Comet 41P/Tuttle-Giacobini-Kresak, 45P/Honda-Mrkos-Pajdusakova, and 46P/Wirtanen: Water Production Activity over 21 Years with SOHO/SWAN.

In 2017, 2018, and 2019, comets 46P/Wirtanen, 45P/Honda-Mrkos-Pajdusakova, and 41P/Tuttle-Giacobini-Kresak all had perihelion passages. Their hydrogen comae were observed by the Solar Wind ANisotropies (SWAN) all-sky hydrogen Lyman-alpha camera on the SOlar and Heliospheric Observer (SOHO) satellite: comet 46P for the fourth time and comets 45P and 41P for the third time each since 1997. Comet 46P/Wirtanen is one of a small class of so-called hyperactive comets whose gas production rates belie their small size.

Comet C/2017 S3 (PanSTARRS): Outbursts and Disintegration.

The Solar Wind ANisotropies (SWAN) all-sky hydrogen Lyman-alpha camera on the SOlar and Heliospheric Observer (SOHO) satellite observed the hydrogen coma of comet C/2017 S3 (PanSTARRS) for the last month of its activity from 2018 July 4 to August 4 and what appears to have been its final disintegration just 11 days before its perihelion on August 15. The hydrogen coma indicated water production had a small outburst on July 8 at a heliocentric distance of 1.1AU and then a much larger one on July 20 at 0.

A Survey of Water Production in 61 Comets from SOHO/SWAN Observations of Hydrogen Lyman-alpha: Twenty-One Years 1996-2016.

The Solar Wind Anisotropies (SWAN) instrument on the SOlar and Heliospheric Observatory (SOHO) satellite has observed 44 long period and new Oort cloud comets and 36 apparitions of 17 short period comets since its launch in December 1995. Water production rates have been determined from the over 3700 images producing a consistent set of activity variations over large parts of each comet's orbit. This has enabled the calculation of exponential power-law variations with heliocentric distance of these comets both before and after perihelion, as well as the absolute values of the water production rates.

Water Production Activity of Nine Long-Period Comets from SOHO/SWAN Observations of Hydrogen Lyman-alpha: 2013-2016.

Nine recently discovered long-period comets were observed by the Solar Wind Anisotropies (SWAN) Lyman-alpha all-sky camera on board the Solar and Heliosphere Observatory (SOHO) satellite during the period of 2013 to 2016. These were C/2012 K1 (PanSTARRS), C/2013 US10 (Catalina), C/2013 V5 (Oukaimeden), C/2013 R1 (Lovejoy), C/2014 E2 (Jacques), C/2014 Q2 (Lovejoy), C/2015 G2 (MASTER), C/2014 Q1 (PanSTARRS) and C/2013 X1 (PanSTARRS). Of these 9 comets 6 were long-period comets and 3 were possibly dynamically new.

Voyager measurements of hydrogen Lyman-α diffuse emission from the Milky Way.

Doppler-shifted hydrogen Lyman-alpha (Lyα) emission from galaxies is currently measured and used in cosmology as an indicator of star formation. Until now, the Milky Way emission has not been detected, owing to far brighter local sources, including the H (hydrogen) glow, i.e.

A warm layer in Venus' cryosphere and high-altitude measurements of HF, HCl, H2O and HDO.

Venus has thick clouds of H2SO4 aerosol particles extending from altitudes of 40 to 60 km. The 60-100 km region (the mesosphere) is a transition region between the 4 day retrograde superrotation at the top of the thick clouds and the solar-antisolar circulation in the thermosphere (above 100 km), which has upwelling over the subsolar point and transport to the nightside. The mesosphere has a light haze of variable optical thickness, with CO, SO2, HCl, HF, H2O and HDO as the most important minor gaseous constituents, but the vertical distribution of the haze and molecules is poorly known because previous descent probes began their measurements at or below 60 km.

Discovery of an aurora on Mars.

In the high-latitude regions of Earth, aurorae are the often-spectacular visual manifestation of the interaction between electrically charged particles (electrons, protons or ions) with the neutral upper atmosphere, as they precipitate along magnetic field lines. More generally, auroral emissions in planetary atmospheres "are those that result from the impact of particles other than photoelectrons" (ref. 1).

Deflection of the interstellar neutral hydrogen flow across the heliospheric interface.

Using an absorption cell, we measured the Doppler shifts of the interstellar hydrogen resonance glow to show the direction of the neutral hydrogen flow as it enters the inner heliosphere. The neutral hydrogen flow is found to be deflected relative to the helium flow by about 4 degrees . The most likely explanation of this deflection is a distortion of the heliosphere under the action of an ambient interstellar magnetic field.

Nightglow in the upper atmosphere of Mars and implications for atmospheric transport.

We detected light emissions in the nightside martian atmosphere with the SPICAM (spectroscopy for the investigation of the characteristics of the atmosphere of Mars) ultraviolet (UV) spectrometer on board the Mars Express. The UV spectrum of this nightglow is composed of hydrogen Lyman alpha emission (121.6 nanometers) and the gamma and delta bands of nitric oxide (NO) (190 to 270 nanometers) produced when N and O atoms combine to produce the NO molecule.

Water production of comet C/1999 S4 (LINEAR) observed with the SWAN instrument.

The SWAN (Solar Wind ANisotropies) Lyman-alpha all-sky camera on the SOHO spacecraft observed the hydrogen coma of comet C/1999 S4 (LINEAR) from the end of May through mid-August 2000. A systematic set of water-production rates was obtained for this well-documented event of complete fragmentation of a cometary nucleus. The observations indicate that the lower limit for the sunlit surface area of the nucleus was about 1 square kilometer before the fragmentation and that the amount of water released throughout the observing period was 3.

Discovery of a comet by its Lyman-alpha emission.

Several searches for near-Earth objects have recently been initiated, as a result of increased awareness of the hazard of impacts on the Earth. These programs mainly search for asteroids, so amateur astronomers can still contribute to the discovery of comets, especially out of the orbital plane of the Solar System. An ideal way to search for comets would be to use a spaceborne instrument capable of imaging the whole sky on a daily basis in a systematic and repeatable way.