The name Proba comes from the Project for On-board Autonomy. It blazes a trail for space engineering in the future, when spacecraft will make operational decisions for themselves. It also typifies a trend towards small but powerful spacecraft, made feasible by advances in miniaturization.

Whilst Proba-1 carries a miniaturized camera to observe the Earth at many selectable wavelengths of light, which has been a big success, Proba-2 will watch the Sun. The main instrument, called SWAP, will make detailed images of the solar atmosphere once a minute, by the light of very energetic ultraviolet rays.

Using techniques of solar observation developed by scientists in Belgium, for ESA's Space Weather project, Proba-2 will give early warnings of eruptions on the Sun that provoke stormy weather throughout the Solar System.

These often damage satellites and engineering systems on the Earth. Proba-2 itself will perform much of the analysis needed to spot the solar storms. Proba-1 already flies a monitor, called SREM, which measures the effect of the solar activity around the Earth.

The lightweight SWAP instrument on Proba-2 will test the latest space technologies to be used in a future instrument to replace a corresponding device on the ESA-NASA SOHO spacecraft, which is at present the world's chief watchdog for the Sun.

"Most of ESA's own missions in the foreseeable future will still require medium-sized or large satellites," notes Frederic Teston, project manager of Proba. "But small satellites have proved their worth for rapid testing of spacecraft techniques and onboard instruments. They can also support dedicated missions very efficiently. And the Proba project keeps us up to date with microsatellite technology in general, which may become a big interest of ESA's Member States."

For example, the uses of micro-spacecraft in interplanetary projects are now being studied by several European institutes and companies. In one case, under an ESA contract, Astrium is examining the feasibility of sending dozens of 20-kilogram spacecraft together, like a swarm of bees, for close-up examinations of many of the objects in the Asteroid Belt between Mars and Jupiter. This proposal is called the Asteroid Population Investigation and Exploration Swarm, or Apies.

The global picture
Even small or poor countries can now run their own space programmes using microsatellites, which are generally defined as having no more than 100 kilograms of mass at launch. Algeria, Turkey and Nigeria have been assisted in their space ambitions by the University of Surrey in the United Kingdom. Their microsatellites are Earth-observing spacecraft with unusually wide fields of view, for a multinational Disaster Monitoring Constellation. China will join the constellation in 2005, and participation by Vietnam and Thailand is expected too.

In normal operation the satellites supply their owners with images of pre-selected areas every four days. In the event of a disaster all of them can work together to provide daily images of the affected area. This service complements ESA's Earth Watching programme, which coordinates the use of imagery from European and US satellites in helping disaster-relief teams.

The British university and its company Surrey Satellite Technology have been building and operating small spacecraft for more than 20 years. Their low-cost and high-speed mode of operation recently won them the contract from ESA to build an early test-bed satellite for Europe's big Galileo system of navigational satellites. Europe's first nanosatellite, meaning a spacecraft weighing less than 10 kilograms, was Surrey's SNAP-1, launched in 2000, with a mass of under 7 kilograms.

"Attitudes have changed rapidly in the past 18 months," comments Professor Sir Martin Sweeting of Surrey Satellite Technology. "People now realise that small satellites can perform important global tasks. Some can be done far more cheaply and quickly than was previously thought possible. And as experts become more familiar with the remarkable capability of these small satellites, the human imagination will come up with applications we haven't even thought of yet."

In the case of Denmark
In 1999 the launch of the 60-kilogram �rsted satellite initiated the Small Satellites Programme of the Danish Space Research Institute. This is an example of the interest in microsatellites within one of ESA's smaller Member States. �rsted and its sister �rsted 2 carry instruments to monitor variations in the Earth's magnetic field.

Now ESA is considering Swarm, a Danish proposal to launch four satellites to operate simultaneously in different orbits, to provide a more comprehensive picture of the response of the Earth's field to solar activity.

"Microsatellites developed by small teams are of course a nice idea for small countries like Denmark," says Flemming Hansen, technology manager for small satellites at the Danish Space Research Institute. "Having our own national spacecraft in orbit heightens public awareness of the value of space projects, and stimulates our industrial contractors.

For newcomers in space business the 'learning curve' is less steep than with big multinational projects. But there are also important scientific and technical advantages with small satellites. You can move much faster from conception to execution, and operate a number of spacecraft together at a reasonable cost."

Small satellites have an educational role. Danish universities have already designed and built, as student projects, picosatellites weighing only 1 kilogram each. Aalborg University (AAU) and the Technical University of Denmark (DTU) made the first two. AAU CubeSat was fitted with a digital camera for Earth observation.

DTUsat was a technically innovative project to deploy a 450-metre copper wire, or 'tether', to interact with electrons and the Earth's magnetic field, so as to generate an electromagnetic force that could drive the satellite out of its orbit.

The two student satellites were launched by a Russian Rockot on 30 June 2003 together with American, Canadian and Japanese picosatellites. For unknown reasons the DTUsat sadly failed.

The AAU Cubesat is alive, but communication difficulties have so far prevented initiation of the planned mission. Undaunted, students at Aalborg are now planning a 1-kilogram lander to send to the Moon, complete with a lunar rover vehicle just 8 centimetres long.

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