Small Satellites May Play Big Role In Future Interplanetary Missions

Proponentsof small satellites say that tiny spacecraft have potentially big roles to playin planetary exploration.

Today'ssmall satellites--generally spacecraft weighing around several hundred kilograms--areconfined largely to low Earth orbit where they perform remote sensing missions,conduct science operations and serve as technology testbedsand communication relays.

But someforward thinkers are already looking ahead to interplanetary missions and seesmall satellites as a good fit with the space exploration agendas outlined bythe world's spacefaring nations.

TheEuropean Space Agency, for example, is taking a look at a low-cost, multiplespacecraft Venus mission that would utilize small satellite technologies,including a small, deployable weather balloon of sorts, to study the planet.The Indian Space Research Organization last year short listed a gravity-mappingnanosatellite for inclusion on its Chandrayaan-1lunar orbiter mission.

AndyPhipps, a senior engineer at the British small satellite company SurreySatellite Technology Ltd., said his team recently completed a so-calledtechnology reference study funded by the European Space Agency to identify thetechnologies and design philosophy needed for the proposed Venus mission.

Phipps saidhis team spent 18 months and several hundred-thousand Euros developing amission concept featuring two orbiters packed with miniaturized instruments anda tiny aerobot that would bedropped into Venus' corrosive atmosphere.

The aerobot, consisting of an instrument-laden gondolasuspended from a balloon, would add about 90 kilograms of mass to one of theorbiters, a data relay satellite that would be placed in a highly ellipticalorbit around Venus. The aerobot would be dropped intoVenus' atmosphere where it would float at an altitude of 55 kilometers,circumnavigating the planet several times during its projected 15- to 22-daymission.

The otherorbiter would be packed with miniaturized instruments and would circle theplanet at a lower altitude, imaging the planet and making scientificmeasurements.

Theproposed orbiters themselves would be relatively small for interplanetaryspacecraft, weighing several hundred kilograms apiece. NASA's MarsReconnaissance Orbiter, in contrast, will weigh nearly 2,200 kilograms atlaunch and require an Atlas 5 rocket to reach orbit.

The twosatellites and the inflatable robotic stowaway would launch on a singleRussian-built Soyuz rocket equipped with an upper stage. The total projectedmission cost, Phipps said, is several hundred-million dollars, or aboutone-tenth of what the U.S.and Europe spent on the Cassini-Huygensmission to Saturn.

Theproposed Venus Entry Probe mission is only one of a half-dozen mission ideasthe European Space Agency is considering as it looks ahead to the 2015-2025timeframe to try to understand what technologies it should be investing in now.

Phipps saidthe technology needs of the Venus Entry Probe mission are considerable andinclude: highly protective cover glass to shield imaging instruments from acidrain; steerable planar array antennas to increase datareturn from the aerobot; higher efficiency solarcells; low-mass structural components that can withstand the planet's corrosiveenvironment; and lightweight thermal protection system for the aerobot's entry vehicle.

Phipps'colleagues will be presenting the Venus Entry Probe mission concept at the 19thAnnual Small Satellite Conference in Logan, Utah, Aug. 8-11.

Alsopresenting at the conference is a group of Canadian scientists and engineersthat have come up with a nanosatellite mission dubbedLunette that would map the gravitational field of the far side of the Moon.

KieranCarroll, a Lunette team member and director of technology development at Gedex Inc., a Toronto-based start-up company specializingin terrestrial gravity mapping for mineral exploration, said better maps of theMoon's irregular gravitational field would shed more light on the lunarinterior, aid the cause of exploration by potentially locating useful resourcesbelow the Moon's surface and help engineers better plan and operate missions inlunar orbit.

Carrollsaid that when spacecraft began orbiting the Moon in the 1960s it became clearjust how lumpy and irregular the Moon's gravitational field is compared to theEarth's. Spacecraft tracking data obtained during the Apollo program and morerecently from NASA's Lunar Prospector mission have produced decent -- yet farfrom perfect -- gravity maps of the near side of the Moon. But gravity maps ofthe Moon's far side, Carrol said, are "largely guesswork at this point" because Earth-based tracking stations lose sight ofspacecraft as they travel over the lunar horizon.

The Lunettemission would solve that problem, Carroll said, by substitutingspacecraft-to-spacecraft tracking for Earth-based tracking. Lunette is afive-kilogram payload that would be added to a low altitude, lunarpolar-orbiting satellite mission such as the Indian Space Research Organisation's Chandrayaan-1, or NASA's LunarReconnaissance Orbiter. The payload consists of a three-and-one-half-kilogram nanosatellite and a small amount of equipment that wouldneed to be left behind on the parent spacecraft for the mapping mission.

The Lunettenanosatellite would be released from its parentspacecraft and then maintain a distance of 100 kilometers. The two spacecraftwould send signals back and forth using low-power transponders. By measuringslight changes in the signal, the differential effect gravity has on eachspacecraft can be measured, enabling scientists and engineers to create adetailed map of the Moon's lumpy gravitational field.

"All thegravity models of the Moon have been done using similar techniques excepttracking stations on the Earth have sent signals to spacecraft at the Moon,"Carroll said. "That's a classic range-rate tracking exercise NASA does onalmost all spacecraft it sends into deep space."

While thattried and true technique works fine for mapping the side of the Moon that facesEarth, it does not work so well for the far side of the Moon, Carroll said."What we aim to do is to do Doppler tracking on the far side of the Moon bytracking between one spacecraft and another."

The IndianSpace Research Organisation short listed Lunette forinclusion on Chandrayaan-1 last year, Carroll said, but had to move on when theteam was unable to secure an immediate funding commitment from the CanadianSpace Agency.

Likewise, thewindow of opportunity for including Lunette on the Lunar Reconnaissance Orbiterhas closed. NASA already has chosen its payloads for the 2008 mission, and theNASA official in charge of the project said it is too late to accommodatesomething like Lunette. "Effectively the door is closed because of the timing,"NASA Lunar Reconnaissance Orbiter program manager Mark Borkowskisaid.

Carrollsaid the team is still trying to line up a funding commitment for the mission,which he said could be done for a "Canadian-sized prize" of just a few milliondollars provided accommodations for the tiny nanosatellitecan be secured aboard some future Moon-bound orbiter.

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