My Science School

You might know about comets – cosmic snowballs of frozen gases, dust and rock that orbit the sun. But have you heard about sungrazing comets? A class of celestial objects, sungrazing comets are comets that pass extremely close to the sun at its perihelion or point of closest approach. When we say extremely close, we refer to a few thousand kilometres from the sun’s surface – a small distance on a cosmological scale.

These sungrazing comets have been observed for many hundred years and there is reason to believe that ancient Greeks – philosopher Aristotle and historian Ephorus amongst them – might well have spotted these during their times. sungrazing comets, however, do not lend themselves exactly to be easily observed.

Space-based approach

It is for this reason that only a few sungrazing comets had ever been spotted from ground-based observatories. It was a whole new snowball game altogether from 1979, however, when satellites and space-based observatories started doing our bidding.

The P78-1 satellite, which carried a white-light coronagraph known as SOLWIND, was launched in February 1979 by the U.S. Air Force. Out there to observe and give us insights into solar physics and operational for about five years, SOLWIND’s most important discovery turned out to be a comet.

Masks for corona

While certainly not the first space-based coronagraph, SOLWIND was surely an update on its predecessors. Designed to look at the solar atmosphere and thus monitor activity in the sun’s outer corona, the SOLWIND had a mask or occulting disk. Using this mask, it was able to create the effect of an eclipse artificially, hiding the bright disk of the sun to better observe its corona.

On August 30, 1979, SOLWIND spotted a comet approaching the sun – rather closely than usual – and recorded the data. Delay in analysing the spacecraft data, however, meant that these images were actually seen only a couple of years later.

Shock to surprise

When Naval Research Lab (NRL) scientists Russ Howard, Marty Koomen and Don Michels first looked at the images taken on August 30-31 1979, they were first horrified. On seeing the huge bright streak appear in the data, they first thought that something had happened to the camera, leading to a reflection inside it. Only on further study did they realise that they were actually staring at a feature that was moving and that it was indeed a comet.

It wasn’t long before they figured out that the satellite had captured a sungrazing comet. The Howard-Koomen-Michels comet was the first comet to be discovered by a space-based observatory – a satellite in this case.

Once the floodgates had been opened, it happened repeatedly as observatories in space made discoveries on a regular basis. In fact, they spot new asteroids and comets almost every week! SOLWIND itself was able to discover a number of other comets before it was eventually destroyed by a ground-based missile in 1985 during a planned Air Force exercise.

Naming convention changes

The flurry of discoveries also led to a change in the naming convention as the International Astronomical Union caught up with the fact that satellite discoveries were the norm and not the exceptions. So rather than naming it after the person who identified objects in the image, it is now named after the satellite or survey that found it in the first place. And this meant that comet Howard-Koomen-Michels officially became C/1979 Q1 (SOLWIND). Space observatories that came later have hundreds and thousands of comets named after them.

The Kreutz sungrazers

German astronomer Heinrich Kreutz studied comets that had been observed until then in the 1880s and 1890s.

Apart from figuring out that some of these were sungrazers and some were not, Kreutz was also able to deduce that many of the sungrazers actually follow the same path or orbit.

It was as if these comets were all broken up fragments of a much larger comet from the past. The original comet and its fragments likely broke up repeatedly as they orbited the Sun and approached it closely.

To honour Kreutz’s work, this group of comets was named as the Kreutz sungrazers.

The Kreutz sungrazers get to within about 50,000 km of the sun’s surface, meaning that they reach the lower layers of the solar atmosphere, or the corona.

All the comets discovered by Solwind, including comet Howard-Koomen-Michels, belong to the Kreutz group.

 

Picture Credit : Google

Before landing humans on our moon, one of the important points of consideration, obviously, were the landing sites. In response to this need of obtaining detailed photographs of potential Apollo landing sites, NASA’s Lunar Orbiter program was born. The plan included a series of three-axis stabilized spacecraft to be inserted into lunar orbit, with each of them comprising a main engine and four solar panels.

The first of these was the Lunar Orbiter 1, designed primarily to photograph the smooth areas of the lunar surface. For this, it was fitted with a Eastman Kodak imaging system that weighed 68 kg.

Spy turns scout

The system, which employed both wide and narrow-angle lenses, had the ability to develop exposed films, scan the images and relay them back to Earth. The Eastman Kodak camera flown on the Lunar Orbiters, in fact, had been originally developed by the National Reconnaissance Office (NRO) and used in spy satellites – a truth that was revealed only after the Cold War ended.

While imaging was the primary objective, the spacecraft was also equipped with other instruments to collect data regarding radiation intensity and micro-meteoroid impact, among others. Launched on August 10, 1966, it was placed in an Earth parking orbit before being fired towards the moon.

Stumbling blocks

During its cruise to the moon, the spacecraft experienced failure to its Canopus star tracker (probably due to stray sunlight) and overheating. The former issue was resolved by navigating with the moon as the reference, and the latter was taken care of by orienting the spacecraft at a different angle off the sun.

On August 14, Lunar Orbiter 1 was injected into an orbit around the moon and began working on its objective of photographing nine potential Apollo landing sites, seven secondary areas and some sites on the far side of the moon. It successfully completed this work by August 28, with over 200 images to boast about.

Even though some of the early high-resolution images lacked quality due to smearing, the mission was largely successful as it was able to capture images covering over 5 million sq.km. of the moon’s surface. While the wide-angle images taken by this system showed resolutions up to 0.5 km, the narrow-angle pictures were accurate up to 60-80 m.

First Earthrise

Among these photos was that of the first Earthrise, captured unintentionally. During its 16th orbit around the moon on August 23, Lunar Orbiter 1 took the first photograph of our Earth taken from the moon. The image, which was shot just before the spacecraft was about to pass behind the moon, shows the crescent of the Earth. The image data was transmitted by Lunar Orbiter 1 and received at the NASA tracking station at Robledo De Chavela near Madrid, Spain.

Lunar Orbiter 1 continued working, turning its attention away from photography and focusing instead on engineering goals from September 16. The spacecraft’s condition, however, deteriorated by October 28, forcing the ground controllers to command it to impact onto the lunar surface.

On October 29, on its 577th orbit around the moon, Lunar Orbiter 1 crashed on the moon’s far side to prevent its transmission from interfering with the Lunar Orbiters to come. By the time the Lunar Orbiter program, which consisted of five orbiters, came to an end, 99% of the moon’s surface was photographed down to a resolution of 1m!

Old and new

As for the first Earthrise photo, it proved to be a remarkable image, despite the fact that the image released then was starkly black, wide and had poor resolution. The full resolution of the image wasn’t obtained from the mission data up until 2008.

The Lunar Orbiter Image Recovery Project at NASA Ames Research Center went about their task then, obtaining original mission data from tapes and restoring it to an operational condition by combining modern electronics with 1960s era parts. The result was a beautiful high resolution image of the first Earthrise photograph, a touched up version of which has been used with this story.

 

Picture Credit : Google