Asteroids, asteroid discoveries and You

Nandivada Rathnasree, ASI POEC

Nehru Planetarium, Nehru Memorial Museum and Library, New Delhi

When was the first discovery of an asteroid? Ceres, the first of the asteroids to be discovered, was detected on the first night of the 19th century. What a start to a new millenium!

It was observed/detected by Guiseppe Piazzi. And shortly thereafter, three others were discovered.

But, how? How did he make this discovery? What are these objects?

Lets first take a look at the origin of the word planets or wanderers. They were observed to be wandering objects in the sky and hence named planets by the Greeks (of the ancient civilization).

Wandering. What kind of wandering? Well, the night sky is filled with stars and these stars rise and set arising from the spin of the Earth. When the Earth completes one spin, you may expect the stars to be back in the exact same location in the sky, right? However, due to the rotation of the Earth around the Sun, there is a small difference in this, and the exact same position in the sky is occupied by the stars four minutes earlier than the time when Earth returns to the same orientation after one spin.

So, that is how stars behave in the night sky over a period of a year.

What about other objects in the night sky? Moon’s behaviour is the most noticeably different, compared to the stars. In a given night, you can detect the Moon’s angular distance from any nearby star change quite a bit. Sometimes the change is such that a nearby star seems to disappear behind the Moon and reappear again on the other side, about an hour later. Such events are Lunar Occultations. So, because of the motion Moon has around the Earth, its movement in the sky over one night is different from the background stars.

Another thing to notice here - the movement of the Moon in the sky arising from its motion around the Earth, is in a direction opposite to that due to the spin of the Earth. Earth’s spin makes objects in the sky appear to move from East to West, and the Moon does do that, rise in the East and set in the West, right? However, its own movement around the Earth would make it move West to East. The result being that the Moon moves East to West, slower than the stars in the sky.

The Moon moves West to East in the sky in a night, with respect to the background stars! Its anomalous motion with respect to stars is the reason why it can sometimes come in front of a star and occult it.

What about planets? They too rise in the East and set in the West on any given night, arising from Earth’s spin. However, their movement arising from their orbit around the Sun makes them move relatively West to East with respect to the background stars, when observed over a few days. This is their Prograde motion. Sometimes, when Earth crosses them in its orbit around the Sun and overtakes them in the orbit around the Sun, the motion of the outer planets becomes East to West with respect to the background stars. This is their Retrograde motion.

Planets, from ancient times, were observed in these wandering motions - prograde to retrograde and back to prograde, making loops in the sky. Any solar system object which had these anomalous motions were thought of as planets, unless they had extended tail like nebulosity and were comets.

When Ceres was first discovered, in an orbit in between that of Mars and Jupiter, where a planet was expected, it was thought to be a planet. Very soon following that Pallas, Juno and Vesta, three other such wandering objects were discovered. It was then thought that perhaps these are fragments of a larger planet which was earlier orbiting between Mars and Jupiter. Modern thoughts about the origin of these swarms of minor planets center around the possibility of a ring of nebular mass which was prevented from becoming a planet due to the gravitational perturbation of Jupiter.

Now, coming to that first discovery by Piazzi. It is celebrated through this painting by Giuseppe Velasco

Where Piazzi is pointing out the newly discovered object.

However, it would definitely not have been anything so easily discernable as in the painting.

His discovery came about in the following way.

Piazzi was involved in a program of accurate measurements of stellar positions. This involved repeated measurements of Geocentric positions of stars.

In this process, through telescopic observations, he noticed a celestial object, whose position seemed to have changed the next night. Continued observations from January 1st to 4th, in that year 1801, convinced him that there was genuine movement of the position of this object with respect to the background stars and thus it was a solar system object, which he presumed maybe a comet.

By January 11th the object’s motion had changed from retrograde to prograde and the absence of any nebulosity detectable, convinced him that this was a new planet.

Within a year of this, the next such object, Pallas was discovered by Olbers. By studying the information available about just these two objects, William Herschel came to the conclusion that these were a new class of Solar system objects being observed and in a letter to Piazzi proposed that they be named asteroids.

