With the end of the monsoon in the southwest US (or at least a break in the monsoon since the rains are back for the next few days), the professional asteroid surveys have wasted no time in getting back to work. Since the start of the month over 50 new near-Earth asteroids have been discovered. Two of these objects will make close approaches to the Earth, within the distance of the Moon. Both flybys will occur on September 8 UT. The short time between flybys is coincidental since the asteroids are not related to one another.
At around 10 hours UT on Sept. 8, 2010 RX30 will pass within 0.0017 AU (149,000 miles/248,000 km) of Earth. For reference the Moon is located at an average distance of 239,000 miles or 384,000 km. We don’t have a direct measurement of its size but based on its brightness (absolute magnitude [H] = 27.1) it is probably between 8 and 25 meters across.
2010 RX30 was discovered by on Sunday morning by Andrea Boattini (Mount Lemmon Survey/Catalina Sky Survey) with the Mount Lemmon 1.5-m telescope just north of Tucson, AZ. A faint 19th magnitude at discovery the asteroid will brighten to magnitude 15.5 a few hours before close approach. (If you’re wondering why it isn’t brightest at closest approach, the phase angle increases rapidly for this asteroid meaning less of the asteroid is illuminated as seen from Earth.) It’s orbit takes stretches from 0.50 AU to 1.15 AU from the Sun and is inclined 4.5° from the plane of the ecliptic. As an Aten-type asteroid it has a semi-major axis (average distance from the Sun) of 0.82 AU which is less than the Sun-Earth distance. The small semi-major axis also means it circles the Sun in less than a year, in this case 0.74 years or just under 9 months.
The 2nd asteroid to buzz the Earth is smaller and will pass even closer to Earth. Similar to 2010 RX30, 2010 RF12 was also found at 19th magnitude on the night of Sept 5 UT by Andrea Boattini from Mount Lemmon. With an absolute magnitude of 28.1, it is likely between 5 and 15 meters. Closest approach will occur around 21 hours UT on Sept. 8 at a distance of 0.00053 AU (49,000 miles or 79,000 km) from the center of the Earth. It will be magnitude 13.8 at its brightest. Visually this is much too faint for all but those with very large telescopes under a dark sky.
As an Apollo-type object is crosses the Earth’s orbit and has a semi-major axis greater than 1 AU. Its orbit stretches from 0.82 to 1.17 AU from the Sun and is inclined 3.5°. With a semi-major axis of 0.997 AU it orbits the Sun once every 1.0 years. Such an Earth-like orbital period means RF12 and the Earth only catch up with each other once every 100 or so years.
Whenever objects like this are found, there are always questions as to how we missed it. The current crop of asteroid surveys only search about 2% of the sky every night. Over time this adds up and most of the larger and most dangerous objects are found. This is the goal of the asteroid surveys; to find dangerous asteroids decades before they are a threat. And they gives us sufficient time to mitigate the danger. But very small objects are too faint to be seen except for a few days before or after close approach. For every 5-20 meter object we find, hundreds to thousands pass sight unseen.
Before you worry about the danger of very small asteroids, know that neither of these objects would have survived in one piece to impact the ground. Both objects would fragment high in the Earth’s atmosphere and mostly burn up. A fraction of the asteroid’s original mass would survive as fist-sized meteorites impacting the ground at terminal velocity (~100 mph). So if 2010 RF12 or 2010 RX30 collided with Earth it would have resulted in a spectacularly bright fireball and a large number of small meteorites. This is not too dissimilar to the impact of 2008 TC3. Check out the Impact Effects page to run the numbers yourself. (I also used a projectile density of 1500 g/cm^3, impact velocity of 17 km/s and impact angle of 45°.)