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Anomaly in spacecraft flybys puzzles scientists

Sept. 22, 2005
Courtesy of SINC 
and World Science staff

The laws of gra­vity don’t seem to be work­ing ex­actly as they should around Earth­—and it’s af­fect­ing our own space probes, though not se­ri­ous­ly, a new re­port ob­serves.

When space agen­cies send probes to ex­plore the So­lar Sys­tem, they of­ten use its plan­ets and moons to help move those crafts along. Yet the pa­pe­r notes that there’s a ti­ny dis­crep­an­cy in the ex­pected ef­fect, ver­sus the ac­tu­al ef­fect, at least when Earth it­self is used for such ma­neu­vers.

Artist's rendition of the Euro­pean Space Agen­cy's Ro­set­ta probe dur­ing a fly­by of Earth. (Cour­tesy ESA/C.Car­reau) 


The use of plan­ets and moons to ad­just a craft’s mo­tion is called a “gra­vity as­sist.” It’s like bounc­ing a space­craft off a plan­et. Be­cause eve­ry plan­et is in mo­tion, this pro­ce­dure will change the speed of the bounced ob­ject—much as a ball, tossed at the front of a speed­ing train, would bounce back faster than the speed you threw it at. 

In the case of a “gra­vity as­sist,” the “bounce” is­n’t quite the fa­mil­iar kind of bounce. The space­craft does­n’t ac­tu­ally tou­ch the plan­et. In­stead, it flies close by. But the plan­et’s gra­vity pro­vides a si­m­i­lar ef­fect to a bounce. In fact, the math is quite sim­i­lar.

And it works quite well. The prob­lem is that the real-life ef­fect seems to dif­fer a bit from what cal­cula­t­ions, based on the known laws of phys­ics, pre­dict. The new pa­pe­r, pub­lished in the August issue of the re­search jour­nal Ad­vanc­es in Space Re­search, re­views the is­sue, and con­cludes that phys­i­cists still haven’t come up with a sat­is­fac­to­ry an­swer. Only mea­sure­ments near Earth have been pre­cise enough to clearly de­tect the ef­fect, ac­cord­ing to au­thor Lu­is Acedo Ro­dríguez, a phys­i­cist at the Pol­y­tech­nic Uni­vers­ity of Va­len­cia in Spain.

He sug­gests that the so­lu­tion might lie in fac­tors such as radia­t­ion from the sun, tides, or “dark mat­ter”—an in­vis­i­ble goop that is be­lieved to pe­r­me­ate ga­lax­ies through­out the uni­verse, though it be­trays its pres­ence only through its gra­vity.

The ti­ny anomaly has been de­tected in near-Earth fly­bys thanks to mon­i­tor­ing sta­t­ions such as that of the NASA in Rob­ledo de Cha­bela in Ma­drid or that of the Eu­ro­pe­an Space Agen­cy in Ce­breros, Spain, Acedo said. Thus, when the Gal­i­le­o space probe flew over Earth in 1990, in­stru­ments de­tected a change of speed of about one-hundredth of a mile or kil­o­me­ter per hour com­pared to what was ex­pected.

“These de­via­t­ions do not se­ri­ously af­fect the tra­jec­to­ries,” Acedo told Spain’s Sci­en­tif­ic In­forma­t­ion and News Serv­ice. Yet it’s “very im­por­tant to clar­i­fy” the causes, he in­sisted, “espe­cially in the cur­rent era of pre­cise space ex­plora­t­ion.” 

Sci­en­tists have pro­posed many hy­pothe­ses, he said. One points to­wards so­lar radia­t­ion be­ing the cause of the change in speed. Oth­ers sug­gest an in­flu­ence from mag­net­ic fields, from tides, or from a blob of dark mat­ter around Earth.

Acedo has pro­posed his own the­o­ry—though he him­self ad­mits it has prob­lems. It in­volves a sup­posed “cir­cu­lat­ing gra­vito­mag­net­ic field,” a force field that would fol­low the Earth’s par­al­lels. The ap­proach can be used to ex­plain the ma­jor­ity of cases, he said, and “E­in­stein’s gene­ral the­o­ry of rel­a­ti­vity pre­dicts the ex­ist­ence of a si­m­i­lar field.” Yet some in­stru­ments that should have de­tected the ef­fect have not.

If that is wrong, he wrote, the anom­a­lous be­hav­ior “must orig­i­nate in some­thing that, al­though com­mon, we have been un­aware of to date, or in an er­ror in the da­ta anal­y­sis pro­grams.”

Mean­while, he said, space probes con­tin­ue to chal­lenge sci­en­tists eve­ry time they pe­rform a “gra­vity as­sist.” One of the last was that of the space­craft Ju­no in last Oc­to­ber, from Earth en route to Ju­pi­ter. NASA has­n’t yet pub­lished da­ta on this jour­ney, but eve­rything in­di­cates that its speed as it flew over our plan­et was once again dif­ferent from es­ti­mates, Acedo said.

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