It was only as more and more objects were detected, that the idea of the presence of a swarm of such bodies came about, and the name asteroids became accepted. They were deduced to be much smaller bodies than the planets.

In essence, the detection of these objects is based on determining changes in their positions with respect to the background stars, analogous to other solar system objects.

Here, for instance is the change in position of Ceres as may have been observed by Piazzi, the first four days of his observations.

He did not image and then detect these position changes with respect to the background stars, but, inferred the same by measuring the position of the new object he noticed, every day. By the way, he named this new object Ceres Ferdinandea.

Modern day detections also use the same principle, of something moving against the background of the stars, however, it is now done using digital images of starfields and using the technique of blinking different images of the same star field to see if there is one object in the field whose position changes.

Can you find out the differences between the two images of tulip fields shown here? It will take some careful scanning of the two images to arrive at that, isnt it?

Instead, take the two individual images and create a gif animation out of them.

It is very easy to see what are the differences between the two images, now, isnt it?

This is the technique to be used, to determine whether a dot of light seen in an image of a region of the sky, has an anomalous movement against the background stars.

Shown here is a gif animation created from the starfield around Ceres, on the evening of January 1st-4th, 1801, the discovery positions, recreated using the Stellarium software. Piazzi detected this movement through measurement of the position of Ceres, not through imaging as can be done now.

Do you know that using this method you could make an asteroid discovery of your own?

You may wish to make amateur surveys of the sky and lookout for new asteroids and comets as amateur astronomers of yore spent years of passion on such nightly surveying.

However, with many objects having already been discovered, the current situation is such that years may frustratingly be spent on such manual surveying of the skies before (or without) finding that one single anomalous dot in the sky which maybe an undiscovered object.

There are many automated surveys of the night skies, towards discovering new asteroids. One such project is the International Asteroid Search Collaboration http://iasc.hsutx.edu 

The International Astronomical Search Collaboration (IASC) is an educational outreach program for high schools and colleges. It provides high quality astronomical data to students around the world. Students are able to make original astronomical discoveries and participate in hands-on astronomy. This service is provided at no cost to the participating schools!

In this collaboration, images from the following telescopes, a 50" f/14 Cassegrain Telescope, Western Kentucky Observatory,  a 24" Nighthawk CC06 Telescope Sierra Stars Observatory, a 32" Schulman Telescope, Mt. Lemmon SkyCenter and a 32" Ritchey-Chrètien Telescope, Tarleton State University are taken and made available to students worldwide to undertake the necessary analysis using software Astrometrica, also provided by the IASC collaboration.

At the student’s end, the images provided from the IASC need to be analysed and ultimately through the blinking of different images and scanning by eye to notice anomalous movements, an opportunity for making new asteroid discoveries is given to the participants. Why don’t you go the IASC website and try to participate in this process?

Do you know that there are more than 50 such asteroid discoveries which have been made by Indian students? The image below shows the evening sky on the 30th of June 2017, the International Asteroid Day. In this sky image, the positions of some of the asteroid discoveries by Indian students are marked with the provisional names given to these asteroids. The dots indicating the asteroids themselves have been made brighter than what they should be, to make them visible on this skymap.

Evening sky map for the 30th of June for Delhi, showing the locations of some of the student asteroid discoveries.

There are also other such possibilities. The website http://www.asterank.com/discover for instance.

In addition to making attempts at discovering asteroids, you may also wish to know more about these interesting objects and what is more, observe a bright, known asteroid on your own, using a telescope.

If you wish to make these observations on the 30th of June 2017, the International Asteroid Day, there are these two asteroids in the evening sky which could be attempted for viewing through a moderate aperture telescope.

Evening asteroids around 8 PM on the 30th of June 2017

Location of Vesta and Juno in the evening skies on the 30th June

Scanning the parts of the sky indicated in the all sky map for the concerned asteroid, with a pair of binoculars using the maps below which give closer views around the asteroid would help with this excercise.