Rogue one pays visit
IN September and October, our solar system had a very unusual visitor: an asteroid, or possibly a comet, that is not a member of the sun’s family.
It arrived from outside the solar system, and it is now leaving. It will never be back.
The object, which is named A/2017 U1, is not large. Even our best telescopes cannot show its size or shape directly, but its diameter is thought to be less than 400m. Calculations by one of the researchers suggest that it could be only about 160m across, based on an estimate of how much light its surface reflects.
The deduction that A/2017 U1 is not a member of our solar system has been possible from an analysis of the shape of its orbit.
Orbits of bodies that repeatedly go around and around their central body — such as the Earth moving around the sun — are elliptical, or oval-shaped. This was a deduction made by the great astronomer and mathematician Johannes Kepler four centuries ago, solving the problem of the observed positions of the planets.
An important quantity associated with ellipses is called their eccentricity. This is a number between 0 and 1, and is a measure of how drawn-out the oval shape is.
If the eccentricity is exactly zero, the ellipse becomes a circle, which can be considered to be a “special case” of an ellipse: one with no eccentricity at all.
In theory, it is possible for an orbit to be circular, but in practice this does not happen. The Earth’s orbit, for example, is very nearly circular.
The orbits of some objects, however, are very noticeably oval shaped. This particularly applies to many asteroids, and especially comets, some of which get very close to the sun and then return again to a very distant point before returning to the inner solar system.
An example of this is the famous Comet Halley, which can pass closer to the sun than Venus, but at its most distant, in the 2020s, it will be farther from the sun than the orbit of Neptune.
If an ellipse is so eccentric so that it is just open-ended — with a tight curve at one end but with the sides not meeting again — it is said to be a parabola.
You can see the shape of a parabola easily by asking a friend to throw a solid ball up at an angle, and carefully watching its trajectory as it rises to a peak height then falls back to the ground.
A parabola is said to have an eccentricity exactly equal to 1.
And yet, there is an even more extreme case: if the orbit is even more open-ended than a parabola, it is said to have the shape of a hyperbola. And it is this feature of the orbit of A/2017 U1 that tells astronomers that it is not from our solar system.
If the object were indeed a member of the sun’s family, it could not be following a hyperbolic path, because there is no hope of it reaching a “most distant point” from the sun and then beginning its journey back again. Measurements of its orbital path have shown it to have an eccentricity of 1.19, which is so much greater than 1 that its non-membership of our system was immediately apparent to astronomers.
So, where did it come from? It would have originated in another star system, but we have no idea which one.
All we know is the direction from where it came. We may never know its true “original” home, but of one thing we can be pretty certain: it will never return there, either. It is a true “rogue” object in interstellar space.
The object was discovered using the Pan-STARRS 1 telescope, which is atop Mt Haleakala on the Hawaiian island of Maui. It is especially designed for this sort of work, in which being able to image the whole sky relatively quickly with a large CCD camera is important.
Hopefully, we shall find more of these objects, but this will always be remembered as the first to be confirmed. Martin George is manager of the Launceston Planetarium (QMVAG).
We may never know its true ‘original’ home, but of one thing we can be pretty certain: it will never return there.