Alien visitors, redux: Why are we seeing them now?

 After my last post on 3I/ATLAS, an interested reader (full disclosure - my wife, Ruth Ehrenkrantz) asked an interesting question: Why are we only seeing the interstellar visitors now?  We have now seen three in eight years, after centuries of nothing?  

 The answer to this question is: technology.  Finding small (asteroids and comets both qualify) objects in our solar system is a matter of taking multiple photographs of the sky, spaced days or weeks apart, and looking for points of light that are moving against the fixed background of stars.    One needs to look at a lot of images to find a few moving objects.   Finding the even rarer (cf. three in eight years) objects that are moving too fast to originate in the solar system is even harder, and requires fast searches of large regions of the sky.

High-speed searching requires automation, to methodically take photos at appropriate intervals and automatically compare the images.  Correcting for different seeing conditions is not trivial - if the atmosphere is a bit more turbulent or hazy, images will look different.      This is technology that has been developed over the past few decade, driven by "Planetary Defense"which even has its own office at NASA.  Despite a name evocative of "Space 1999," this office is focused on a more prosaic concern, the possibility that an errant asteroid or comet might hit the Earth, ala the Tunguska meteorite or, worse, the asteroid that wiped out the dinosaurs (as shown in the visualization below, from New Scientist magazine). 

One well known search program is Asteroid Terrestrial-impact Last Alert System (ATLAS), which found 3I/ATLAS.  It uses five robotic telescopes, like the one shown above to search around the ecliptic plane (the plane containing the orbits of the planets around the Sun), looking for objects that might come close enough to Earth to pose a danger.  When it finds a moving object, it will follow it to determine its trajectory, and project its movement into the future, to see if it will come close to Earth, or even might hit it.  Of course, the further one projects into the future, the larger the uncertainty.  

Sometime - three times so far - these automated searches will find objects that are moving too fast to be gravitationally bound to the Sun.  It is not at all surprising that our observations of these extra-solar objects has started in the last decade, as searches like ATLAS have ramped up.   

Incidentally, although 3I/ATLAS looks like a typical comet, that has not stopped some astronomers - notably Harvard's Avi Loeb - from speculating that it may be an extra-terrestrial probe, on the basis that it is coming surprisingly close to several planets (not Earth, though).   Other astronomers continue to study it, and nothing too surprising has been observed.  Recently, it has been found to contain water, for example.