On June 19th, 2016 we learned about

A nearby asteroid nearly shares Earth’s orbit around the Sun

Orbits seem like they should be so straight-forward, but even a basic ellipse can be confusing. From our perspective on Earth, objects in the space can appear to move across the sky in a way that suggests they’re all circling our own planet, just like the Moon. This geocentric model of the universe has long been discredited, but even today you can find objects that loop around the Earth’s position almost as strictly as our own Moon, which is truly in orbit of our planet. An asteroid discovered in April demonstrates how overlapping orbits can make a cosmic neighbor look like a planet’s satellite.

Overlapping ellipses

The asteroid, called 2016 HO3, never strays too far, or close from the Earth. As we orbit the Sun, HO3 is always there with us, maintaining a distance between 8 to 25 million miles away. Like the Moon, HO3’s relative position in the solar system dances back and forth with the Earth, partly thanks to the Earth’s gravity keeping the rock from drifting too far ahead or behind us. We complete one orbit around the Sun a mere 16.6 hours before HO3 does, and yet this doesn’t all add up to having a second, smaller moon. You just have to look past our geocentric position, and consider the asteroid’s orbit from the Sun’s perspective.

HO3 and the Earth are both orbiting the sun, almost like two cars racing around a track. They’re basically neck and neck, with one occasionally passing on the inside or outside of the bigger overall orbit, but neither is strictly circling the other. In fact, if you looked from HO3’s perspective, it might appear that the Earth was circling the asteroid instead, although the Earth’s gravitational pull is probably influencing the asteroid’s movement much more due to their relative masses.

Too small to see, but not to visit

Asteroid 2016 HO3 really isn’t that big, which is why nobody had really noticed this co-traveler that’s been staying relatively close to us for so long. It’s estimated to be no more than 328 feet in diameter, and only reflects one-millionth the light of event the faintest star in the sky at night. While the Pan-STARRS observatory could pick it up, no human eyeballs were going to see this rock zipping by any time soon, even if it’s been doing so on such a regular schedule.

This doesn’t mean that HO3 is nothing but a novelty though. It’s close, predictable path actually means that it might be an easy destination to land future probes on, requiring relatively little fuel to reach and intercept. With interest in landing astronauts on asteroids down the line, this particular rock might save us the trouble of dragging one in from further out in space.

Source: Another Moon for Earth? Well, Not Really, but It Depends on Your Point of View. by Phil Plait, Bad Astronomy

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