Celestial Ships In The Night

Perhaps you heard about the asteroid that passed close by the Earth recently? On the morning of July 3rd, the asteroid designated 2004 XP14 zoomed by at a distance only slightly further away than the Moon's orbit - approximately 270,000 miles. In astronomical terms, that's very close, indeed. As chance would have it, North America was about the best place to be to view the fly-by, so many amateur astronomers had an excellent chance at a first-hand look. Since the asteroid is only 800 meters (1/2 mile) across, even at it's closest approach it was expected to brighten to only 11th magnitude - quite faint for amateur observers. I certainly couldn't see such a faint object from the city. But luckily I was to be in Mammoth Lakes, high in the Sierra Nevada mountains on that evening. Could I find it myself? I took it as my next observing challenge!

So how can one distinguish the asteroid from all the other dim stars in the sky? By it's movement! Due to it's proximity at closest-approach, 2004 XP14 should move across the sky at a rate noticeable through my telescope at modest magnification. However, one still has to be looking in exactly the right spot at exactly the right time! Advance preparation is the key to success in astronomical endeavors such as this.

The first step was to determine the asteroid's path across the portion of the sky visible from my location, synchronized with the time during which it would take that path. The calculations required to accomplish this belong to the mathematical discipline known as astrodynamics. It's an exceedingly complex, calculation-intensive study. In other words, it's hard. Luckily, JPL provides access via the Internet to a program which is designed to solve just this sort of problem. It's called HORIZONS and it can be found at JPL Solar System Dynamics.

HORIZONS Output for 2004 XP14To use it, one simply provides parameters, such as the object in question, one's location, a date/time range and a time interval. With all the necessary information, HORIZONS crunches the numbers and returns a report listing a series of position "snapshots" at the specified intervals, during the specified time range. I generated the report on the right for the period of time in which the asteroid would be at least 30 degrees above the horizon at my location. Turned out the window of opportunity was from 2:45am to 3:45am. After that, the sky would begin to lighten with the approaching dawn. At 5-minute intervals, this meant I had a dozen points along the asteroid's path at which I could point my telescope in the hopes of spotting it.

Tracking Asteroid 2004XP14The numbers HORIZON spits out are great, but they're impossible to visualize. In order to get a visual frame of reference, I entered the points, measured in Right Ascension (RA) and Declination (DEC), into my planetarium program, The Earth Centered Universe. At each point I plotted a circle, centered on the point and with a diameter equal to the field of view of my preferred eyepiece. The resulting plot at left shows the circles laid out against the sky. The center of the bottom circle represents where the asteroid would be at 2:45am - the top circle an hour later. (The image is actually flipped horizontally, to reflect the apparent motion in my telescope. If you looked at the sky directly, the asteroid followed a path upwards and to the left.) You can see that the 5-minute interval between points spaced the circles to overlap slightly. This way, I had a continuous series of visual fields to traverse. This plot also showed that the asteroid would (roughly speaking) traverse each field of view in approximately 5 minutes.

2004 XP14 Telescope CoordinatesSo now I had a good mental picture of the asteroid's motion and a series of continuous fields of view in which to hunt. I figured that if I couldn't find it in an hour, I wasn't going to, dawn or no dawn! The final preparatory step was to enter the dozen coordinates into the telescope's computer. This was tedious, but not difficult. The picture at right shows my worksheet. The ID #'s were important, since they were the key to recalling the coordinates later.

Here's where the power of a "GoTo" telescope shows itself. Once the coordinates are entered and the telescope is precisely aligned to the sky, I simply direct it to center on the coordinate of interest and the scope moves there. However, since I'm not altogether familiar with the performance of my new telescope, I wasn't sure how accurately it would center at the right locations. This was going to be a good test.

At 1:45am, the alarm rousted me out of bed. I'd already packed the car, so I was quickly on my way across (a deserted) town to the dark, open spaces on the east side. There was a steady breeze blowing off the mountains, but it was relatively warm. I found a good spot with unobstructed views to the northeast. The scope was setup and aligned with the sky, and my watch synchronized, with 10 minutes to spare. At 2:35am, I pointed the scope at the first point and waited. Knowing that the asteroid would cross the eyepiece in approximately 5 minutes, I knew that it would enter the field 2 1/2 minutes before and exit 2 1/2 minutes after the calculated center time. So, if everything was precisely correct, the asteroid should enter that first field of view at approx. 2:42 and leave it at approx. 2:47.

The view through the eyepiece revealed a sparse number of dim stars. Luckily, this was not a crowded part of the sky. The minutes ticked by and I strained to detect any motion in the pattern of faint dots - nothing. Once my watch indicated 2:47, it was time to move to the next (2:50) coordinate. The scope purred a short distance and I began looking again. Again nothing. The faint stars - a new pattern - remained stubbornly fixed. But about the time I was beginning to wonder if I could really see the asteroid at all, I noticed a change. In the upper third of the eyepiece one of the faint points of light was definitely shifting relative to its neighbors! The motion was slow - detectable over about 10 seconds - but it was steady - a sure sign of celestial motion! I nudged the scope to center the target in the eyepiece. Yep, it was definitely moving, but was it the asteroid? It was approximately the predicted magnitude, it was moving in the right direction and at the right speed. Everything agreed with expectation. Yep, I'd found 2004 XP14! I was thrilled.

Now it may seem strange that a grown man would experience such joy from a dim point of light silently crossing the sky in the dead of night. But such is the quirky nature of sky watchers everywhere. As I stood there following 2004 XP14 glide slowly across the star field, I thought of all the other astronomers all over the country doing exactly the same thing at exactly the same time. I knew they were enjoying the hunt and cheering the moment of discovery much as I did. And at that moment, standing out there in the chill morning hours, I didn't really feel at all alone. I was together with all those like-minded astronomers, together with a small rock speeding silently by Earth on its own celestial journey - two ships passing in the night, so to speak. Most of all, I felt that ultimate togetherness - the one that, time and time again, drags us out of warm beds at ridiculous hours to peer at the heavenly wonders all around us - the profound realization that we are home in the cosmos. Once again, I was reminded that it is us and we are it.

After a half hour of following this silent visitor's steady passage, my cold hands and feet told me it was time to wrap it up. But first, I could not pass up the chance to visit some of my old favorites of the summer night sky. I slew across to the Hercules Cluster (M13) and its beautiful companion cluster M92, both now well west in the sky. Then down to the Lagoon Nebula (M8) and Trifid Nebula (M20) in the heart of the Milky Way. Up again to the Ring Nebula (M57), high overhead in Lyra, and finally across to the magnificent Andromeda Galaxy. Once again, my old friends reminded me how spectacular the summer sky really is!