How to Spot an Asteroid with Mobile Astronomy Apps

As they orbit the sun, the larger asteroids in the main belt between the orbits of Mars and Jupiter can be viewed in backyard telescopes, if you know where to look. The SkySafari 5 app allows you to search for asteroids by name and display their location on any date. In September 2016, asteroid Pallas — indicated by the white crosshairs — is traversing the small constellation of Equuleus, located in the eastern evening sky. Its looping trajectory is shown here for an 18-month period.

Credit: SkySafari for Android and iOS

Yesterday, NASA launched its OSIRIS-REx mission to visit asteroid Bennu. Once there, its spacecraft will take a sample of the asteroid and return the material to Earth. In this edition of Mobile Astronomy, we'll focus on asteroids, and how you can see one in a backyard telescope with the help of an astronomy app.

Our solar system is littered with bits and pieces of rocky and metallic material that either was never used up during the initial planets' formation 4.6 billion years ago, or that has broken off of larger bodies during subsequent collisions. The sizes range from small particles up to individual objects that are 1,000 kilometers in diameter. 

The larger bodies are of most interest for several reasons: We might wish to visit them to harvest the minerals they are made of, and we would want our spacecraft to avoid accidentally colliding with one. And we definitely want to keep an eye on any that might collide with the Earth, like Bennu! [OSIRIS-REx: A Video Primer on NASA's Mission to Study Asteroid Bennu]

What's an Asteroid?

During the early formation of the solar system, as the primordial gas cloud collapsed and the density of material increased, atoms and molecules of heavy elements (i.e., heavier than helium) clumped together into small fragments. These in turn attracted more material through mutual gravitation. Most of the growing lumps of matter coalesced into planets or moons, while the rest became inert fragments orbiting the sun in interplanetary space. It turns out that the maximum size for the objects is about 1,000 kilometers (620 miles), and there are relatively few of those. As the size decreases, the population rapidly rises, to such an extent that the smallest ones are too numerous to count.

A large percentage of the bodies ended up orbiting in a large belt between the orbits of Mars and Jupiter. Those are the ones we refer to as asteroids. Other pieces were eventually shepherded into the stable gravity wells, called Lagrange points, around planets. Jupiter, with its huge mass, has collected a great many of them, known as Trojans. The rest orbit the sun in roughly circular orbits of various sizes and tilts, occasionally being redirected into a new orbit by a close encounter with a larger object or violently crashing into the surface of a planet or moon. The Earth and moon bear the scars of these past collisions. The meteorites that streak through our atmosphere as shooting stars and land on the ground come from the same material, and are composed mainly of objects that are less than a meter in diameter. 

The number of known asteroids increases as their size decreases. The term asteroid covers objects down to 1 meter in diameter.

Credit: Marco Colombo, DensityDesign Research Lab

Researchers refer to all of these bodies as minor planets, regardless of where they are located. Because some objects, called Near Earth Objects or NEOs, pose a small risk of striking the earth, amateur and professional astronomers have been detecting, tracking and tabulating them for many years. The Minor Planet Center is the worldwide central repository for all of that data, and it is operated by the International Astronomical Union and NASA. At present, more than 721,700 objects have been discovered, of which nearly 14,700 are NEOs. Asteroid Bennu, the destination of OSIRIS-REx, is a NEO that might impact the Earth in the next century. NASA and other agencies are working on ways to deflect NEOs. Comets, which are primarily icy and have highly elongated orbits, are not counted as minor planets. 

The MPC website has a lot of information about asteroids, geared both toward professionals and the public. The site's new Asteroid Data Explorer, featuring informative videos and more, is geared toward teachers. 

Spotting an Asteroid

You don't need to be a professional astronomer or have special telescopes to see an asteroid for yourself! The objects in the asteroid belt, like planets, shine with reflected sunlight, so they can be spotted visually if they are not too remote or too small. As they orbit the sun, they move across the sky, tracing a path through the distant, fixed stars.

Take a photograph, or make a sketch, of a patch of sky, and then do it again a few days later. Any object that has moved is either a comet or asteroid. Astronomers use the brightness of the object and the speed and direction of its motion, to determine its orbit and size. The first asteroids discovered were the largest ones, since they were easiest to detect in early telescopes.

The first — and largest — asteroid discovered was Ceres, named after the goddess of agriculture (think "Cereal"). Between Jan. 1 and Feb. 11, 1801, Italian astronomer Giuseppe Piazzi watched it move through the stars of Taurus. You can use your astronomy app to see what Piazzi saw. Set your sky-charting app to 8 p.m. on Jan. 1, 1801, and search for Ceres. (Your app might bring up the name (1) Ceres, as asteroids are given numbers in order of discovery.) By advancing the date through February 11, 1801, you will see Ceres move in a looping arc to the right of Taurus the Bull's triangular face, below the Pleiades cluster. I find that it helps to center your app on any nearby star, and then let Ceres move across the display as you change the date. Notice that toward the end of the observing period, Piazzi would have seen Mars enter the field on a converging path with Ceres.

