An extraterrestrial probe is a hypothetical interstellar probe constructed by an extraterrestrial civilization. There has been much speculation that such objects may be present within the solar system and therefore be a legitimate target for SETI (Search for Extraterrestrial Intelligence). See, for example, Bracewell probes (also known as "messenger probes") and von Neumann probes. However, although a few small-scale searches have actually been carried out (see references below), by Robert Freitas, Francisco Valdes, and others, for alien objects in Earth orbit, lunar orbit, and the Earth-Moon libration points, there has been a reluctance, and indeed a certain amount of hostility on the part of mainstream (microwave) SETI researchers, to engage in a committed effort. A more balanced outlook, though, appears to be emerging, in which searches may be carried out both for signals and material transmissions.
In 2004, a paper was published in Nature, by Christopher Rose, an electrical engineer at Rutgers University, Piscataway, New Jersey, and Greg Wright, a physicist from Antiope Associates in Fair Haven, New Jersey, which presents a new argument for the efficiency of sending probes to gather and transmit data rather than communicating via interstellar radio messages.5 For more about their analysis, go to the external link below.
1. Freitas Jr., Robert A.and Valdes, Francisco. "The Search for Extraterrestrial Artifacts," Acta Astronautica, 12, No. 12, 1027-1034 (1985).
Abstract: The rationale for the use of interstellar artifacts by intelligent life in the universe is described. The advantages of using interstellar probes as a means of exploration and communication are presented and shown to be significant enough to counter the time, energy, and technology arguments generally raised against contact via extraterrestrial artifacts. Four classes of artifacts are defined: Those seeking contact, those seeking to avoid contact, those intended to provide a passive technological threshold for detection, and those for which detection is irrelevant. The Search for ExtraTerrestrial Artifacts (SETA) is based on the latter two classes. Under the assumption that an extraterrestrial probe will be interested in life in our solar system, a near-Earth search space is defined. This search space is accessible to us now with ground and satellite observing facilities. The current observational status of SETA is reviewed and contrasted with the achievable detection limits for the different parts of the search space.
2. Freitas, R. A. "The Search for Extraterrestrial Artifacts (SETA)," Journal of the British Interplanetary Society, 36, 501-506 (1983).
Abstract: The Artifact Hypothesis states that an advanced extraterrestrial intelligence has undertaken a long-term programme of galactic exploration via the transmission of material artifacts. An attempt to verify this hypothesis experimentally, the search for extraterrestrial artifacts (SETA), is proposed to detect such evidence in the Solar system by telescopic, radar, infrared, direct probe, or other available means.
3. Tough, Allen. "Small Smart Interstellar Probes," Journal of the British Interplanetary Society, 51, 167-174 (1998).
Abstract: Humanity is making rapid progress in computers, robotics, nanotechnology and space exploration. Consequently, within 200 years, we will be likely to launch small interstellar probes containing highly advanced computers. Perhaps other civilizations, more advanced than ours, launched intelligent machines long ago to explore parts of our galaxy. One of their tiny probes may have already reached our planet in order to observe or monitor us. Our current scientific search for extraterrestrial intelligence should be expanded by adding a sophisticated search for such a probe. Two SETI "declarations of principles" have been developed to cover the search for radio and laser signals originating many light-years away. Some SETI scientists have assumed that these two declarations also apply to the scenario of discovering a nearby probe but, in fact, the fit is not very good. A separate set of "Procedures Following Detection of An Interstellar Probe" has been drafted.
4. Steel, Duncan. "SETA and 1991 VG," The Observatory, 115, No. 1125, 78-83 (April 1995).
Abstract: A ~ 10-metre object on a heliocentric orbit, now catalogued as 1991 VG, made a close approach to the Earth in 1991 December, and was discovered a month before perigee with the Spacewatch telescope at Kitt Peak. Its very Earth-like orbit and observations of rapid brightness fluctuations argue for it being an artificial body rather than an asteroid. None of the handful of man-made rocket bodies left in heliocentric orbits during the space age have purely gravitational orbits returning to the Earth at that time, and in any case the a priori probability of discovery for 1991 VG was very small, of order one in 100,000 per anmun. In addition, the small perigee distance observed might be interpreted as an indicator of a controlled rather than a random encounter with the Earth, and thus it might be argued that 1991 VG is a candidate as an alien probe observed in the vicinity of our planet.
5. Rose C., and Wright G. Nature, 431, 47-49 (2004).