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Theoretically, If an asteroid on a collision course with our planet could be slowed sufficiently it could be made to pass behind our planet as it orbits the Sun.

The Vera C. Rubin Observatory, also known as LSST, with its 8.4 m mirror, the width of a tennis court, and the worlds largest 3.2 gigapixel camera, roughly the size of a small car, will image the entire sky every 3 or 4 nights in such a way as to detect objects that change in position and/or brightness. It is estimated the LSST will discover one or two new impacting small asteroids per year and provide several days of lead time for follow up telescopes to track an impactor into the atmosphere to find meteorites whose chemical composition will tell us about where they originated.

My Catalina Sky Survey teammate Greg Leonard's discovery of, 2018 FO5, a three football field diameter asteroid that can come closer than our Moon is a reminder that we need to keep searching for the many thousands of Potentially Hazardous asteroids larger than 500 feet in diameter which are likely to exist.

After completing a sample and return mission to the asteroid Ryugu (ree-oo-goo) and a fly by of asteroid Torifune (toh-ree-foo-neh), the Japanese asteroid exploring spacecraft Hayabusa2 (high-yah-BOO-sah-too )is on its way to fly by the rapidly spinning tiny asteroid 1998 KY2 .

My Catalina Sky Survey teammate Jess Johnson was very surprised when the large comet with a long stringy tail, which he has just imaged with our 60 inch telescope on Mt. Lemmon, Arizona, was an unknown object. Jess quickly emailed his discovery observations along with a description of the comet's coma and tail to the Minor Planet Center. Over the next eight days astronomers at 11 different observatories around the world tracked Jess's new comet. Scientists at the Minor Planet used these data to calculate it's path around the Sun, verify it to be a comet, and give it the name C/2018 F3 (Johnson). We now know that Comet C/2018 F3 (Johnson) had crossed the orbit of Pluto in 1997 and had reached it's closest point to the Sun 217 days previous to Jess's discovery observations. Further, about two weeks before Jess spotted it , moving through the constellation of Leo, Comet 2018 F3 (Johnson) had streaked unnoticed past the Earth. Unfortunately Jess's comet is likely to be visible only in electronic camera images and in 2037 will once again cross Pluto's average distance from the Sun on a path which will take it into truly deep space. Until a few years ago all of the comets human astronomers had observed originated in the distant regions of our solar system, the Kuiper Belt or the Oort Cloud . Perhaps C/2018 F3 (Johnson) is an interstellar visiting Comet and is thus a sample of a place currently beyond our reach.

Viewed in a telescope an asteroid appears as a moving star like point of light while  a comet often appears to be a fuzzy point of light with a tail.  The orbit of an asteroid is governed force of gravity exerted by the Sun, planets, and in some cases our moon. In addition the Yarkovsky Effect can cause the orbit of a spinning asteroid to shift slightly over millions of years as the heat it absorbs from the Sun radiates back into space. Comets on the other hand can have orbits which change erratically as its nucleus emits small rocket like bursts when frozen gases trapped within it are heated by the Sun.

On Saint Patrick's Day, while searching for Earth approaching asteroids with the Catalina Sky Survey's 60 inch telescope on Mt. Lemmon, Arizona I spotted a slightly enlarged star like object with a tiny tail moving through the constellation of Virgo. After checking to see that it was not a known comet, I sent my discovery observations along with a description of it's coma and tail to the Minor Planet Center. Turns out that it had been observed as a moving point of light, but not noticed to be a comet, by the PanSTARRS group in Hawaii 3 month's earlier. Given the PanSTARRS historical observations along with those from other observatories around the world it only took scientists at the Minor Planet Center 2 days of additional observations to verify it to be a newly discovered comet. Since the naming rules state that a comet should be named after the first observer that identified it to be a comet it was given the C/2018 F1 (Grauer). In December of 2018 and July of 2019 Comet C/2018 F1 (Grauer), which I share with my wife Annie Grauer, will be closest to Earth on it's 5,555 year orbital path about our Sun. Unfortunately it is not likely to be visible to the unaided eye, appearing only as faint moving comet in the night sky on electronic camera images, leaving us to speculate on the condition of humanity when Comet C/2018 F1 (Grauer) returns in the year 7573 AD.

Recently a pair of JPL SynTrack Robotic automated telescopes located at a high altitude low light pollution site in California provided 4 tracking data points taken less than 30 minutes apart which indicated that a small space rock with a diameter smaller  the height of a human being was on a collision course with Earth.

To give you an idea of the asteroid traffic in our neighborhood, on a mostly cloudy night, through holes in the clouds, in a space of less than 2 hours, my Catalina Sky Survey teammate Richard Kowalski posted 7 new close approaching asteroid discoveries on the Minor Planet Center's Near Earth Object Confirmation Page

The northern lights or Aurora Borealis have fascinated humans throughout the ages. Today aurora watching is a multimillion dollar industry in places like Alaska, Canada, Norway, Sweden, Finland, Greenland, and Iceland. Auroras are manifestations of the interaction between the solar wind, radiation, and our atmosphere we call space weather.

The large dangerous asteroids that we don't know about are what keep asteroid hunters going to the telescope.

As the Earth orbits the Sun at 18.5 mi/s it sweeps up about 100 tons of micrometeorites and space dust every day of which some 14 tons eventually makes it to the ground. You can even find some of these particles of space dust by using a magnet on material in the rain gutters attached to your roof.

Tonight, even though the chances are extremely slim, an asteroid hunter could find a sizable asteroid on a collision course with planet Earth. If we have decades warning, the potential impactor's arrival time could be changed by impacting the dangerous object with a high velocity mass or if we have less time we will need to nuke it which will either blow it to bits or give it a rocket like push.

When accurately recorded historical observations do not agree with what we what we see today it is often very rewarding to find out why. In modern telescopes we see Theta Eridani to a be pair separated by about 425 times our distance from the Sun orbiting a common center of gravity. Detailed observations of the spectrum of one of these show it to be itself a pair of stars orbiting each other every 4.1 days. One of this very close pair, as it is aging, dumps matter onto its companion causing it to brightened significantly. In the past this mass transfer rate was much higher than it is today and the naked eye brightness of Theta Eridani is fainter to us than it was long ago.

In the future one can envision a space mining family boarding a descendent of the SpaceX Falcon Heavy Launch Vehicle for a trip into low Earth orbit. A few hundred miles above the Earth's surface they would rendezvous with their asteroid mining spacecraft which will be their home for the next five years or so.