CARAVEL: A Solar-Sail-Based Nanosatellite Mission to a Near-Earth Asteroid

Kieran Carroll of Gedex and the UTIAS Space Flight Laboratory discussed the CARAVEL mission, a solar-sail-based nanosatellite mission to a near-earth asteroid at the 2012 Canadian Space Summit.

Abstract: The price tags for all past planetary missions have been extremely high, in the range of hundreds of millions to billions of dollars for each mission. Costs that high have kept most countries, including Canada, from attempting stand alone planetary missions, opting instead to make minor contributions to expensive missions led by those countries/space agencies (NASA, ESA, etc.) who can afford them. While some types of planetary missions are inherently that expensive, recent technology developments have opened up the possibility of carrying out certain types of missions at a much lower cost.

CARAVEL (Canadian Asteroid Rendezvous And Visit Enabled by Lightsail) is a mission concept that was studied recently at the Space Flight Laboratory (SFL) at UTIAS, whose general objective is to formulate a scientifically useful planetary exploration mission whose cost is low enough to be easily affordable within the Canadian Space Agency's base budget. CARAVEL's specific objective is to perform one or more slow flybys of a near Earth asteroid, capturing detailed images of the asteroid and transmitting them to Earth.

CARAVEL's mission definition intertwines three distinct strategies to make the low cost part of the objective achievable. The first of these is to choose an exploration target that is particularly close to the Earth. Distance between Earth and the exploration target is a mission cost driver, because communications over long distances is expensive in several ways. The larger the distance, the higher the power needed by the transmitter aboard the spacecraft; high power drives up the size, mass and cost of the power subsystem on the spacecraft, making this subsystem more expensive, and also increasing the mass of the overall spacecraft, which makes the in space propulsion subsystem larger, making the overall launched mass larger and more costly. CARAVEL minimizes the communications range by targeting asteroid 2004 GU9, an Apollo family asteroid which is a quasi satellite of Earth, whose range to Earth would not exceed 50 million km during the mission. 2004 GU9 has an orbital inclination of 13.6 degrees, and an estimated size of 200 metres.

The second low cost enabling strategy is to design CARAVEL using "nanosatellite technology." In SFL's practice, this means designing the mission and system using the "Microspace philosophy," the engineering management and design approach used in all of SFL's nanosat missions, with heritage extending back to the MOST microsat mission, and beyond that to many AMSAT missions. The main objective of that approach is to develop highly capable and reliable space mission at the lowest possible cost. One of the notable outcomes of this approach is spacecraft that are very small and low in mass, making use of the same sort of very up to date, "Commercial Off The Shelf" electronic components which have enabled very small, highly functional consumer electronic devices. The resulting low spacecraft mass significantly reduces launch cost.

The third strategy is to employ solar sailing technology to transit from Earth to the target asteroid. In particular, CARAVEL's design employs essentially the same L'Garde solar sail design being developed for use in NASA's 2014 Sunjammer mission, a square solar sail 35 metres on a side. A major cost driver for all previous planetary missions has been the need to buy a dedicated launch vehicle, to launch the exploration spacecraft on a trajectory towards its destination planet or asteroid. Many much mission have also required a chemically propelled upper stage and/or a chemical or ion engine on board propulsion system with substantial delta V capability, all of which are typically quite expensive. A solar sail propulsion system can be low enough in mass and capable enough in propulsion capability, to open up the possibility of reaching interplanetary space via a "hitch hiker" secondary payload launch the class of launch which essentially defines the microsat and nanosat classes of spacecraft, due to its much lower cost.

This talk will provide an overview description of the CARAVEL mission and system design, with a particular focus on the challenges posed by communicating at planetary distances using nanosat grade equipment.