Abstract: Extraterrestrial technology may exist in the Solar System without our knowledge. This is because the vastness of space, combined with our limited searches to date, implies that any remote unpiloted exploratory probes of extraterrestrial origin would likely remain unnoticed. Here we develop a probabilistic approach to quantify our certainty (or uncertainty) of the existence of such technology in the Solar System. We discuss some possible strategies for improving this uncertainty that include analysis of moon- and Mars-orbiting satellite data as well as continued exploration of the Solar System.
If extraterrestrials exist in our galaxy, then might they try observing us? This question was raised by Ronald Bracewell  shortly after Cocconi and Morrison’s suggestion  to search for extraterrestrial broadcasts. It is certainly possible that extraterrestrials could be observing us remotely; after all, near-future human technology includes the prospects of observing atmospheric spectra of extrasolar terrestrial planets  and exploring nearby star systems with unpiloted remote exploratory probes [4, 5, 6]. One possible scenario for human exploration of space begins with the discovery of an Earth-like planet around a nearby star, with an exploratory probe sent as a follow-up mission. It is at least plausible that extraterrestrials might also adopt a similar strategy [7, 8]. If so, then extraterrestrial technology could be hiding in our own Solar System.
We have obviously not yet discovered any technology of extraterrestrial origin, but how sure can we be that the Solar System contains none of these artifacts? In this paper we develop a probabilistic approach to this ques- tion. Certain regions of space have been searched sufficiently to discover such technology, if it exists, but much of the Solar System remains unex- plored. It is completely possible that extraterrestrial technology could be actively observing us without our knowledge, while it is also possible that defunct extraterrestrial technology exists on planetary surfaces or interplan- etary space. Therefore, it is of interest to calculate a likelihood that a given region of space is in fact absent of extraterrestrial technology. We begin our argument by first clarifying the types of extraterrestrial technology we con- sider. We then develop our probabilistic analysis and discuss its implications for future searches of the Solar System.
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 an3’ 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.