Lawrence Garrett
206 River Road
Fairfax, Vermont, USA 05454
Lgasteroid@earthlink.net

WELCOME SPACEGUARD INDIA ASTEROID OBSERVERS!

In this age of giant telescopes like the Kecks, space telescopes such as Hubble and deep space probes such as NEAR, the minor planets have moved from star-like points of light to front-page news as they approach Earth only a few lunar-distances away.

Interests in these space rocks are at an all-time high for both professional and amateur astronomers as well as the general public. But little known to the layman, these objects can be seen with small telescopes and binoculars. During favourable oppositions, asteroid 4 Vesta is visible to the naked eye. This article aims to promote visual observation of such "space rocks" and to guide observers from locating the very brightest to spotting the faintest near-Earth objects that make headlines!

1. Equipment

A small telescope or tripod-mounted binoculars (50mm+ recommended), plus a good knowledge of the night sky are a must to begin asteroid hunting. Sky and Telescope as well as other astronomy magazines and journals publish charts for bright asteroids such as 1 Ceres, 2 Pallas, 3 Juno and 4 Vesta. The "Big Four" is a perfect project for those seeking their first asteroid sighting.

Observers should look for objects above the limiting magnitude of their equipment to avoid mistaking a background star for the target asteroid. The Observers' Page at Spaceguard India will post charts for locating brightest asteroids currently visible. We hope to post successful observations on this page and offer a certificate of achievement for those observing at least 12 asteroids. Join our "Big Four Club" by spotting these as well. Observing asteroids can be fun. Even without large and expensive telescopes, locating bright asteroids - combined with learning - is an excellent and well-rounded observing programme. Remember, asteroids are very under-observed objects, join our ranks tonight!
Levels of visual minor planet observing fall into three categories - first sightings, intermediate and advanced.

1.First Sightings:

If possible, first-time asteroid observers should attempt to observe one of the "Big Four", 1 Ceres, 2 Pallas, 3 Juno or 4 Vesta.

Observers with dark skies and 7x50mm binoculars should be able to track these when they are 9th magnitude or brighter. Binoculars have the advantage of "correct images", correctly oriented North-South and East-West. Telescope-users must remember which way North-South and East-West are in their telescopes.

Make sure that you print and HOLD your charts to match the view in your telescope/binoculars. A good starting project is to make "EASYVIEW Star chart table" as an observing aid.
All asteroid charts show the path of a moving target against the "fixed" background stars. As you are looking for the "extra star", count the days from the start of the track to match the current day. These charts are in Universal Time, not your local time. You will need to do the correction in your local time to match this date.

Circle this position without drawing over any field stars and head out to the telescope. In most cases, brighter asteroids (9th magnitude or brighter) will show right up. You then must watch to see if the suspected asteroid moves along the track in the direction predicted to confirm its planetary nature. Should it cloud over, you can come back the next night to see if your "star" has moved; just draw all the stars near its predicted position.

This is how Clyde Tombaugh discovered Pluto, taking images over a few nights apart. With binoculars, you might not see the asteroid motion the same night, unless placed very close to background stars. In a telescope at 50x or more, the motion should show in an hour or more. The higher the power, the easier it is to detect the motion of the asteroid.

A good tip is to notice any field stars and imagine them to be in a line that the asteroid will cross in its motion. You will be able to watch it cross the "finish" line in this easy way, in three steps. First, before the line, then right on the line (three in a row visible) and finally across the line. Many shapes of field star patterns can both help and hinder viewing asteroid motion. Master this, and get ready to move on the really faint objects!

Intermediate Asteroid Observing:

An excellent astronomy club project and challenge.

Once the basics of finding your correct field of view, and tracking a few bright targets are routine, moving fainter towards your viewing limit becomes your next goal. Depending on your telescope size, and just how dark your skies are, viewing asteroids to the brightness of Planet Pluto can be considered a good measure of skies and observer! Indeed, Pluto should become a target if you have a 8 inch (20cm) or better yet 10 inch telescope (25cm). Finder charts for this planet appear in Sky and Telescope on a yearly basis, but home computer programs that can produce custom finder charts are the best methods for success. With these programs, charts can be made to match the high powers used at the telescope, usually 150x or greater to spot Pluto. Unlike asteroids, you will not see Pluto move in a few hours, but will need to return the next clear night to see its motion. In even a week, the motion will have not carried it too far from where you observed it. Spotting where it has moved from and it is now, will give you a "feel" for its speed in space, as well as the knowledge you have spotted this distant Kuiper-belt object!

