46P/Wirtanen

46P/Wirtanen is a small short-period comet with a current orbital period of 5.4 years. It was the original target for close investigation by the Rosetta spacecraft, planned by the European Space Agency, but an inability to meet the launch window caused Rosetta to be sent to 67P/Churyumov–Gerasimenko instead. It belongs to the Jupiter family of comets, all of which have aphelia between 5 and 6 AU. Its diameter is estimated at 1.4 km. In December 2019, astronomers reported capturing an outburst of the comet in substantial detail by the TESS space telescope.

Discovery
46P/Wirtanen was discovered photographically on January 17, 1948, by the American astronomer Carl A. Wirtanen. The plate was exposed on January 15 during a stellar proper motion survey for the Lick Observatory. Due to a limited number of initial observations, it took more than a year to recognize this object as a short-period comet.

Perihelion passages
The July 2013 perihelion passage was not favorable, only reaching a magnitude of 14.7. Between January 23 and September 26 of 2013, the comet had an elongation less than 20 degrees from the Sun.

On 16 December 2018 the comet passed 0.07746 AU from Earth, marking one of the 10 closest comet flybys of Earth in 70 years. The comet reached an estimated magnitude of 3.9, making this pass the brightest one predicted, and the brightest close approach for the next 20 years. The comet experienced six outbursts, with the comet brightening by −0.2 to −1.6 magnitudes.

The 2018 close approach, combined with Wirtanen's brightness provides an opportunity to study a potential future spacecraft mission target in detail. A worldwide observing campaign was organized to capitalize on the favorable circumstances of the 2018 apparition.



Exploration proposals


The comet was the target for the proposed Comet Hopper mission, which reached the finalist stage in the NASA Discovery program. It was one of only three missions in that selection to have a more detailed study. The selection process was ultimately won in 2012 by the InSight mission, a Mars lander. The Comet Hopper was designed to use the ASRG, the Advanced Stirling Radioisotope Generator.

The Comet Hopper mission, if it were selected, would have had multiple science goals over the 7.3 years of its nominal lifetime. At roughly 4.5 AU the spacecraft would rendezvous with Comet Wirtanen and begin to map the spatial heterogeneity of surface solids as well as gas and dust emissions from the coma - the nebulous envelope around the nucleus of a comet. The remote mapping would also allow for any nucleus structure, geologic processes, and coma mechanisms to be determined. After arriving at the comet, the spacecraft would approach and land, then subsequently hop to other locations on the comet. As the comet approached the Sun, the spacecraft would land and hop multiple times. The final landing would occur at 1.5 AU. As the comet approached the Sun and became more active, the spacecraft would be able to record surface changes.

Also, 46P/Wirtanen was the original destination of the European Space Agency's Rosetta spacecraft mission, but launch delays meant that the comet was no longer easily reachable and another periodic comet, 67P/Churyumov–Gerasimenko, was chosen as the mission's target instead.

2023
Close approaches to Jupiter in 1972 and 1984 moved the comet's orbit closer to Earth, and as of epoch 2018 the comet has an Earth–MOID of 0.071 AU. In 2023 Earth passed through a denser part of the 1974 meteoroid stream than Earth did in 2007. As a result a shower with radiant in the southern constellation of Sculptor was observed with a zenithal hourly rate (ZHR) of $1.4 km$ and was given the name λ-Sculptorids. The meteors made atmospheric entry (Ve) at a relatively slow 15 km/s and as a result the mean mass of the meteoroids observed was about 0.5 grams, about 10 times higher than that of other meteor showers.

2012
Russian forecaster Mikhail Maslov had predicted that the Earth's orbit would cross Comet Wirtanen's debris stream as many as four times between December 10 and December 14, 2012. As there had not previously been an encounter with this debris stream, it was not certain whether or not a meteor shower would be visible from Earth, but there was speculation that a shower with as many as 30 meteors per hour might occur.

Observers in Australia reported that on the night of December 14, 2012, as many as a dozen meteors were seen emanating from the predicted radiant in the constellation of Pisces.