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Get ready for 2014 JO25, the biggest asteroid to fly this close to Earth since 2004. Good news — even a 3-inch telescope will show it!

Every week, a handful of new Earth-approaching asteroids are caught in a net of robotic telescopes and join the ranks of nearly 16,000 other fly-by-night space boulders. Among their number is one 2014 JO25, discovered in May 2014 by astronomers at the Catalina Sky Survey (CSS) near Tucson, Arizona.

Observations made by NASA's NEOWISE mission have pegged the asteroid at roughly 650 meters (2,000 feet) across and twice as reflective as the Moon. That and its orbit are about all we know about this speeding space mountain for the moment.

That should change very soon. Asteroid 2014 JO25 will be making a close approach to Earth on April 19th. Because of its size and proximity, it will be bright enough to spot in a small, backyard telescope and moving fast enough to see in real time.

Closest approach occurs around 12 UT (7 a.m. CDT) April 19th when it zips by at 1.8 million km (fewer than 1.1 million miles) away, or about four times the distance to the Moon. When darkness falls in Europe and Africa that evening, the asteroid will shine at its peak magnitude of +10.7 along the Ursa Minor–Draco border. Several hours later, North American observers can catch it rolling west across Coma Berenices a hair fainter, between magnitude +10.8 and +11.0.

Small asteroids outnumber big ones by several orders of magnitude, making this pass of 2014 JO25 a must-see event. The last time an asteroid this size or larger blew by Earth was in September 2004, when 4179 Toutatis, a 5-km-tall bowling pin, came within about four lunar distances. Not until 2027 will we have another shot at seeing a big rock tumble by. That year, 800-meter-wide 1999 AN10 will cruise within one lunar distance of the hairs on your head.

Telescopes across the planet will be trained on 2014 JO25. Astronomers will seek to determine a rotation rate by watching as the asteroid's light varies in a regular way. Sometimes these variations are obvious visually at the telescope. Watch for changes in brightness as you follow 2014 JO25's progress across the sky. If it's spinning fast enough, you might even catch a couple rotations and see the light variations repeat.

One thing's for sure: During early evening hours on the 19th, the asteroid will cover ½° of sky in just under 30 minutes, or 1′ (arcminute) every minute. That's fast enough to see it creep across the field of view in real time.

Reflectance spectroscopy, where astronomers use a spectrograph to examine how the asteroid absorbs sunlight depending upon the minerals that compose its regolith (soil), can reveal the object's surface composition. Radar observations are planned at NASA's Goldstone Radar in California between April 16–21 and at Arecibo Observatory in Puerto Rico April 12–21. Radar images will show the shape, allow determination of the rotation rate, and reveal surface details as small as a few meters (yards).

Hazardous to your health?

The April 19th encounter is the closest this asteroid has come to Earth for at least the last 400 years and will be its closest approach for at least the next 500 years. Although 2014 JO25 will fly safely past Earth, it's on the list of PHAs (Potentially Hazardous Asteroids), those rocks that are big enough and occasionally pass close enough to Earth to be of concern. PHAs have diameters of at least 100-150 meters (330-490 feet) and pass less than 0.05 a.u (7.5 million km / 4.6 million miles) from our planet. As of March 2017, we know of 1,786 of them. No known PHA is predicted to impact Earth for at least the next 100 years.

2014 JO25 will makes its first appearance for most of us low in the northern sky in Cepheus during evening hours across the Americas on Tuesday, April 18th. At magnitude +13, it will be dim. Over the next 24 hours, this monster rock will sprint southwest across 80° of sky. Come nightfall on April 19th, it will be beautifully placed for viewing in Coma Berenices, a little brighter than magnitude +11, and quickly slowing down. The following evening, April 20th, the asteroid only gets as far as the Cup of Virgo, spinning away a few degrees north of Gamma (γ) Virginis at nightfall. It will be shining at magnitude +12 — still an easy find in a 6 or 8-inch telescope.

That gives us three nights of choice viewing. I've set the charts above for the central United States at latitude 40° N on the best night. As near-Earth asteroids go, 2014 JO25 isn't coming unusually close, so parallax (the apparent shift in the position of a nearby object against the sky background depending on your observing location) won't be objectionable. The asteroid's path will shift about 11′ over 90° of latitude (between 45° N and 45° S) or ~7″ (arcseconds) per degree of latitude. If you're south of 40° N, the path shifts slightly north; if you're north of 40 N°, the path shifts slightly south. Keep that in mind when using the charts.

Freestyle it

While the charts will work for most observers, you can always prepare your own. Here are two easy ways:

1. Download the free home planetarium software Stellarium for Windows or Mac. Once you've chosen your city, click on the configuration icon (lower left side of screen), select Plugins, scroll, and select Solar System Editor. Click Configure and select the Solar System tab. Select Import orbital elements in MPC format, then Asteroids. Under Select bookmark, choose the PHA option and click Get orbital elements. Scroll through the list and find 2014 JO25, then click Add to list. Next, close the dialog boxes, choose a time you want to view the asteroid, and click Search window (magnifying glass in lower-left corner of screen). Type in the name 2015 JO25 and the program will take you straight to it. I know — sounds like a lot of steps, but it's painless. And accurate.

2. If you already have a program like Starry Night, Megastar, etc. that can plot an asteroid path, just go to the Minor Planet and Comet Ephemeris Service, type 2014 JO25 in the open box, scroll to the end of the page, select your software. and click on the Generate ephemerides/HTML button. Save the file that pops up on your screen into your program, open the program, select the asteroid, and create a custom map with time intervals and a magnitude range to your liking.

You've got time to plan for the big night, so print out the maps and play with the software so you can be ready to board the asteroid express Wednesday evening. If bad weather intervenes, don't give up. Astrophysicist Gianluca Masi will share live, real-time images and commentary on his Virtual Telescope Project website beginning April 19th at 19:30 UT (2:30 p.m. CDT).

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