ESA Science News

10 Nov 1999

Mercury pass in front of Sun will get SOHO even better data

The celestial encounter between Mercury and the Sun on 15 November could help scientists gather more accurate information on the solar
atmosphere than ever before. This type of planetary transit is a rather uncommon event that happens at intervals of 7, 13, or 33 years.

People in some areas of North and South America will be in the best position to see Mercury since it will pass in front of the solar disk right at
around sunset. Mercury will be invisible to the naked eye.

Protect your eyes!

The risk to your eyes in trying to observe this event is the same as for the 11 August solar eclipse! Observers must observe the same safety
precautions. To prevent irreparable eye damage, certified filters must be installed on binoculars and telescopes and cameras.

The so-called Mercury transit is expected to begin at 21:15:01 UTC and conclude at 22:06:47 UTC — almost 52 minutes. In northeastern
Australia, Papua New Guinea, and Hawaii, observers should be able to see at least part of the transit. The planet will appear similar to a small
sunspot, just like a little dot sailing from left to right. In this particular case, Mercury will pass across the northernmost edge of the Sun in
what’s called a grazing transit.

From an astronomical point of view, the event is only mildly interesting for scientists, but it is a unique opportunity for those conducting
research through ESA’s Solar and Heliospheric Observatory (SOHO). For the first time since the spacecraft was launched in late 1995,
scientists will be able to take advantage of a Mercury transit to improve the quality of data gathered by SOHO on the rarefied solar atmosphere,
called the corona.

From the perspective of the SOHO spacecraft, 1.5 million kilometres from Earth, the closest Mercury will get to the Sun will be at an angle of
about 92 arc seconds. This means that SOHO will not see Mercury crossing the Sun’s disk — the planet will pass just above the disk, right in
front of the corona. By so doing, the planet will intercept a small part of the coronal light. Two instruments aboard SOHO, the Extreme
Ultraviolet Imaging Telescope (EIT) and the Coronal Diagnostic Spectrometer (CDS) will observe the phenomenon.

In theory, all the coronal light blocked by Mercury should look like a couple of perfectly black pixels in the images sent to Earth both by the
EIT and CDS instruments.

“We know that whatever is not black is scattered light inside the instrument,” said Bernhard Fleck, SOHO’s Project Scientist. “We need to
distinguish between light from the solar disk and that from the corona. That will allow us to get more refined measurements of the corona.”

The difficulty in obtaining accurate data from coronal measurements is caused by the amount of light coming from the solar disk (much
brighter than the dim corona) that bounces around inside the instruments. This scattered light is also referred to by scientists as stray light.

“Now, with the Mercury transit, we can measure exactly how much of the light coming from the corona is real light and how much is stray
light from the disk.” said Fleck. By finding the exact amount of stray light reaching the instruments, “the key result will be a significant
improvement in the quality of the data we gather, more meaningful and more reliable” Fleck said. “Then we’ll be able to make real statements.”

Mercury is the planet closest to the Sun. Its orbit brings it to a minimum distance of 46 million kilometres from our star and to a maximum
distance of 69.8 million kilometres. Mercury has a heavily cratered appearance, very similar to that of our moon, and has no atmosphere.
Temperatures on its surface can reach 400 degrees centigrade.

“Mercury’s transit in front of the Sun is quite symbolic: It marks a celestial encounter between present and future objectives of our Science
Programme,” said Jean-Pierre Lebreton, Study Scientist for ESA’s Mercury sample return mission, an advanced study carried out after the
Venus Sample Return Mission study, in preparation for the long-term exploration of the Solar System.

A mission to Mercury, called BepiColombo, is one of the ESA science programme “cornerstone” missions, to be launched in 2009. According to
preliminary studies completed in April 1999, it would include a planetary orbiter, a small magnetospheric orbiter and a surface element for
in-situ analysis of surface samples.