~NA~~~~v~o~L.~~~7~~~Y~~~~~3 ----------------------IN~E~WNSSAA~NWO~VfflliEWS-------------------------------------15 predominant insertion opposite apurinic distinguish a newly replicated DNA strand ween the Northern and Southern Hemi­ sites in single-stranded <I> X174 DNA and from the parental strand by the former's spheres. In their model, the force of the Kunkel (University of North Carolina) ob­ paucity of methylated adenines and acts by albedo as the satellite crosses the ter­ tained a similar result with heat-treated correcting replication errors in the newly minator of the Earth's shadow in the Nor­ single-stranded M13 mp2 DNA. This is synthesized strand against the 'correct' se­ thern Hemisphere is not balanced by the consistent with in vitro observations on quence in the parental strand (Meselson, force of the albedo as the satellite crosses DNA polymerase I by Loeb and by Strauss Harvard University). The same pheno­ the terminator in the Southern Hemisphere (University of Chicago). In terms of selec­ menon has now been shown to operate in because of the asymmetry of the albedo tive advantage during evolution this would cell-free extracts of E.coli (Lu, Duke between the two hemispheres. When it is seem sensible as the lesions most likely to be University). The in vitro activity is depen­ winter in the Northern Hemisphere, it is encountered by a DNA replication fork in dent on the state of the dam methylation of summer in the Southern Hemisphere. significant numbers are sunlight-induced the DNA and is deficient in extracts of the Moreover, the distribution of oceans and photoproducts involving adjacent mutator strains mut-H, mutL and mutS. continents is different in each hemisphere. thymines. Preferential insertion of deoxy­ The correction process involves a repair­ Anselmo eta/. 6 show that the asymmetry in adenosine in such a circumstance would synthesis event of considerable length (up the reflective properties of the Earth bet­ result in conservation of the genetic infor­ to 1,000 nucleotides) which probably stret­ ween the Northern and Southern mation rather than mutation. ches between the mismatch site and the Hemispheres can produce a variable ac­ Although untargeted mutations were nearest dam methylation site. Both these celeration of the Lageos satellite very frequently found in infections of UV­ sites (relatively remote from one another) similar to that observed. Over several years irradiated bacteria by bacteriophage they have to be present for the repair synthesis to however, this effect averages to zero and were presumed to be less important in occur, which raises intriguing possibilities therefore cannot explain the basic accelera­ bacteria themselves because of the for the mechanism of action. D tion. Nonetheless, considering the simplici­ effectiveness of the bacterial mismatch­ Bryn Bridges is at the MRC Cell Mutation Unit, ty of the model it seems likely that the correction system. This system is able to University of Sussex, Brighton BNJ 9QG. north-south asymmetric albedo is the cause of the variations in the acceleration that are seen on Lageos. Further, the model Celestial mechanics predicts several other periodic components that are seen in the acceleration data. J. Morgan 7 has further suggested that Acceleration on Lageos spacecraft asymmetries in the Earth's albedo in local from David E. Smith time could cause the observed constant component of the acceleration of Lageos. SINCE the launch of the Lageos spacecraft celerating in a manner consistent with drag In his model, the albedo of the Earth at in May 1976 into a near-circular orbit at (neutral or charged particles) but at a much dawn is different from the albedo at dusk, 1 6,000 km altitude, the spacecraft orbit has higher rate than expected • Attempts to ex­ thus causing an acceleration (or decelera­ been decaying at an average rate of about 1 plain this fact 2 were initially directed tion) of the spacecraft. If the asymmetry in mm per day. This decay causes the satellite towards the possible existence of helium at albedo with local solar time is preserved to accelerate in its orbit at a rate of about the satellite's altitude of 6,000 km but ran over extended periods oftime, as the model 3 x to-12 m s -2, which is one to two orders into the difficulty of requiring very large suggests, then it could, in principle, pro­ of magnitude larger than was expected. exospheric temperatures. Neutral hydro­ vide the constant acceleration that is The cause of the acceleration (orbit decay) gen was also ruled out as the primary cause observed. Morgan's model has not yet 2 3 is unknown but calculations and of the drag • and attention became focus­ undergone the testing required to establish theoretical studies of the changes in the ed on the effect of charged particles. As the the magnitude of the local time asymmetry spacecraft orbit suggest that charged­ spacecraft moves through the atmosphere that would be needed to sustain the acceler­ particle drag and Earth albedo radiation it becomes charged through collisions with ation. pressure (sunlight reflected from the Earth charged particles and interacts electromag­ Recent tracking data have suggested that back onto the spacecraft) are the most like­ netically with particles at a distance, the satellite is no longer accelerating but ly explanations. Recently, the orbital decay thereby losing energy. Although we do not rather decelerating, causing the satellite or­ seems to have ceased and even reversed, at know the charge on the spacecraft it seem­ bit, at least temporarily, to stop decaying least temporarily, so that the orbit may be ed that charged-particle drag could ac­ and expand. Verification of this change is 3 5 beginning to increase again in size. If this is count for the observation - • extremely important because, if true, the case, charged particle drag cannot be Several years of laser tracking of Lageos charge drag cannot be the only cause of the the sole explanation and Earth albedo have shown systematic variations in the ac­ acceleration and an additional force, such becomes a very probable second cause. celerations about the mean of around 100 as albedo, must be introduced. A tem­ The Lageos spacecraft is a sphere 60 em per cent and with several different periods. porary change in the albedo such that the in diameter carrying 426 laser corner re­ The largest-amplitude period is about 285 acceleration becomes a deceleration is a flectors on its surface. The precision-built days, half the time the orbit takes to precess much more acceptable explanation since spacecraft was designed to be a near­ in space with respect to the Sun. Although the albedo is closely related to the vagaries perfect target for ground-based laser­ the variations in the acceleration are of clouds and the weather and is known to tracking systems that could measure the significant, none has been successfully cor­ vary geographically and seasonally. 0 spacecraft's distance (range) to better than related with standard atmospheric indices, a centimetre. The measurements were to be such as 10.7 -em solar radiation or David E. Smith is Project Scientist for the used to determine precisely the relative planetary indices reflecting particle fluxes NASA Crustal Dynamics Project and Head of positions of the laser-tracking stations for in the atmosphere. The existence of a the Geodynamics Branch, Goddard Space estimating crustal movements, such as tec­ strong 285-day period in the acceleration Flight Center, Greenbelt, Maryland 20771. tonic plate motion, and the rotation of the however, is a clear indication that the ef­ I. Smith, D.E. & Dunn, P. Geophys. Res. l.e/1. 7, 437 (1980). Earth on its axis. fect, at least in part, is related to the 2. Rubincam, D.P. Geophys. Res. /.ell. 7, 468 (1980). Because of its design, high altitude and geometry of the orbit with respect to the 3. Afonso, G., Barlier, F., Berger, C. & Mignard, F. C. r. hebd. !Seanc. A cad. Sci., Paris 2908, 445 (1980). very accurate tracking, Lageos has the Sun and therefore of solar origin. 4. Mignard, F. Pap. presented at the Space Geodesy Coloq., most precisely known orbit of any artificial Recently, Anselmo et a/. b have inves­ Cannes, France (November 1980). 5. Rubincam, D.P. Celest. Mech. 26, 361 (1982). satellite. Within weeks of its launch it tigated the effect of Earth albedo on 6. Anselmo, L. eta/. Astr. Astrophys. 117,3 (1983). became clear that the spacecraft was ac- Lageos and, in particular, variations bet- 7. Morgan, W.J. Personal communication (1983). 0028.0836/83/270015-01$01.00 © 1983 Macmillan Journals Ltd .