It’s 50 years since the first interplanetary probe, and this year’s stunning pictures from across the solar system show how far space technology has come
This image gives us an unprecedented view of Earth at night. Ribbons of light from cities stretch across the shadows of the world’s great land masses and reveal the extent of our species’ conquest of the planet. The picture is one of a series known as the Black Marble photographs, released this month by Nasa.
The US space agency created them from data gathered between April and June by the Suomi NPP satellite, which sweeps over the poles at a height of 500 miles, covering the entire planet as it revolves beneath. Each picture is a composite of several photographs taken on different cloud-free nights using the satellite’s visible infrared imaging radiometer suite; they are remarkable for the detail they provide of Earth at night.
The Black Marble photographs were just one of several sets of stunning photographs generated by Nasa spacecraft last year. Other probes sent back enthralling images of Mars, Saturn, the Moon, Mercury and other planets, revealing remarkable information and unexpected details of these distant worlds. Has Nasa become, along with everything else, the world’s best photographic agency?
The publication of these interplanetary photographs marks the 50th anniversary of the first successful flight of an interplanetary probe and demonstrates the dramatic changes that have been made to America’s fleet of robot spacecraft in that time. Launched on 26 August 1962, the US probe Mariner 2 swept past Venus at a distance of 22,000 miles in December that year. Its radiometers revealed a world with cool, thick clouds and a very hot surface. It returned no images of Venus, however, for the craft was not fitted with a camera.
Today Nasa spaceships bristle with them. The US robot rover Curiosity, which landed on Mars this summer, has a total of 17 on board. These recorded the craft’s descent to the surface of the red planet and have since provided detailed images of every manoeuvre the rover has made.
Curiosity landed on Mars in August and has since been subjecting rocks and soil to a detailed examination. The craft has a laser to vaporise slivers of rocks and analyse their chemical composition; a robot arm to pulverise pieces of stone; and an oven in which soil and rock samples are baked and tested for the presence of organic carbon. In addition, Curiosity’s cameras have returned glorious images of the terrain and hills as the robot rover trundles along on a journey of investigation that has been designed to discover if Mars ever possessed the right atmospheric and geological conditions to support life.
In contrast to Curiosity, which has subjected Mars to an investigation that is up close and personal, most other US probes are surveying their targets from a distance. An example is Nasa’s Messenger probe, which has been in orbit around Mercury, the closest planet to the Sun, for almost two years. Daytime temperatures there can reach more than 400C. Despite the searing heat, however, Messenger last month sent back data which showed that ice and frozen organic material exists in craters permanently shadowed in Mercury’s north pole.
“It’s not something we expected to see, but then of course you realise it kind of makes sense because we see this in other places,” planetary scientist David Paige of the University of California told Reuters at the time. Other probes have shown that our own Moon has ice in craters at its poles, for example.
The discovery of the ice and frozen organics on Mercury is a technical triumph. The planet’s orbit approaches to within 46 million kilometres of the Sun (compared with the Earth, which reaches 147m km at its closest). At this distance, the radiation from the Sun and its intense gravitational pull make it difficult to manoeuvre and observe Mercury. In fact, Messenger is the only spacecraft to achieve orbit round the planet. It used radar imagery to detect ice and organics. These icy deposits are thought to have been dumped on Mercury by comets in the distant past and their discovery helps explain how such material might have appeared on Earth and how life could have evolved here.
Equally surprising were the results produced by two Nasa satellitescalled Ebb and Flow, collectively known as the Gravity Recovery and Interior Laboratory (Grail) mission. They have been measuring tiny fluctuations in the gravitational field of the Moon and have produced a detailed map of rock densities there. The map shows that, in its first billion years, the Moon was fractured repeatedly by impacts of unexpected violence from asteroids and other remnants from the early years of the solar system.
On Earth, the shifting of tectonic plates and churning of our planet’s molten mantle have wiped out most evidence of these early asteroid bombardments. Grail has now revealed that conditions in the early solar system must have been far more violent than we once thought and has shown that our planet must have gone through some particularly violent formative years.
The photographs and information sent back by Messenger, Grail and Curiosity show worlds that are very different from our own. By contrast, those returned last month by the probe Cassini of Titan, the largest moon of distant Saturn, have a very familiar look. They show images of a river valley that stretches for more than 400 km from its “headwaters” to a large sea. The valley crosses Titan’s north polar region and runs into Ligeia Mare, one of the three great seas in Titan’s high northern latitudes. The river has tributaries and in some places meanders just like those on Earth. On a smaller scale, it could be confused – from these images – with the Nile delta.
Appearances would be deceptive, however. This river is no flowing waterway like those on Earth. It is made up of liquid hydrocarbons including methane and ethane. Titan may be the only other world in the solar system that has open stretches of liquid on its surface but that liquid is very different from the rivers of Earth.