Do not be surprised if the object you thought was Pluto, has not moved at all, and was a background star. The Hubble guide star catalog (used in most home programs), does not include all the stars down to Pluto's brightness, and uncharted field stars are bound to turn up. This is routine for asteroids as well and Pluto. You must draw in the missing stars, and then return to confirm your suspected planet or asteroid. This makes finally finding Pluto and asteroids, all the more rewarding, and challenging!! Do not expect instant success! Just how clearly you can see this nearly 14th magnitude major planet will be a good measure of the asteroids you should be able to track in your equipment.

In a subject seldom covered, holding your star charts to match your view in the telescope's high power field becomes a must for success. But how, when the stars move hour by hour and asteroids are at many places in the course of the night (not mention cold hands and wind). I again suggest my own observing aid, the "EASYVIEW Starchart table".

http://www.lpl.arizona.edu/~rhill/alpo/minplan/easy/easyview.html

In this age of computer pointed telescope, electronic imaging of the skies, and very large visual telescopes, finding Pluto still ranks as an excellent observing feat. So practice on Pluto and the faintest asteroids you can see, the fast moving near-Earth objects await!!

Advanced Asteroid Observing:

While the vast majority of stargazers will consider even seeing Pluto as advanced, much greater challenge in spotting very small and fast asteroids lies ahead. With your first observations of the "Big Four" simple date plotting was enough to spot these at low power. Fainter "Pluto Class" objects require charts plotted to each hour. But spotting NEAs will require charts made showing five-minute intervals in time. Spotting an asteroid, which is moving as much as two full moon diameters per hour, can be equally frustrating and rewarding.

Viewing at the very limit of any telescope leads to new demands on the asteroid observer, for example the "period of visibility" of your target asteroid. As you approach your sky and telescope-viewing limit, you will notice the very faintest stars are not visible 100 per cent of the time. Using "averted vision" (looking slightly to the side but paying attention to the target area) will make objects become more visible. With practice, an observer can see it 5 to 10 per cent of time, or seeing the asteroid just one-half to 1 second in a 10-second viewing period. Advance observers such as Gerard Faure and Roger Harvey reach nearly 16th magnitude using this method.

Viewing on the true cutting edge of the sky brings into play the slightest change in sky steadiness and haze. And rapid motion of a very fast-moving asteroid will raise this limit.

So, with an asteroid visible for very short periods of time, looking in the correct spot becomes much harder than before. And just how close the approach to the asteroid itself is, will also enter into its position in the sky.

While there are no hard and fast rules, observers who use home computer programs to make their charts, will find this to be no problem as long as the target asteroid remains 10 or more lunar-distances from earth. Closer than this, time plotting on some programs becomes a problem. For example, a chart may come out correct in the motion track but the asteroid in the sky may be ahead or behind in time (as compared to the chart). To detect if your chart is far off in its predicted positions, you can check the predicted position online at the Minor Planet Center website at:

http://cfa-www.harvard.edu/iau/mpc/html.

Use the Minor Planet Ephemeris Service:

http://cfa-www.harvard.edu/iau/MPEph/MPEph.html.

To compute a very precise predicted position of your asteroid, enter your longitude, latitude and altitude in the observing form boxes, and request the output. Remember to ask the correct universal time, day or you might be looking a day in the wrong position. You should go to these positions and see where your software places the same minor planet. In most cases they should be close, but it is possible even good programs can break down and give positions off in time (like an hour or two ahead or behind).

Getting these time corrections right are a must for "stealth" near-earth asteroids, those not directly visible in the eyepiece and very close to the Earth. After you have the correct positions from the Minor Planet Center, you can create new charts tuned to these positions by moving your plotting track back and forth in time to match close as possible. If your chart was 10 minutes ahead, start your plotting time 10 minutes early and mark the time right on the chart to avoid any problems.

This is chart making at the cutting edge. And last, the asteroid's magnitude itself might be off, perhaps enough so it's never seen or brighter than predicted and easier.

Indeed, most asteroids this faint cannot be +/- 0.3 magnitude, yet another factor in near-Earth observing. They can vary in light over time as well, from brighter to fainter than predicted. It is to little wonder that very few people see these objects, which range in few meters rather than miles, objects that you may end up travelling a distance many, many times their diameter to observe.

Closing:

I hope my review of asteroid observing From Vesta To Potentially Hazardous Asteroids, will be a much help to those of you who care to take the time to seek out these minor solar system members.

Clear skies!

Lawrence Garrett
Assistant Coordinator
ALPO Minor Planet Section/
SPACEGUARD INDIA
Visual Observing Coordinator