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August Asteroids: 3 Juno and 7 Iris at Opposition

This month two of the brightest asteroids, 3 Juno and 7 Iris, will be at opposition in our skies, giving a great opportunity for asteroid hunters to track down these lumps of space rock.

Bear in mind though that you (almost certainly) won’t be able to see them with the naked eye, and that you’ll need binoculars on a tripod or a telescope to find them properly. And even then they’ll just look like very faint stars. But they’re not stars; they’re asteroids, lumps of rock in our solar system orbiting the Sun between Mars and Jupiter.

How big and bright are they?

3 Juno and 7 Iris are amongst the largest of the asteroids, a few hundred kilometres along any one axis. This might seem pretty big but they’re tiny compared to the planets, and so don’t reflect nearly as much light back to us, and are therefore much fainter.

Their magnitudes vary depending on how far away they are from us. They both vary between around seventh and eleventh mag; at their brightest 3 Juno is magnitude 7.4 and 7 Iris is magnitude 6.7. This only occurs under perfect conditions, and this year’s oppositions for both asteroids won’t have them presenting their very brightest aspect. The generally-accepted view is that the human eye can only see down to magnitude 6, but in exceptional circumstances – very dark skies free from light pollution, and very good atmospheric conditions – and with exceptional eyesight, you might just be able to see 7 Iris when it’s closest to us, and at its brightest.

When can I see them?

They’re visible all month but the best time to look at them is when they’re at opposition. That means they’re directly opposite the Sun in the sky, and therefore rise as the Sun sets and set as the Sun rises, getting to their highest due south around midnight.

3 Juno reaches opposition on Sunday 4 August 2013 and it’ll brighten up to magnitude 9. You’ll need a scope, a good star map, and patience to track it down.

7 Iris reaches opposition on Friday 16 August 2013, and it’ll be brighter than 3 Juno, but still not near its best, gaining magnitude 8 during this year’s opposition. Again, a good star chart and telescope is needed.

Where can I see them?

They are both visible in the lower part of the southern sky in the constellation of Aquarius, but you’ll need very detailed star maps to help you find them. 7 Iris is only a degree or so away from the brightest star in Aquarius, β Aquarii, on the night of opposition, making it a bit easier to find. The British Astronomical Association computing section has downloadable star-charts to help you find these asteroids, and others.

How will I know that I’m looking at an asteroid?

The short answer is: you won’t, at least not at first. Asteroids, even the brighter ones like 3 Juno and 7 Iris, will only ever appear as tiny specks of light when seen through a telescope, just like the millions of other tiny specks of light, the stars. However if you observe them over the course of a number of nights around opposition, and mark their position on a star map, then you’ll notice that their position changes relative to the “fixed” stars, as they circle the Sun and move through space.

Don’t be put off if you don’t manage to find them. While you’re out hunting for them don’t forget you can check out lots of other amazing sights through your telescope. Why not have a go at finding the Ring Nebula in Lyra, high overhead this month.

Good luck, and happy asteroid-hunting!

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