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Every couple of weeks stories circulate online about some asteroid that's going to skim past Earth. Usually it's about a real object that really will pass fairly close to Earth. Sometimes, though, a conspiracy group will insert it into their version of a "government cover-up" story.
The latest subject is the binary asteroid 66391 (1999 KW4), and its orbit is VERY well constrained. It will safely pass Earth on May 25 2019 at a distance of 13.48 lunar distances. Here are some resources and tools anyone can use to verify or disprove these kinds of claims.
If you ever wonder what's headed our way, this is the "Close Approach" page for NASA's Center for Near-Earth Object Studies (CNEOS) website. There's a lot of good basic information about near-Earth object discovery and programs to assess impact threats. If you notice in the screenshot below, there are actually 8 other previously-discovered asteroids that will pass closer than 1999 KW4 in the next 60 days. While 1999 KW4 is certainly the biggest at 1.3 km diameter, none of them are going to be very close.
As the name might suggest, asteroid 1999 KW4 was first discovered in 1999. There have been over 3200 optical observations of 1999 KW4 in the past 20 years, which you can view publicly on ESA's Near-Earth Objects Dynamic Site (NEODyS). Notice that 1999 KW4 was observed on 9-10 May 2019 by Observatoire des Makes (observatory code 181), which is on the French island of Réunion off the coast of Madagascar.
In addition to viewing each observation of an object submitted to the database, you can view information about an object's orbital parameters and generated ephemeris data. Ephemerides plot the trajectory of an object across the sky for a particular location and time on Earth. Astronomers use this information to know where to point their telescopes. NASA's Jet Propulsion Laboratory Small-Body Database Browser also serves a similar function.
There was also an extensive radar observation campaign of 1999 KW4 in the year 2001, which is why we know that it looks kinda like a walnut and has a small moon.
It will be observed again by radar in the coming weeks, and here's some info on NASA's planned observation campaign.
Another good site is A/CC News, which compiles a daily "traffic report" and all the electronic data that professional and amateur astronomers both utilize and generate when observing asteroids and comets.
I hope this gives you some good resources and tool for fact-checking the next "Will this asteroid hit us??" story. If I've left anything off, please feel free to contact me on Twitter @asteroidanalyst or email at firstname.lastname@example.org
Let's keep learning. Let's keep growing!