From Summertide by Charles Sheffield:
ARTIFACT: SENTINEL.
UAC#: 863
Galactic Coordinates: 27,712.863/16,311.031/761.157
Name: Sentinel Star/planet association: Ryders-M/Sentinel Gate
Bose Access Node: G-232 Estimated age: 5.64 ± 0.07 Mega-years
Exploration History: Sentinel was discovered in Expansion Year 2649 by human colonists of the trans-Orionic region. First entry attempt, E. 2674, by Bernardo Gullemas and the crew of exploration vessel D-33 of Cyclops class. No survivors. Subsequent approaches attempted E. 2682, E. 2695, E. 2755, E. 2803, E. 2991. No survivors.
Sentinel warning beacon set in place, E. 2739; monitoring station established on nearest planet (Sentinel Gate), E. 2762.
Physical Description: Sentinel is a near-spherical inaccessible region, a little less than one million kilometers across. No visible internal energy sources, but Sentinel glows faintly with its own light (absolute magnitude +25) and is visible from every point of the Ryders-M system. The impassable surface of Sentinel readily permits two-way passage of light and radiation of any wavelength, but it reflects all material objects including atomic and subatomic particles. There is photon flux only from the interior, with no particle emission. Laser illumination of the interior is possible, and reveals a variety of structures at the center of the sphere. The most prominent such feature is "The Pyramid," a regular tetrahedral structure which absorbs all light falling onto it. If interior distances within Sentinel have meaning (there is evidence that they do not -- see below) then the Pyramid would be approximately ninety kilometers on a side. No increase in temperature of the Pyramid is detectible, even when incident absorbed radiation is at the gigawatt level.
Path length measurements using lasers show that the interior of Sentinel is not simply-connected; minimal light travel time across Sentinel is 4.221 minutes, compared with a geodesic travel time of 3.274 seconds for equivalent distance across empty space remote from matter. For light incident normally on the Sentinel "equator" travel times across Sentinel are infinite, or certainly in excess of a thousand years. Red shift and grazing incidence laser beams indicate that no mass is present within Sentinel, a result that is inconsistent with the observed interior structure.
Sentinel holds a precise distance of 22.34 a.u. from the Ryders-M primary star, but is not in orbit about it. Gravitational forces and radiation pressure forces either are exactly compensated by some unknown mechanism in Sentinel, or do not act on the structure at all.
Physical Nature of Sentinel: According to Wollaski'i and Drews, Sentinel takes advantage of and is built around a natural anomaly of space-time and possesses only weak physical coupling to the rest of the universe. If so, this is one of only thirty-two Builder artifacts that were created with the use of preexisting and natural features.
Sentinel topology appears to be that of a Ricci-Cartan-Penrose knot in 7-space.
Intended Purpose: Unknown. However, it is conjectured (by analogy with other Builder artifacts, see Entry 311, 465, and 1223) that the Pyramid may possess near-infinite information storage capacity and lifetime. It has therefore been suggested (Lang, E. 4130) that the Pyramid and possibly the whole of Sentinel form a Builder library.
Excerpt from Summertide by Charles Sheffield, ©1990 Charles Sheffield.
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