New Delhi, Oct 6: Scientists at  National Aeronautics and Space Administration (NASA) and European Space Agency have spotted the return of Comet 96P by using the Solar and Heliospheric Observatory (SOHO) – making it the spacecraft’s most frequent cometary visitor. On October 25, Comet entered the lower right corner of SOHO and before leaving on October 30, it skirted up and around the right edge.

This is not the first time SOHO has spotted comet 96P. It spotted it way back in 1996, 2002, 2007 and 2012.

At the same time, comet 96P passed through a second NASA mission’s view: STEREO – short for Solar and Terrestrial Relations Observatory – also watched the comet between October 26-28, from the opposite side of Earth’s orbit. It is extremely rare for comets to be seen simultaneously from two different locations in space, and these are the most comprehensive parallel observations of comet 96P yet.

The combined observations will help in learning more about the composition of the comet and its interaction with the solar wind, the constant flow of charged particles from the Sun.

Both missions gathered polarisation measurements of the comet; these are measurements of sunlight in which all the light waves become oriented the same way after passing through a medium – in this case, particles in the tail of the comet. By pooling the polarisation data together, scientists can extract details on the particles that the light passed through. “Polarisation is a strong function of the viewing geometry, and getting multiple measurements at the same time could potentially give useful information about the composition and size distribution of the tail particles,” said William Thompson, STEREO chief observer at NASA’s Goddard Space Flight Center in the US.

Comet 96 also known as comet Machholz, for amateur astronomer Dan Machholz’s 1986 discovery of the comet – completes an orbit around the Sun every 5.24 years.

It makes its closest approach to the Sun at over 17 million kilometres – a very close distance for a comet. In 2012, when comet 96P appeared in SOHO’s view, the amateur astronomers who were studying SOHO’s data discovered two tiny comet fragments some distance ahead of the main body, signalling the comet was actively changing.

Now, they have discovered the third fragment which is another breadcrumb in the trail that indicates comet is still evolving. Comet 96P fascinated scientists as it has an unusual composition and is the parent of a large diverse family, referring to a group of comets sharing a common orbit and originating from a much larger parent comet that over millennia, broke up into smaller fragments.

Comet 96P is the parent of two separate comet groups, both of which were discovered by citizen scientists studying SOHO data, as well as a number of Earth-crossing meteor streams. By studying the comet’s ongoing evolution, scientists can learn more about the nature and origins of this complex family.