantitative tests of Einstein’s theory of gravity, general relativity (GR), by cryogenic gyroscopes in Earth’s orbit. Data from four gyroscopes gave a geodetic drift-rate of −6601.8 ± 18.3 marc-s yr−1 and a frame-dragging of −37.2 ± 7.2 marc-s yr−1, to be compared with GR predictions of −6606.1 and −39.2 marc-s yr−1 (1 marc-s = 4.848 × 10−9 radians). The present paper introduces the science, engineering, data analysis, and heritage of Gravity Probe B, detailed in the accompanying 20 CQG papers.

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The Gravity Probe B spacecraft was launched on 20 April 2004 to measure the geodetic and frame-dragging effects predicted by the theory of general relativity. A cryogenic optical telescope was used to establish the inertial reference frame for the measurements by tracking a reference or guide star. The motion of this star was independently checked by reference to background galaxies. With the mission now over, we describe the design, construction and evaluation of the optical and electrical performance of the telescope, comparing ground and flight results. We find that the pointing noise was sufficiently low to meet the mission requirements and in fair agreement with extrapolations from ground tests. Due to slight defocusing, the linear range of the telescope output was significantly wider than expected.

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is NASA’s airborne observatory. Observing with SOFIA has different challenges than observing from a ground based, or even space-based observatory. Although pointing SOFIA is similar to pointing an alt/az telescope, positioning the telescope requires not only the telescope assembly but also the aircraft. SOFIA’s telescope assembly can move in altitude nominally between 20∘ and 60∘. Since the telescope is pointed out the left side of a modified Boeing 747, the azimuth is determined by the aircraft heading. As a result, observing plans become the basis for a flight plan, and the science observation and aircraft operations are intrinsically linked. This paper will discuss flight planning and execution on this unique observatory.

SOFIA, the Stratospheric Observatory for Infrared Astronomy, is an airborne observatory that will study the universe in the infrared spectrum. A Boeing 747-SP aircraft will carry a 2.5 m telescope designed to make sensitive infrared measurements of a wide range of astronomical objects. In 2008, SOFIA's primary mirror was demounted and coated for the first time. After reintegration into the telescope assembly in the aircraft, the alignment of the telescope optics was repeated and successive functional and performance testing of the fully integrated telescope assembly was completed on the ground. The High-speed Imaging Photometer for Occultations (HIPO) was used as a test instrument for aligning the optics and calibrating and tuning the telescope's pointing and control system in preparation for the first science observations in flight. In this paper, we describe the mirror coating process, the subsequent telescope testing campaigns and present the results.