46P/Wirtanen talking points for use in proposals

Here are some facts and interesting pieces of information that can be incorporated into observing or grant proposals to help justify studies of this comet. Feel free to use whichever items help to support your arguments, and adapt them as necessary.

Physical Properties - Not much is currently known

Comet Wirtanen is a Jupiter family comet with a relatively small nucleus (0.6 km effective radius; Lamy et al. A&A 335, L25-29, 1998; Boehnhardt et al., A&A 387, 1107-1113, 2002).

Comet Wirtanen is a member of the small family of hyperactive comets (which also includes 103P/Hartley 2 and 21P/Giacobini-Zinner). Its water production rate is higher than would be expected, based only on its nucleus size and standard water vaporization models (Cowan and A'Hearn, 1979). These models suggest that Wirtanen should be active over ~100% of its surface area, which is not likely. Instead, we expect that, like Hartley 2, its nucleus probably contains hypervolatile ices, and when they sublimate, they entrain sub-surface water ice grains and drag them into the coma (Protopapa, et al., Icarus 238, 191-204, 2014). Subsequent sublimation of these grains in the coma dramatically increases the water production, producing the comet's apparent hyperactivity.

Wirtanen's water production peaks at ~2×1028 molecules/s a few days after perihelion (around the time of closest approach to the Earth). Kobayashi & Kawakita (Pub. Ast. Soc. Japan 62, 1025-1033, 2010) combined measurements from multiple apparitions, and found that Wirtanen's production is repeatable from one apparition to the next, with little evidence for secular variation.

Wirtanen's rotation state is not well known. Two sparse lightcurves have been measured, with periodicities of 7.6 hr (Meech et al., A&A 326, 1268-1276, 1997) and 6.0 hr (Lamy et al., A&A 335, L25-29, 1998). These are incompatible with each other, suggesting that one or both measurements is incorrect, or else the nucleus is changing its rotation state. Samarasinha et al. (Planet Space Sci 44, 275-281, 1996) also argued that Wirtanen could be in a state of non-principal axis rotation. The 2018 apparition promises to provide excellent conditions for resolving this issue.

An Accessible Comet - Likely to be a future spacecraft target

Wirtanen's orbit makes it readily accessible for spacecraft missions. It was the original target of the Rosetta mission (a delay in the spacecraft launch date forced a change in the target); It was the proposed target for the Comet Hopper Discovery mission (Phase A study, unselected); and it has likely been the unannounced target of other missions. This history and accessibility means that Wirtanen is a strong candidate for selection as a future mission target. Knowledge of a spacecraft target's physical characteristics can dramatically reduce cost and risk, because the spacecraft design and mission profile need to encompass a smaller range of unknowns. Wirtanen's 2018 apparition offers the best opportunity for decades to learn learn about this comet, allowing more detailed characterization of a cometary spacecraft target than has ever before been possible, which will facilitate the design and planning of any future mission.

Comparative Comet Studies

From the basic information that is known about comet Wirtanen (see Physical Properties), it is a near twin of comet Hartley 2, the 2010 target of the Deep Impact Extended Investigation (DIXI). Studies of comet Wirtanen around the time of its close approach to Earth will have spatial resolutions high enough to permit valuable comparisons to the DIXI measurements and to observations of other spacecraft targets. Observations throughout the apparition will provide a long-duration characterization that can be compared to many other comets at a variety of heliocentric distances.

Excellent Observing Conditions Around Close Approach

Wirtanen makes an historically close (0.0775 AU, or 30 Lunar distances) approach to the Earth on 2018 Dec 16 only four days after perihelion. This is the 10th closest comet approach in the modern era, but Wirtanen is predicted to be brighter than all others except, possibly, IRAS-Araki-Alcock.

Wirtanen is predicted to reach naked eye brightness (Seiichi Yoshida's prediction) and could remain so for a few weeks around close approach. This will allow studies to obtain high signal-to-noise measurements that can be used to investigate details not available from fainter comets.

The observing conditions around the Earth encounter will approach those of a distant spacecraft flyby, with spatial resolutions as good as 57 km/arcsec, allowing most studies to investigate the comet's innermost coma (and radar measurements will resolve the nucleus). Furthermore, the proximity to Earth will allow numerous large telescopes and a variety of instruments, many of which could never be flown on spacecraft, to study the comet.

Wirtanen is near opposition around the time of close approach, so it will be visible all night for weeks from both the Northern and Southern hemisphere. Because its orbit tracks the Earth's for many months, Wirtanen will remain observable for between 2 and 8 hours per night for over a year.

Rare Events Around Close Approach

Wirtanen's close approach also provides a rare scenario for studies that use radial velocities as a descriminator in investigating the physics and chemistry of the comet's coma. During its approach to the Earth, Wirtanen will exhibit a radial velocity of -6 km/sec ten days before close approach. That velocity will have shifted to +6 km/sec ten days after approach. This range of velocities will occur while the comet is at essentially the same heliocentric distance (~1.0), allowing the effects introduced by the radial velocity to be isolated.