PHAs or Potentially Hazardous Asteroids zing through Earth’s neighborhood nearly every day. Over the weekend, six small asteroids including two PHAs came within 16 to 3.5 times the distance of the moon. The largest, 2002 AJ29 was all of 2,100 feet across, a real whopper. Two more will be making close shaves very soon: 2018 CC tomorrow Feb. 6 at 3 p.m. (Central time) and 2018 CB at 4 p.m. on Friday, Feb. 9.
PHAs are defined as asteroids that are at least 328 feet (100 meters) across that approach within 4.6 million miles (7.4 million km) of Earth. Because asteroid orbits can change over time due to gravitational effects from the sun and planets, a PHA could potentially impact the planet sometime in the future.
That future looms large and distant; at least for now, no known PHA is predicted to impact Earth. There are only probabilities, and those probabilities usually drop close to zero once we have a good idea of a newly-discovered asteroid’s orbit. Astronomers determine those details by routinely photographing the asteroid, noting its precise position with relation to the background stars. The more observations, the better we know the orbit and the better we can predict the object’s future behavior.
It’s a blast to track Earth-approaching asteroid in a telescope. Even the little ones can get bright enough to spot when they’re really close. Generally, the closer they are, the faster they zip across the sky. I’ve looked at several where I’ve had nudge the tube in the asteroid’s direction of motion every couple minutes to keep up. All these quick, come-and-go objects look exactly like stars in even the largest telescopes. Only radar can reveal their shapes and textures.
When they’re too faint for beginning or amateur astronomers to track them, several websites offer live, online views from big scopes with a good view. One of those is Gianluca Masi’s Virtual Telescope Project based in Italy which is also connected to the Tenagra Observatories in Arizona. Masi, an astrophysicist, loves to share asteroids, eclipses and other celestial events with as many people as possible, especially those without a telescope or clouded out by bad weather.
Both 2018 CC and 2018 CB are small, 55 and 75 feet (17 and 23 meters) respectively, and will be too faint for most observers to see in a telescope, although 2018 CB will briefly climb to magnitude 13 for you hard-core trackers out there. No worries though. Masi will live stream both close approaches on his site at the following times:
2018 CC will pass only 119,040 miles from us or half a moon’s distance, while 2018 CB really gets in tight with a minimum distance of just 43,000 miles (69,000 km) around 4 p.m. (22:00 UT) on Friday the 9th. That’s not far beyond the bounds of the geosynchronous satellite belt. Because both asteroids will be traveling around 36,000 mph (58,000 km/hour), they won’t be pulled in by the gravity of the Earth. These space rocks have places to go! Still, Earth’s gravity will alter their orbits to some degree, so each will leave the planet’s vicinity on a somewhat different track than that on which they arrived.
Check out this video created by Tom Ruen showing Earth as seen from asteroid 2018 CB during Friday’s flyby.
New Earth-approachers are discovered all the time and old ones keep making the rounds. At least 10 more will be passing our way through the remainder of the month. Check out the live views and be sure to wave as they go by.