It's been so long since a bright comet graced our night sky. Comet ATLAS might be the one to break the monotony!
Comet 2019 Y4 ATLAS (hereafter, comet ATLAS) is a long period comet swooping down from above the solar system. Before closest approach to the Sun it will be well placed for observers in the northern hemisphere. Comet ATLAS will be closest to Earth on May 23rd (117 million km from Earth) and at perihelion (closest to the Sun) on May 31st (38 million km from the Sun).
Comet ATLAS is observable from UK (and latitudes roughly equivalent) until around the end of the third week in May. Here's a star chart I compiled showing the path of the comet while it remains visible from the northern hemisphere.
At the start of April the comet is visible all night for many in the northern hemisphere. It begins this period in the constellation Camelopardalis (a very dim pattern of stars roughly between the seven stars of The Plough and the W-shape of Cassiopeia). The comet is tracking south and eventually passing through Perseus and into Taurus.
There's a lot of excitement (and uncertainty) over how bright Comet ATLAS will become over the next couple of months. Estimates range from magnitude -1 (exceptionally bright; like Sirius or Canopus) to magnitude +4 (think Andromeda Galaxy brightness).
In theory the brightness of an object depends on its distance from both the Sun and the Earth. The correlation is not so straightforward for comets. As they near the Sun their surfaces warm and begin to emit gas and dust, rapidly becoming enveloped by an atmosphere (called a coma). Gas atoms in the coma become excited and begin to glow. The solar wind and radiation (light) pressure both act on the coma to create a tail (often separated into two tails: ion and dust tails) which point away from the Sun. How much gas and dust is released varies enormously from comet to comet! All of these factors influence the overall brightness in way which makes them seem unpredictable at times.
Making predictions about the future brightness of a particular comet requires measurements of its current brightness. Many professional and amateur astronomers carry out measurements (both visually and with cameras) in order to provide a light curve for a particular comet. If the comet has a short period it will have been observed at a previous return and measurements from last time may provide a guide as to how the comet will behave this time. In the case of comet ATLAS there is speculation that it may be a fragment from the Great Comet of 1844! Measurements of the orbit of ATLAS suggest an orbital period of about 5,000 years. If ATLAS is a fragment of the 1844 comet - it separated from it many centuries ago. The fact that ATLAS may be a fragment of a "Great" comet is also cause for excitement!
Whilst there is excitement about the potential of ATLAS becoming a bright comet recent observations suggest the comet has stopped getting brighter. This from noted comet observer and discoverer Terry Lovejoy:
Others remain optimistic, pointing out that other comets have exhibited temporary slowdowns in their climb to maximum brightness such as Comet ISON back in 2013.
I saw the comet through the telescope eyepiece for the first time last week. It was little more than a fuzzy blob of light! I'm hoping to get pictures between now and the end of May but for the next week or moonlight will make that difficult. ATLAS is currently very high in my sky all night so the COVID-19 lockdown will not prevent me observing it from my garden. As the comet gets brighter and drops into the northwest sky...well, I have to hope things change enough that I can travel to a better spot. I'll keep you posted!