Hubble's Exciting Universe: Surveying the Solar System
Like a surveillance camera, Hubble has monitored the often-turbulent weather on most of the planets in our solar system. Unlike distant stars and galaxies, our neighbors routinely undergo noticeable changes in their appearance in weeks or months. These weather observations yield valuable insights into Earth’s own atmospheric dynamics.
The opening chapter in our civilization's initial reconnaissance of the solar system, beyond the Earth-Moon system, began in November 1965 when the Soviet Union launched the probe Vernera 3 to Venus. Since then the United States and the Soviets launched many dozens of probes that visited every major planet, culminating with the Voyager 2 flyby of Neptune in 1989, a year before the launch of the Hubble telescope.
So, the question is, why do we need Hubble if we've visited all these far-flung worlds and seen them close-up? Hubble provides for multiple decades of observations of all classes of objects out to the very fringe of the solar system. The telescope is also used to view them at multiple wavelengths across the electromagnetic spectrum. Hubble complements, enhances, and helps prepare for new planetary missions. Hubble came along right on time to introduce a unique synergy with planet missions, right on the cusp of the initial survey of the solar system.
Hubble provides a global context, a holistic view, for understanding these worlds. The flyby missions were brief snapshots of alien worlds, like a cruise-liner passenger taking tourist shots of Caribbean islands as the ship passes by them. But most of the solar system planets are dynamic, with deep atmospheres undergoing turbulent seasonal changes. And these Hubble observations offer a context for future visiting spacecraft, and those still operating in situ, especially the suite of current Mars landers. Over the years, Hubble has supplied astronomers with weather reports, such as erupting dust storms, that helped guide planning the operations of surface landers and rovers.
Hubble's ultraviolet sensitivity has been a bonanza for studying auroras on the outer planets that are powered by immense magnetic fields. Researchers operating the Cassini spacecraft, which orbited Saturn for 18 years, relied heavily on Hubble observations to understand the interaction of the solar wind with Saturn's upper atmosphere. Hubble observations of Jupiter auroras helped astronomers guide NASA's orbiting Juno spacecraft, which does not have ultraviolet sensitivity to photograph the auroras.
Thirty years of Hubble observations have allowed astronomers to track the changing shape and size of Jupiter's iconic Great Red Spot, a storm big enough to swallow the Earth. In July 1994, Hubble documented the never-before seen string of comet collisions with Jupiter that resulted from the tidal breakup of Comet Shoemaker-Levy 9. The Earth-sized impacts unleashed a tremendous amount of energy and debris that blackened Jupiter’s atmosphere for months. Hubble gave astronomers their best view ever of a once-in-1,000-years event. The dynamic event put astronomers on alert that they had better keep an eye on comet impacts throughout the solar system, and made them assess how Earth could be threatened in the future.
Moons and Minor Bodies
The Voyager and Galileo missions offered dramatic evidence that the Jovian moon Europa has a subsurface ocean entombed under a thick crust of ice. Where there's water, there could be life. One scientist even speculated that the Voyagers might catch a snapshot of geysers on Europa. Such activity turned out to be so elusive that astronomers had to wait more than three decades for the peering eye of Hubble to monitor the moon for signs of venting activity. Several plumes of water vapor were photographed towering many miles above the surface. This evidence could guide a future spacecraft to fly by at low altitude through the plumes to sample chemicals in its subsurface ocean without having to land on the surface and drill through the ice.
Astronomers used Hubble to watch auroras glowing above the icy surface of the moon Ganymede. The auroras are tied to Ganymede’s magnetic field, which descends right down to the icy moon's core. A saline ocean would influence the dynamics of the magnetic field as it interacts with Jupiter's own immense magnetic field, which engulfs Ganymede. Because telescopes can't look inside planets or moons, tracing the magnetic field through auroras is a unique way to probe the interior of another world.
In preparation for the 2005 New Horizons flyby of Pluto, researchers used Hubble to identify potential navigational hazards to the spacecraft. In the process, several small moons were discovered orbiting Pluto and its binary companion object Charon. Astronomers took advantage of Hubble's sharp view to make a detailed map of the dwarf planet’s surface, which guided close-up reconnaissance photos taken by New Horizons.
Hubble was enlisted to do a very deep survey on the edge of our solar system to identify any object New Horizons could pass by and photograph on its outbound leg through the solar system. This was no small task, even for Hubble's eagle-eye vision. It was a “needle-in-the-haystack” search to identify an object not much larger than Manhattan Island and as black as charcoal. It would be about as faint as the glow of a flashlight located at the Moon’s distance from Earth. In 2014, Hubble succeeded in pinpointing an object, called MU69, that is part of a population of icy bodies in the Kuiper belt, a ring of icy debris on the rim of our solar system. The New Horizon probe flew by the primordial, dumbbell-shaped object in January 2019, capping off 50 years of solar-system reconnaissance.
Hubble solar-system observations have several bizarre cases of asteroids disintegrating before our eyes. Some are the victim of collisions; others slowly spin up until they slough off material. Prior to the Hubble observations, asteroids were thought largely to be boringly placid. Hubble unveiled a dynamic asteroid belt. Hubble has made complementary observations in support of the Dawn asteroid mission, and comet flybys.
Ambitious missions to the planet are planned for the 2020s. Hubble will continue to support these voyages of discovery by keeping an eye on planetary destinations. Hubble’s planet-wide coverage will work in support and do parallel observations as these missions unfold.