FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1960
1960 - 0437.PDF
1GHT, 1 April 1960 437 TYPICAL HYPERBOLIC TRAJECTORY DIRECTION OF SUN issiles and Spaceflight . . . IONEER 5 AND ITS ORBIT I AUNCHED from Cape Canaveral by a three-stage Thor Able at' p.m. GMT on March 11, Pioneer 5 achieved an orbit around the in very close to that scheduled. In this article, D. E. Smith, BSc,FRAS, and A. E. Smith, AllnfSc, GradRAeS, describe the payload of •as new Son satellite and discuss current launch and orbital techniquesfor solar probes. PIONEER 5 is a 26-inch diameter instrumented sphere weigh-ing 901b. It carries five basic research instruments plus thenecessary electronic equipment for storing and transmitting the instrument readings back to Earth.As might be expected, instruments for the furtherance of radia- tion studies make up quite a high proportion of the payload. Anionization chamber and a Geiger-Miiller tube measure the total radiation flux encountered and weigh approximately 21b. Theyare particularly sensitive to medium-energy radiation. High-energy radiation is measured by a counter consisting of seven cylindersfilled with argon gas, and these are arranged so that one is encircled by the other six. The striking particles penetrate one or morecylinders, depending upon their energy, and ionize the argon, causing an electrical "blip." The weight of this unit is 51b. A magnetometer weighing only lib is included to determinethe strength and direction of magnetic fields in space; this instru- ment is no doubt included because of the known relationshipbetween radiation intensity and magnetic field patterns. Also included in the payload, accounting for almost another pound,is a micrometeorite counter. This instrument not only measures the number but also the momentum of meteoric dust particlesstriking the probe. To enable the significance of the instrument readings to be • properly understood, a half-pound photo-electric cell, known as an aspect indica-tor, is made to trigger an Hectrical impulse wheneveri- it points straight at the Sun. . Pioneer 5 carries two radiotransmitters, one of 5W and the other of 150W power.Both transmit on a fre- quency of 378Mc/s andobtain their power from batteries charged by 4,800solar cells mounted on four "paddle-wheels" measuringapproximately 18 X 14in, which protrude from thecentral sphere. The trans- mitters operate on commandfrom a ground tracking station and relay back toEarth the stored instrument readings. The satellite's receiver, therefore, is listening out thewhole time. The 5W transmitter is expected to have a range of i about five million miles, and it is hoped that the 150W transmitter"will enable the probe to relay back information up to about 50 million miles. This latest US achievement may perhaps be better understoodby discussing some of the launch and orbital problems associated with solar probes. As might be expected, probes which leave theEarth in the same direction as the Earth's motion round the Sun enter orbits lying outside that of the Earth, because their velocityabout the Sun has been increased. Probes launched in the opposite direction have their velocity relative to the Sun decreased andhence fall into orbits closer to the Sun. A solar probe is therefore launched so as to leave the Earth's sphere of influence with a largevelocity component in the opposite direction to the Earth's motion. (The Earth's sphere of influence is the sphere inside which theEarth's gravitational attraction predominates, and outside which the centre of attraction is the Sun. The radius of this sphere isabout 575,000 miles.) To escape from the Earth the probe's velocity at burnout mustbe greater than the escape velocity and this makes the probe follow a hyperbolic trajectory within the Earth's sphere of influ-ence. Thereafter the trajectory is an ellipse about the Sun. Should the velocity at burnout be less than the escape velocity the probewould either return to Earth and burn up in the denser layers of the atmosphere, or become another Earth satellite in an ellipticalorbit. The particular hyperbolic trajectory followed by the probe depends upon its velocity, range and direction of travel at burnout. AROUND SUN DIRECTION OF EARTH'S ROTATION Fig 2. Initial path of a solar probe launched into a typical hyperbolic trajectory (dotted section shows continuation of hyperbolic curve) Perihelion of ^Pioneer / (46m miles from Earth; Appro* portions of Lunik I and Pionetr E7 on April 1st I960 Perihelion of Pioneer Iff Aphelion of Pioneer 2 Position of Earth at launch of Pioneer 2 Position of Wnus when Pioneer 2 is at Perihelion Positions of Earth and Pioneer Y. on 1st April I960 // _ , /—A—Position of Eort Jyjf/ I when Pioneer S Aphelion of Lunik t Fig 1. The orbit of Pioneer 5 in relation to those of other bodies in the solar system. Distances indicated from the Sun are measured in millions of miles. Planetary orbits are actually slightly eccentric Similarly the orbit around the Sun which the probe enters dependsupon its velocity and direction of travel as it leaves the Earth's sphere of influence. The greater the velocity component (relativeto the Earth) in the opposite direction to the Earth's motion, the closer the resulting orbit will be to the Sun. This componentcan be increased slightly by taking advantage of the Earth's rota- tion on its axis, and this is achieved by turning the probe in aneasterly direction shortly after launch. It is interesting to note that the combined effect of the easterly turn after launch and theshape of the hyperbolic trajectory means that the local time at launch must be between 6 a.m. and 12 noon (this is illustratedin Fig 2). The planned burnout velocity of Pioneer 5 was a little over25,200 m.p.h. This in conjunction with an 8 a.m. (local time) launch from Cape Canaveral would have placed the probe in a295-day orbit intersecting the orbit of Venus at its perihelion (its closest approach to the Sun). In reality the probe's burnoutvelocity was 24,869 m.p.h. and, in consequence, it entered an orbit with a period of 312 days and a perihelion distance of74.9 million miles. This means that its nearest approach to the orbit of Venus will be about 7.7 million miles. It will be noted from Fig 1 that the perihelion position of theorbit of Pioneer 5 is very slightly less than 180° away from the launch position—measured in the direction of motion. This is aresult of the probe having a small velocity component towards the Sun when it left the Earth's sphere of influence. Had there beenno such component, the perihelion would be exactly 180° away and the aphelion (greatest distance from the Sun) would becoincident with the probe's point of exit from the Earth's sphere of influence. Similarly, if the velocity component towards the Sun hadbeen negative the perihelion position would have been slightly greater than 180° and the probe would in fact have passed throughaphelion shortly after leaving the Earth's sphere of influence. A complicating factor is the slightly eccentric orbit of the Earth.At present the Earth is moving away from the Sun and this motion is naturally possessed by the probe at the time of its launch fromthe Earth. An advantage of the perihelion position being less than 180°is that the probe should still be within radio range of the Earth when it passes through this position. The distance between theprobe and the Earth at this time is predicted to be about 46 million miles, and this will occur in mid-August this year. If the peri-helion position had been only a few degrees greater than 180 it is likely that the probe would have been greater than 50 million milesaway—the estimated range of the high powered transmitter—when it passed through perihelion. Pioneer 5 is the third artificial Sun satellite but the first solarprobe. The other two satellites, Lunik 1 launched on January 2, 1959, and Pioneer 4 launched on March 3, 1959, were launchedin the direction of the Earth's motion round the Sun and hence lie in orbits almost entirely outside that of the Earth (these orbitsare shown in Fig 1). These satellites ceased transmitting before they reached the edgeof the Earth's sphere of influence, and so Pioneer 5 is the first satellite to transmit information back to Earth whilst actually inorbit round the Sun. Pioneer 5 passed out of the Earth's sphere of influence and into its Sun orbit on March 15, 1960, approxi-mately five days after launch. It is not expected to come within a few hundred thousand miles of the Earth for perhaps thousandsof years, but it will come within radio range again in mid-1965. If its transmitters still transmit on command at that time, thesatellite will surely hit the headlines again.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
