Planets and Comets Can't Hide from SOHO's Eye

By: Bob King February 3, 2021 7

With the help of the Solar Heliospheric Observatory you can not only keep track of the planets in the daytime sky but maybe even discover a comet.

Daytime planet fest — The Solar and Heliospheric Observatory can photograph rare events that can't be seen from the ground, like this grouping of four planets and the Pleiades near the Sun that happened on May 15, 2000.
*ESA / NASA / SOHO*

Nature is replete with rhythms. Earth spins, planets revolve, variable stars pulsate, and the moon waxes and wanes. Many of us watched Jupiter and Saturn squeeze together during the recent Great Conjunction, then followed them until they disappeared in the solar glare in January.

Jupiter and Saturn near solar conjunction — SOHO's LASCO C3 coronagraph made this photo of Jupiter and Saturn on January 26, 2021, when they were within 2° of the Sun. Horizontal lines through the planets, called *pixel bleeding*, are from overexposure. Cosmic rays from the Sun and other sources leave bright trails.
*ESA / NASA* */ SOHO*

Proximity to the Sun during solar conjunction temporarily interrupted our view of the planets. Broke the rhythm as it were. Saturn and Jupiter won't return to visibility until the end of February at dawn. But why wait? I got on the web, dialed up the Solar and Heliospheric Observatory (SOHO) site, and watched each planet sidle up to the Sun during their transition from the evening to morning sky.

The Solar and Heliospheric Observatory keeps watch 24/7 on the Sun from its hovering post at the first Lagrangian Point. Other similarly stable Lagrangian Points are also shown. Not to scale.
*NOAA with SOHO inset by ESA*

Launched in December 1995, SOHO is a joint venture between NASA and the European Space Agency (ESA). It orbits near the first Lagrangian Point (L₁), a region where the gravitational tugs of the Sun and Earth are at equilibrium, providing a 'parking place' for the spacecraft. L₁ is located 1.5 million kilometers inside Earth's orbit, an ideal spot for SOHO to keep a continuous watch on our tempestuous star.

Among its suite of instruments is the Large Angle and Spectrometric Coronagraph Experiment (LASCO) that comprises three telescopes dubbed C1, C2, and C3, each of which employs an opaque occulting disk to block the Sun from view. On Earth, the atmosphere scatters sunlight and makes the sky blue, but in airless space, the coronagraphs capture crisp views of the solar corona as well as coronal mass ejections — powerful eruptions of plasma that occur in the wake of solar flares and prominence ejections. They also record everything else crossing the field of view including stars, planets, asteroids, comets, and even a few deep-sky objects.

LASCO C1 went belly-up in 1998 after a bad command caused a loss of communication and other issues, but C2 and C3 came through unscathed. LASCO C3 has a field of view about 16° across, equal to 45 million kilometers at the distance of the Sun, or half the diameter of Mercury's orbit. The C2 coronagraph focuses in more narrowly; its field of view encompasses about 3°.

SOHO transit almanac — This diagram shows the paths of celestial objects that will cross the eye of the LASCO C2 and C3 coronagraphs in 2021. Each line or arc represents the path of a single object. Outer planets and stars always move from left to right across the view (because SOHO tracks the *Sun*), while the inner planets Mercury and Venus can travel in both directions — from left to right when passing between the Earth and Sun and right to left when orbiting around the backside of the Sun.
*Worachate Boonplod*

To anticipate what comes and goes under the coronagraphs' gaze, Thai amateur astronomer and SOHO comet hunter Worachate Boonplod created a 2021 coronagraph transit calendar and list of events (below). Each line in the image above represents the path of an object transiting either coronagraph's field of view. Arrows indicate the direction of motion. Objects labeled 323P, 342P, and C/2020 S3 are comets. The limiting magnitude for the LASCO C3 instrument is about 8.0 – 8.5 and 8.5 – 9.0 for LASCO C2.

Visibility in LASCO C3	Object	Magnitude	Direction
Jan. 19 – Feb. 8	Jupiter	–1.9	Left to right
February 4 – 12	Mercury	+5	Left to right
Feb. 22 – Apr. 25	Venus	–4	Right to left
March 2 – 19	Neptune	+8.9	Left to right
Mar. 30 – Apr. 15	1 Ceres	+9	Left to right
April 11 – 25	Mercury	–2	Right to left
Apr. 22 – May 9	Uranus	+5.9	Left to right
June 5 – 16	Mercury	+5	Left to right
July 25 – Aug. 9	Mercury	–2	Right to left
Sept. 14 – Nov. 1	Mars	+1.7	Left to right
October 6 – 13	Mercury	+5	Left to right
October 18 – 20	342P/SOHO	+7	Left to right to left
Nov. 11 – Dec. 14	Vesta	+7.5	Left to right
Nov. 15 – Dec. 13	Mercury	–1	Right to left

This is a list of transits that will be visible in the C3 coronagraph in 2021.

Bookmark the SOHO images site, and when you're in the mood for some daytime observing or to see any of the events listed above, click on either the blue LASCO C3 or red LASCO C2 photo for the most current high-resolution image. To see archived photos, click on the orange More 512 × 512 links below the images. You can also watch an animation of each day's image-take at the LASCO coronagraph site.

SOHO soronal mass ejection and new comet — Recently discovered Comet SOHO (C/2020 X3) moves sunward from the lower left in this LASCO C2 time lapse from December 14, 2020. At the same time, the Sun launches a coronal mass ejection. The comet, about the size of a semitruck, was traveling at 724,000 kilometers an hour and later disintegrated due to the intense solar radiation. Worachate Boonplod discovered it a day earlier.
*ESA / NASA / SOHO / Karl Battams*

A number of amateur astronomers routinely monitor SOHO images to find new comets in the Sun's vicinity that are otherwise invisible in daylight from the ground. More than 4,100 have been discovered to date including 600+ by Boonplod. Most of them are Kreutz sungrazers, fragments of a much larger comet that broke up centuries ago that continue to orbit the Sun.

To participate and potentially spot a new comet, check out The Official Guide to SOHO Comet Hunting or participate in The Sungrazer Project. If you regularly monitor the photos you'll also be among the first to catch sight of coronal mass ejections, massive outbursts of solar plasma launched into space that can spark spectacular auroras.

Jupiter in the C3 coronagraph — February 1st was Saturn's last day in the LASCO C3 field, but Jupiter remains in this C3 photo taken early on February 3rd, along with Iota (ι) (magnitude 4) and Upsilon (υ) Capricorni (magnitude 5).
*ESA / NASA / SOHO*

Comments

Anthony Barreiro

February 3, 2021 at 3:42 pm

It didn't make sense that outer planets would move through the field of view from left to right, i.e. east to west, when they're orbiting the Sun from west to east, so it seemed that they should be moving right to left. It took me a minute of waving my hands around while visualizing the orbits of an outer planet *and the Earth* to realize that the outer planet appears to move from left to right because the Earth is moving faster than the outer planet. Sort of the inverse of why the outer planets appear to move retrograde when they're at opposition. It also helped to remember that the outer planets are left of the Sun in evening twilight before conjunction, and right of the Sun during dawn after conjunction, at least as seen from Earth's northern hemisphere.

Such a complicated and interesting solar system!

SOHO is a venerable instrument. Twenty-five times around the Sun!