Between Jan. 1 and Feb. 11, 1801, Italian astronomer Giuseppe Piazzi discovered the first asteroid, which was named Ceres after the goddess of agriculture. He tracked the motion of the object as it moved through the stars of Taurus the Bull. You can simulate his discovery by selecting those dates in your astronomy app.

Credit: SkySafari for Android and iOS

 On March 28, 1802, while trying to confirm Ceres' orbit, German astronomer Heinrich Wilhelm Matthias Olbers discovered the second asteroid, (2) Pallas, which is 512 kilometers (318 miles) in diameter. It's also a nice target for you to try and observe in September, although a bit dim for the smallest telescopes. Use your app to find the little constellation of Equuleus the Little Horse. (It is just to the upper right of Pegasus.) The bottom star in Equuleus is a naked-eye star named Kitalpha. On Sept. 9, 2016, (2) Pallas is located 50 arc-minutes (just less than a full moon's diameter) to the upper right of Kitalpha. As the days pass, the asteroid will move down and to the right, slowly pulling away from the star. 

Try to spot and track the asteroid in your telescope. In the SkySafari app, you can simulate your telescope and eyepiece, and display a circle indicating the field of view. At low power, your telescope should fit both the star and the asteroid in the field of view, but remember that your telescope will probably flip the image left to right. If so, you can enable the image flipping in your app to match it. (Using the moon is a good way to figure this out. Then write it down, as it will always be the same for that telescope.) 

Put the star near one edge of your field of view and then look for the asteroid. The two objects are the brightest ones in that patch of sky. Make a sketch of all the major "stars" in the circle. The next evening, or a few days later, repeat the exercise and compare the sketches. The object that moves, or disappears out of the field, is the asteroid. 

A close-up view of the path of asteroid Pallas as it passes near the naked-eye star on the dates around Sept. 9, 2016. The blue circle represents the field of view of a typical backyard telescope at low power.

Credit: href="http://skysafariastronomy.com/">SkySafari for Android and iOS

As it happens, you can also observe Ceres for yourself this autumn. As September opens, Ceres is situated in the constellation of Cetus the Whale, and it rises about 10:30 p.m. EDT. At a visual brightness of magnitude 8.2, Ceres is within reach of a modest backyard telescope. Even better, a month from now, Ceres will rise at about 8 p.m. local time and will have brightened to magnitude 7.6. SkySafari and other astronomy apps can display the path of the object, so you can see where it will be on a given date.

The SkySafari 5 app provides users with observing and technical information about the major asteroids and information about their discovery.

Credit: SkySafari for Android and iOS

The Asteroid Sample-Return OSIRIS-REx Mission

NASA chose asteroid (101955) Bennu for its OSIRIS-REx asteroid mission for several reasons. The asteroid is relatively easy to travel to in a reasonable amount of time, it is massive enough to have sufficient gravity for the spacecraft to be pulled close to it, its slow rotation allows the spacecraft's sample arm to touch it briefly, and it is one of few targets with a primordial composition that will yield important information about the early solar system.

After launch, the spacecraft will spend two years matching speed and orbit with Bennu, then spend more than a year mapping the asteroid from a distance of approximately 5 kilometers (3.1 miles) to plan the sample-collection rendezvous. In July 2020, the spacecraft will position itself a few meters above the surface and extend a sample collecting arm to collect anywhere from 60 grams to 2 kilograms of surface material. In September of 2023, a capsule containing the sample will crash-land on earth, where it will be retrieved by excited scientists. [Infographic: How NASA's OSIRIS-REx Asteroid Mission Works]

When an asteroid is large enough, we can bounce radar off it to measure its distance and estimate its size and shape. This has been done in great detail for Bennu, with the use of the giant radio telescope in Arecibo, Puerto Rico. Unfortunately, Bennu can't be observed with amateur telescopes, but you can see where it is if you are an early riser. Use your astronomy app to find the dim constellation of Cancer, which sits low in the eastern predawn sky. The asteroid is presently sitting in the lower part of the constellation, and is heading lower into Leo.

On Sept. 8, 2016, the OSIRIS-REx spacecraft departs for a rendezvous with asteroid Bennu. Once there, it will study the asteroid for a year, and then collect a sample and return it to earth in 2023.

Credit: NASA

Going Beyond

When NEOs occasionally fly past us, usually well beyond the moon's orbit, they move across the sky very rapidly, showing movement against the stars over the course of just a few minutes. Most of these are far too small to see visually, or with small telescopes, but a rare few are large enough. Asteroid Tracker for Android and iOS is an app that lists the upcoming passes, including the object's name, closest distance and mass. It also lists the predicted impact hazard level. The website Slooh.com frequently streams live broadcasts of the passes as well as many other interesting astronomical events, complete with video feeds through telescopes and commentary by experts.

In future editions of mobile astronomy, we'll look at photographing objects with your smartphone, some cool astronomy virtual-reality apps and hardware, how to use astronomy apps in the classroom and more. Until then, keep looking up!