Sen—When you think of asteroids, you probably mentally picture a cluttered band of rocks orbiting the Sun between Mars and Jupiter, what astronomers call the Main Belt.
There are two problems with that picture: One is that the belt isn’t cluttered; there are a billion rocks or more orbiting in that region, but space is big. On average there are millions of kilometers between decent-sized asteroids.
The other problem? Not all asteroids live there. Some orbit the Sun on paths that take them nowhere near the main belt, or even anywhere near Mars.
Some of them come near Earth, of course. In a fit of brilliant coinage, we call those Near-Earth Objects, or NEOs. But even those have a variety of orbits, and we group them by the orbital type.
Those that stay outside Earth’s orbit (technically speaking, they have a perihelion—closest distance to the Sun—greater than 1 Astronomical Unit (or AU, the size of Earth’s orbit) but still get near Earth are called Amor asteroids, named after the first one in the class discovered, 1221 Amor. Nearly 4,000 Amors are known.
Apollo asteroids cross Earth’s orbit, and have orbital diameters larger than Earth’s. About 7,000 of these are known.
Then there are the Atens, which have orbits with diameters smaller than Earth’s, but which also have aphelia (farthest distance from the Sun) of more than 0.983 AU. Many of these can get close to Earth, and in fact about a hundred are labeled as “potentially hazardous”, which means sometime, hopefully in the far future, they can potentially hit the Earth.
Obviously, the more we know about these, the better.
It’s not clear how many Atens there are, though about a thousand are known. The problem is one of geometry: They spend a lot of time inside Earth’s orbit, so from our viewpoint they’re near the Sun in the sky. That makes them tough to observe from the ground.
But of course, we need not just look from the ground. We have space-based observatories, too, like the Wide-field Infrared Survey Explorer, or WISE. It’s on a polar orbit, scanning the skies with its modest 40-centimeter-wide telescope. It launched in 2009, and for over a year scanned the sky in infrared light, mapping nebulae, galaxies, and asteroids. It ran out of coolant in 2011 and was put into hibernation. However, as part of NASA’s push to look for near-Earth asteroids, WISE was switched back on, resurrected, in 2013 and renamed NEOWISE.
Because it’s in space, it can see objects closer to the Sun than we can from Earth (no atmosphere means no bright blue sky during the day). Asteroids are typically pretty dark, making them hard to spot in visible light. But they’re warm, and glow in the infrared, making them much easier targets for NEOWISE. Infrared light gives better clues about the sizes of asteroids too, making NEOWISE the premier tool for classifying these objects.
Now that all that preface is out of the way, let me direct your attention to the image at the top of this article, a composite of several recent NEOWISE observations. A newly discovered asteroid, 2015 QM3, is circled in the image as it moved across NEOWISE’s field of view. I found out about it when Amy Mainzer, the Principle Investigator of NEOWISE, tweeted that image.
The orbit of 2015 QM3, which takes it close to the three innermost planets in the solar system. Graphic by NASA/JPL-Caltech.
QM3 is pretty interesting. It’s an Aten, with an orbit that takes it just outside Earth’s, and drops it to just inside Mercury’s orbit. It never gets terribly close to Earth, Venus, or Mercury; at least, not close enough to worry about an impact any time soon.
But interestingly, it can pass close enough to all three planets to get slight tugs from them gravitationally. For example, in 1982 it came within about 9 million kilometers of Earth, and in 2017 it’ll pass just 7 million km from Venus.
Over time—millions of years, certainly—these tugs are enough to shift its orbit, so it’s a relative newcomer to the path its on. I was curious about how many such objects are known, and when I asked Dr Mainzer she was literally looking that up when I sent her the question (great minds…). She told me that only 16 other objects are known with similar orbits; that is, with perihelia less than 0.33 AU, and aphelia less than 1.083.
That makes QM3 a rare beast indeed… but that’s not surprising. Over time, anything on such an orbit is bound to get tossed about by the inner planets’ gravity, and very quickly on a cosmic timescale; the orbital periods of these objects are less than a year, so encounters are frequent. Some time in the future it will have a close pass with Earth, or Venus, or Mercury, and its orbit will change. Perhaps it’ll get closer to the Sun, perhaps farther, and it’s also possible the orbit might circularize a bit, changing both its peri- and aphelia.
It’s hard to say just yet for this rock, since it’s only been observed for a short time. The longer the observation baseline, the better we can predict its orbit in the future.
Lots of questions remain. How did this asteroid find its way on this path? What is its future? How many more are there like it? Mind you, while 16 asteroids like QM3 are known, we have no idea how many there actually are out there. Astronomers can run simulations to make estimates, which Mainzer is doing.
It’s surprising how much we don’t know about the Solar System, even in space near the Earth. We’re still meeting the neighbors! But we’re getting out there, exploring and learning. The more we do, the more we discover… and of course, that’s where the fun is.