From a sling to a galaxy

Gravity assist. A probe that passes behind a moving planet takes a little of the planet's orbital speed and leaves faster relative to the Sun. Voyager 1 launched on 5 September 1977, passed Jupiter on 5 March 1979 and Saturn on 12 November 1980, and left the heliosphere on 25 August 2012 [1]. Voyager 2 launched on 20 August 1977 and flew by Jupiter (9 July 1979), Saturn (25 August 1981), Uranus (24 January 1986) and Neptune (25 August 1989) [2]. It used a once-every-175-years alignment of the outer planets, the Grand Tour [3].

The edge of the bubble. The solar wind flows out until the pressure of the gas between the stars stops it. Voyager 2 crossed the termination shock at 84 AU in August 2007 and the heliopause, about 119 AU out, in 2018 [4]. Gravity reaches much further: the Oort Cloud starts at about 1,000 AU [4].

Other worlds. A planet does not just orbit its star: both orbit their barycentre. The barycentre of the Sun and Jupiter lies just outside the Sun's surface [5]. We detect planets from the tiny wobble of the star's light. By 17 September 2025 the NASA Exoplanet Archive listed 6,000 confirmed planets [6].

The galaxy. The Sun is one of a few hundred billion stars in the Milky Way, about 28,000 light-years from the centre according to ESA [7]. It orbits at about 230 km/s, one lap in about 230 million years [8]. Relative to the cosmic microwave background, the Sun moves at 369.82 km/s [9].

What if the orbits stopped? Orbital speed is what keeps a planet from falling in. A body let go from rest at distance r falls to the centre in 1/(4√2) = 0.177 of its orbital period. For the Earth that is 365.25 / 5.657 = 64.6 days.

The monster and the missing mass. Sagittarius A* has about 4 million solar masses and was imaged by the Event Horizon Telescope; the image was released on 12 May 2022 [10]. The star S2 orbits it in 16 years, reaching 7,650 km/s, 2.55% of c [11]. Far from a galaxy's centre the stars should orbit more slowly if only visible matter counted; they do not. About 27% of the universe is dark matter against about 5% ordinary matter, a ratio of about 5.4 : 1 [12].

Sources

1. Voyager 1 (NASA Science), https://science.nasa.gov/mission/voyager/voyager-1/
2. Voyager 2 (NASA Science), https://science.nasa.gov/mission/voyager/voyager-2/
3. The Grand Tour (NASA Science), https://science.nasa.gov/image-detail/grand-tour/
4. Oort Cloud and Scale of the Solar System (infographic) (NASA Science), https://science.nasa.gov/resource/oort-cloud-and-scale-of-the-solar-system-infographic/
5. What Is a Barycenter? (NASA Space Place), https://spaceplace.nasa.gov/barycenter/en/
6. 2025 Exoplanet Archive News (NASA Exoplanet Archive), https://exoplanetarchive.ipac.caltech.edu/docs/2025news.html
7. Galaxies and the Expanding Universe (ESA), https://sci.esa.int/web/education/-/36827-galaxies-and-the-expanding-universe?fbodylongid=1732
8. Solar System: Facts (NASA Science), https://science.nasa.gov/solar-system/solar-system-facts/
9. Planck 2018 results. I. Overview (dipole, ar5iv HTML) (Planck Collaboration / ar5iv), https://ar5iv.labs.arxiv.org/html/1807.06205
10. Astronomers reveal first image of the black hole at the heart of our galaxy (eso2208) (ESO), https://www.eso.org/public/news/eso2208/
11. Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole (abstract) (GRAVITY Collaboration / arXiv), https://arxiv.org/abs/1807.09409
12. Dark Matter (NASA Science), https://science.nasa.gov/dark-matter/