Astronomers hit a 1.6-kilometer rock* recently: they observed the 1,000th asteroid close to Earth through radar.
That’s a lot of rocks. But it’s only a fraction of what’s there.
Asteroids near Earth (or NEA) are almost the ones that are advertised: asteroids approaching Earth. Specifically, they are defined as any asteroid with an orbit that falls about 145 million kilometers from the Sun (1.3 times the distance from Earth to the Sun).
As of September 2, 2021, 26,520 NEAs were known. Nearly a thousand are 1 km in size or more and 10,000 more than 140 meters in diameter. The rest are smaller.
If they pass close enough to Earth, a few million kilometers or so, they can be observed by radar. Astronomers use giant radio telescopes (such as NASA’s Goldstone facility and the defunct and unfortunate Arecibo Observatory) to send pulses of radio waves that hit the asteroid and bounce off Earth. It can be used to get an extremely accurate distance and speed for the rock, and if it is big enough and close enough you can also determine its size and shape (it’s a complicated process; my friend and colleague Emily Lakdawalla has an excellent slow page describing it on the Planetary Society website).
The lucky 1,000th NEA observed that this way was the 2021 PJ1, a small rock probably 20 to 30 meters wide (therefore slightly larger than the one touched Chelyabinsk, Russia in 2013). In August, PJ1 passed just over 1.6 million miles from Earth and the Goldstone plate was used to take a look. It’s too small to get a picture of, but the orbit was refined, which is great.
The first asteroid to be observed in this way was Icarus 1566, a robust asteroid more than a kilometer wide seen in 1968. Observing a thousand more since then is an astonishing success.
I will add that a week after observing PJ1 they also observed 1,001c NEA: 2016 AJ193, which is 1.3 km wide and passed us 3.4 million kilometers. It rotates once every 3.5 hours, as can be seen in the previous image series or in this animation:
Cooooool.
Now the not so fun part. A subset of NEAs are PHAs, potentially dangerous asteroids. They are defined as those that have an orbit that takes them less than 7.5 million kilometers from Earth’s orbit and have a size or more than 140 meters. Over time, the orbits of asteroids can change due to the gravity of the planets and the effects of sunlight, so the definition of distance may seem quite large at first. In addition, it can be difficult to nail the orbit accurately, so some confusion is allowed in the definition. The definition of size is because at 140 meters the impact energy of such a beast becomes regional rather than just local, making them a decent threat.
Astronomers are closely monitoring the PHAs. The biggest threat now lies in Bennu, the 500-meter-wide pile of debris recently visited by the OSIRIS-REx spacecraft. If it came to devastation, it would be considerable, but the chances of impact are low: even adding up all the possible times it may affect us (the first approach of this known type is in 2178, the last in 2290) is only 1 in 1,800. Perhaps best stated is a 99.943% chance Lady the Earth in each of these 157 close approaches during the date range. So it’s not too big a business.
Statistically speaking, approximately 90% of all PHAs greater than 1 km have been found, but this percentage is much lower for those of 140 meters or more. The good news here is that some fairly large sky surveys will go online soon and detect many more, and NASA is building a new spacecraft called the NEO Surveillance Mission to observe and characterize them even better.
Finding more may not sound like good news, but the thing is they are there. If we find them, we will know more and if we detect one with our name, maybe we can take some action to avoid an impact. NASA and the JPL are also building a mission called DART to investigate the possibility of pushing an asteroid into a safe orbit, if necessary.
Knowledge is power. In this case, knowledge can literally save humanity.
So this 1,000th asteroid with a little radar love is good. When it comes to threats like these, it’s much better to look up instead of having your head in the sand.
*Here’s a whole metric, folks.