The Early History of Heliospheric Science and the Spacecraft That Made It Possible

Ram pressure in astronomy and engineering

The ram pressure of a moving fluid, P r , is the rise in pressure at a stagnation point relative to the upstream pressure. In astronomy, it is used to calculate the interaction of stellar winds with planets and to quantify the effects of ram pressure stripping. On aeroplanes and in wind tunnels, it is measured with a pitot-static tube, an inexpensive device with no moving parts that was invented in 1732. Up through the mid-1960s, across both astronomy and engineering the ram pressure of a moving gas and its momentum flux, ρ u 2 , where ρ and u are the upstream mass density and flow speed, were properly treated as related but distinct quantities. This relationship may be expressed as P r = Sp   ρ u 2 , where Sp is the dimensionless Spreiter number, which ranges between 0.5 and 0.88 for a monatomic gas, depending on the upstream Mach number, Ma . Unfortunately, by the early 1970s, in astronomy ram pressure was defined to be the momentum flux and Sp was fixed to be unity and forgotten as a parameter. This article seeks to raise awareness of this issue, and to review the determination of Sp for subsonic and supersonic flow.

Dust in and Around the Heliosphere and Astrospheres

Interstellar dust particles were discovered in situ, in the solar system, with the Ulysses mission's dust detector in 1992. Ever since, more interstellar dust particles have been measured inside the solar system by various missions, providing insight into not only the composition of such far-away visitors, but also in their dynamics and interaction with the heliosphere. The dynamics of interstellar (and interplanetary) dust in the solar/stellar systems depend on the dust properties and also on the space environment, in particular on the heliospheric/astrospheric plasma, and the embedded time-variable magnetic fields, via Lorentz forces. Also, solar radiation pressure filters out dust particles depending on their composition. Charge exchanges between the dust and the ambient plasma occur, and pick-up ions can be created. The role of the dust for the physics of the heliosphere and astrospheres is fairly unexplored, but an important and a rapidly growing topic of investigation. This review paper gives an overview of dust processes in heliospheric and astrospheric environments, with its resulting dynamics and consequences. It discusses theoretical modeling, and reviews in situ measurements and remote sensing of dust in and near our heliosphere and astrospheres, with the latter being a newly emerging field of science. Finally, it summarizes the open questions in the field.