Two comets have the potential to reach naked-eye visibility in 2018. Hyperbolic comet C/2017 S3 (PanSTARRS) comes to perihelion in mid-August and short-period comet 46P/Wirtanen makes a very close approach to Earth in mid-December.
Comet C/2017 S3 PanSTARRS
The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) is located at the Haleakalā Observatory on the Hawaiian island of Maui. Its major goal is to discover Earth-approaching objects, particularly those which may pose a danger to our planet. Because it is looking for relatively rapidly moving objects against the background stars, it has detected quite a few comets. The survey team in charge of the telescope discovered comet C/2017 S3 (PanSTARRS) on 17 August 2017. The orbital elements (as of May 2018) are as follows:
| eccentricity | 1.000 |
|---|---|
| inclination | 99.04° |
| perihelion distance | 0.208 au |
| perihelion date | 15 August 2018 |
With an eccentricity of just over 1, comet C/2017 S3 (PanSTARRS) is on a hyperbolic orbit. This is its first and last visit to the inner solar system. The high inclination indicates that the comet is coming in at almost a right angle to the ecliptic. It will make its closest approach to the Sun on 15 August at a distance of 0.2 au. This is about twice as close as the planet Mercury ever gets to our star.
The MPC predicts that the comet may become as bright as fourth magnitude with JPL forecasting a dimmer fifth magnitude. In either case, it should be visible in binoculars. It is a morning sky object in early August, skimming past the bright stars Castor on 4 August and Pollux a little over a day later. Observers in northern latitudes will be battling both morning twilight conditions and low altitudes as the Sun is not far from the constellation of Gemini. Southern hemisphere comet chasers are unlikely to spot this visitor. The comet vanishes in the solar glare well before perihelion, not reappearing until much later in the year as a rapidly fading telescopic object.
Comet 46P/Wirtanen
Comet 46P/Wirtanen is named for its discoverer, American astronomer Carl A. Wirtanen. He found the object on a photographic plate on 17 January 1948. This plate was exposed two nights earlier as part of a stellar proper motion survey conducted at Lick Observatory on Mount Hamilton in California. The orbital elements (as of October 2018) are as follows:
| eccentricity | 0.659 |
|---|---|
| inclination | 11.75° |
| semi-major axis | 3.091 au |
| period | 5.43 years |
| perihelion distance | 1.056 au |
| perihelion date | 12 December 2018 |
| closest approach to Earth | 0.077 au |
| date of closest approach to Earth | 16 December 2018 |
46P/Wirtanen is a short-period comet with a perihelion distance just outside the orbit of Earth. The orbit is highly elliptical and inclined more than 11° to the ecliptic. Compare these numbers with those of hyperbolic comet C/2017 S3 (PanSTARRS).
46P/Wirtanen was the original target of the Rosetta mission. The spacecraft was to orbit the comet and place a lander on the surface. However, the launch window was missed and the European Space Agency instead aimed Rosetta at Comet 67P/Churyumov-Gerasimenko where it completed a highly successful mission between August 2014 and September 2016.
This is the twelfth return of the comet since its discovery and a particularly good apparition. (The 1980 perihelion was unobserved because the comet was too close to the Sun to be seen.) The comet comes to perihelion on 12 December and is closest to the Earth (approximately 30 lunar distances away) just four days later. Because perihelion occurs just outside Earth's orbit, this means that the comet is actually at opposition in December and visible for most of the night. Some sources predict that the comet may become as bright as third magnitude although others are much less optimistic, forecasting eighth magnitude at best. Binoculars or a small telescope should reveal the comet in either case. Racing northwards through the constellations Eridanus, Taurus, and Auriga, it it is well-placed for viewing from both hemispheres, with the south favoured early in December and the north later in the month. The comet is found near the stars ο Tauri and ξ Tauri on 12–13 December. It is just west of the Pleiades open cluster on 16–17 December and has a close encounter with first-magnitude star Capella on 23–24 December.
